A variable-frequency / line-frequency automatic switching energy-saving system and method

Abstract

The application discloses a variable-frequency-power-frequency automatic switching energy-saving system and method, and belongs to the technical field of motor control, which comprises the following steps: in the variable-frequency starting stage, a motor is driven to run at variable speed by a frequency converter; when the motor reaches full frequency, high power and stable operation, a control device controls the output voltage of the frequency converter to be synchronous with the grid voltage, and the switching to the power-frequency grid is completed when the phases are consistent; when the load decreases and needs to be reduced, the control device starts the frequency converter to make the output voltage of the frequency converter synchronous with the counter electromotive force of the motor in the power-frequency operation, then performs a back switching operation, the motor in rotation is smoothly connected by the frequency converter, and speed reduction control is implemented, and finally the variable-frequency speed regulation operation state is returned; the application realizes automatic selection of the optimal operation mode according to the actual load, avoids energy loss of the frequency converter in the high-speed area, and ensures that the impact on the motor and the grid in the switching process is minimum.

Description

Technical Field

[0001] This invention belongs to the field of motor control technology, specifically an energy-saving system and method for automatic switching between variable frequency and power frequency. Background Technology

[0002] Variable frequency drives (VFDs) have been widely used in speed control of high-power equipment such as fans and pumps. By changing the motor speed, they adapt to load changes and achieve significant energy-saving effects. However, when the production process requires the equipment to run at full load, the motor itself is already in power frequency operation, but the current still needs to flow through the IGBT power module and filter circuit inside the VFD. The VFD will generate a certain amount of power loss in this process. This loss mainly includes switching loss, conduction loss and its own auxiliary power consumption. Under the current operating mode, even when the equipment is under full load for a long time, it is still powered by the frequency converter. This additional energy consumption is wasted and does not meet the requirements of lean management and green energy saving. An intuitive improvement idea is to add a power frequency bypass to the frequency converter and switch to power frequency operation when under full load. However, direct switching has technical risks: because the phase of the 50Hz voltage output by the frequency converter is randomly different from the phase of the power frequency voltage of the grid, if the switch is made directly at an asynchronous moment, it is equivalent to two asynchronous power sources being connected to the grid at an instant. This will generate an inrush current of up to 5-8 times the rated current, which can easily cause the circuit breaker to trip, the contactor contacts to weld, or even damage the motor and the frequency converter. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention proposes an energy-saving system and method for automatic switching between variable frequency and mains frequency, comprising: during the variable frequency start-up phase, the motor is driven by the variable frequency drive for speed regulation; when the motor reaches full frequency, high power and stable operation, the control device controls the output voltage of the variable frequency drive to synchronize with the grid voltage, and completes the switch to the mains frequency grid when the phases are consistent; when the load decreases and speed reduction is required, the control device starts the variable frequency drive to synchronize its output voltage with the back electromotive force of the motor running at mains frequency, and then performs a back-switching operation, whereby the variable frequency drive smoothly takes over the rotating motor and implements speed reduction control, finally returning to the variable frequency speed regulation operation state; this invention realizes automatic selection of the optimal operating mode according to the actual load, avoids energy loss of the variable frequency drive in the high-speed range, and ensures that the impact of the switching process on the motor and the grid is minimized.

[0004] To achieve the above objectives, the present invention provides the following technical solution:

[0005] An energy-saving method for automatic switching between variable frequency and mains frequency includes:

[0006] When the main circuit contactor is closed and the power frequency bypass contactor is open, the frequency converter is soft-started and the motor is driven to run in the frequency conversion speed regulation operation state.

[0007] In the variable frequency speed regulation operation state, the control device monitors the motor operating frequency and actual power. When the motor operating frequency reaches the set full frequency value, the actual power meets the rated power threshold and continues for a preset time, it is determined that the variable frequency to power frequency switching condition is met.

[0008] Based on the judgment result, the control device sends a synchronization command to the frequency converter, the frequency converter enters the tracking mode, and the control device compares the phase difference between the output voltage of the frequency converter and the grid voltage in real time. When the phase difference is less than the set safety angle threshold, the main circuit contactor is disconnected, and after a preset dead time delay, the power frequency bypass contactor is closed to switch the motor to direct operation on the power frequency grid.

[0009] When the motor is running under the drive of the power frequency grid and the control device determines that the speed needs to be reduced according to the load demand, the control device starts the frequency converter and makes its output voltage track the back electromotive force of the motor under the current power frequency grid drive. After the output voltage of the frequency converter is synchronized with the back electromotive force of the motor, the power frequency bypass contactor is disconnected and the main circuit contactor is closed. The frequency converter captures the rotating motor and smoothly reduces the speed, and re-enters the variable frequency speed regulation operation state.

[0010] Specifically, the closing of the main circuit contactor and the opening of the power frequency bypass contactor are controlled by the controller outputting high and low level signals. The controller first outputs a high level to the main circuit contactor coil to make it energize, and then outputs a low level to the power frequency bypass contactor coil to make it disconnect. After the contactor status feedback signal is sent back to the controller, the controller starts the inverter soft start program.

[0011] Specifically, the logic for determining whether the frequency conversion to power frequency switching condition is met is as follows:

[0012] Three key judgment parameters are preset inside the control device; the key judgment parameters include the full frequency value of the motor operating frequency, the rated power threshold of the actual power of the motor, and the preset duration; the control device is a programmable logic controller, an embedded microprocessor, or a control unit integrated inside the frequency converter.

[0013] The control device collects the motor operating frequency signal and the actual power signal from the frequency converter in real time, and converts them into operating frequency value and actual power value through the analog-to-digital converter inside the control device;

[0014] The control device compares the converted operating frequency value with the set full frequency value. If the operating frequency value is greater than or equal to the full frequency value, a signal indicating that the frequency meets the standard is generated. At the same time, the actual power value is compared with the rated power threshold. If the actual power value is greater than or equal to the rated power threshold, a signal indicating that the power meets the standard is generated.

[0015] The control device performs a logical AND judgment on the frequency compliance flag and the power compliance flag. The control device starts the internal timer to start accumulating time only when both the frequency compliance flag and the power compliance flag are true. If either compliance flag becomes false, the internal timer is reset to zero.

[0016] The control device compares the accumulated time of the internal timer with the preset duration in real time. When the accumulated time of the internal timer is greater than or equal to the preset duration, it determines that the motor is running stably at full frequency and at a state greater than or equal to the preset percentage of rated power.

[0017] Based on the positive result of the duration determination, the control device generates a switching enable signal internally, indicating that the variable frequency to power frequency switching condition has been met.

[0018] Specifically, the synchronization command is a digital signal or a communication message containing a mode switching command. The synchronization command directly triggers the inverter's operating mode to change from V / F control or vector control to the tracking mode.

[0019] Specifically, when the frequency converter enters tracking mode, the control device compares the phase difference between the frequency converter output voltage and the grid voltage in real time, including:

[0020] When the control device sends a synchronization command to the frequency converter, the frequency converter enters the tracking mode and activates its internal integrated grid voltage sampling circuit to sample the grid voltage in real time.

[0021] The inverter uses the grid voltage it samples as a reference signal to start the internal phase-locked loop control unit. The phase-locked loop control unit dynamically adjusts the phase and frequency of the drive carrier signal of the inverter's own power devices through feedback regulation to synchronize it with the grid voltage.

[0022] The control device calculates the phase difference between the inverter output voltage and the grid voltage by reading the real-time phase data calculated by the phase-locked loop control unit or by obtaining the zero-crossing time difference between the inverter output voltage and the grid voltage through an independent phase detection circuit.

[0023] Specifically, when the motor is running under the drive of the power frequency grid and the control device determines that a speed reduction is needed based on the load demand, the control device starts the frequency converter and causes its output voltage to track the back electromotive force of the motor under the current power frequency grid drive, including:

[0024] The control device receives an external speed reduction command through its communication interface or determines through its internal logic that when the actual power of the motor is continuously lower than the preset cut-off power threshold and the preset cut-off judgment time is reached, it generates an internal speed reduction command.

[0025] Based on the internal deceleration command, the control device starts the frequency converter, and the frequency converter outputs a pre-excitation voltage with an amplitude lower than the rated voltage to the motor terminals.

[0026] The inverter uses its output pre-excitation voltage as a reference and detects the back electromotive force generated by the motor when it rotates under the drive of the power grid through its internal detection circuit. It then automatically adjusts the frequency, phase, and amplitude of its output voltage based on the frequency and phase of the back electromotive force until it is synchronized with the back electromotive force of the motor. The back electromotive force of the motor is the residual voltage on the motor terminals.

[0027] The control device or frequency converter monitors the phase difference between its output voltage and the back electromotive force of the motor in real time. When the phase difference is less than the set synchronization tolerance angle threshold, synchronization is determined to be complete.

[0028] Specifically, the process of generating the internal deceleration command includes:

[0029] The control device simultaneously activates the external command path and the internal judgment path; the external command path listens for external deceleration command signals from the operation interface through the communication interface of the control device; the internal judgment path obtains the real-time actual power value of the motor under the drive of the power frequency grid through the data acquisition system of the control device, and compares it with the internally preset back-cut power threshold.

[0030] In the external command path, when the communication interface detects a level transition or an external speed reduction command signal, a hardware interrupt is generated or a status flag is set. The central processing unit of the control device verifies the validity of the external speed reduction command signal to confirm whether the external speed reduction command signal is valid.

[0031] In the internal judgment path, the control device compares the real-time actual power value with the back-off power threshold. When the real-time actual power value is lower than the back-off power threshold, the internal condition satisfaction flag is set. At the same time, the control device starts a timer to accumulate the duration of condition satisfaction. The control device compares the accumulated duration of the timer with the preset back-off judgment duration in real time. Only when the accumulated duration is greater than or equal to the preset back-off judgment duration is the internal condition satisfaction flag finally confirmed.

[0032] The control device performs a logical OR operation on the outputs of the external command path and the internal judgment path to generate an internal deceleration command.

[0033] Specifically, after the output voltage of the inverter synchronizes with the back electromotive force of the motor, the power frequency bypass contactor is disconnected, and then the main circuit contactor is closed, allowing the inverter to capture the rotating motor and smoothly reduce its speed, including:

[0034] After the synchronization is completed, the control device disconnects the power frequency bypass contactor, and after a preset second dead time delay, closes the main circuit contactor to complete the switching of the motor power supply circuit from the power frequency grid to the frequency converter.

[0035] After the main circuit contactor is closed, the frequency converter automatically performs frequency tracking based on the detected back electromotive force of the motor, so that its output frequency is synchronized with the actual rotation frequency of the motor.

[0036] After completing frequency tracking, the inverter reduces its output frequency according to the preset deceleration curve, thereby causing the motor speed to decrease synchronously.

[0037] When the output frequency of the frequency converter drops to the preset target operating frequency, the speed reduction process ends, and the motor enters the speed regulation operation state driven by the frequency converter.

[0038] Specifically, the main circuit contactor and the power frequency bypass contactor are provided with a double interlock, including an electrical soft interlock implemented by the control device program and a hardware hard interlock composed of the contactor auxiliary contacts.

[0039] An energy-saving system with automatic switching between variable frequency and mains frequency includes: a control module, a power switching module, a variable frequency drive module, and a signal detection module;

[0040] The control module is used to execute control logic, process sensor signals, and make switching decisions;

[0041] The power switching module is used to change the power supply path of the motor under the instruction of the control module, and to complete the switching between the frequency conversion circuit and the power frequency circuit.

[0042] The variable frequency drive module is used for soft start and speed regulation of the motor in variable frequency mode, and to operate in tracking mode during switching.

[0043] The signal detection module is used to monitor the motor's operating frequency and actual power.

[0044] Compared with the prior art, the beneficial effects of the present invention are:

[0045] This invention proposes an energy-saving system for automatic switching between variable frequency and power frequency, and optimizes and improves its architecture, operation steps and processes. The system has the advantages of simple process, low investment and operating costs and low production and working costs.

[0046] This invention proposes an energy-saving method for automatic switching between variable frequency and mains frequency. By continuously monitoring the operating frequency and actual power of the motor, the motor is automatically switched from variable frequency operation to mains frequency operation only when the stringent conditions of full frequency, high load and continuous stability are met. This avoids the additional losses caused by the frequency converter operating in the high-speed range, and directly utilizes the mains frequency power supply, thereby improving the system's operating efficiency under stable high load conditions.

[0047] This invention proposes an energy-saving method for automatic switching between variable frequency and mains frequency, ensuring the safety and smoothness of the switching process. Whether switching from variable frequency to mains frequency or back from mains frequency to variable frequency, strict voltage synchronization control is used, including synchronization with the grid voltage and with the motor back EMF, to ensure that the switching occurs at the moment of phase alignment. A necessary dead zone delay is also set to effectively avoid current surges. In particular, during the back-switching process, the frequency converter can capture the rotating motor without shock and achieve smooth speed reduction, ensuring equipment safety and process stability, and forming a highly efficient energy-saving operation closed loop. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of an energy-saving method for automatic switching between variable frequency and mains frequency according to the present invention;

[0049] Figure 2 This is a flowchart illustrating the principle of an energy-saving method for automatic switching between variable frequency and power frequency according to the present invention.

[0050] Figure 3 This is a diagram of an energy-saving system architecture for automatic switching between variable frequency and mains frequency according to the present invention. Detailed Implementation

[0051] Example 1:

[0052] Please see Figure 1 and Figure 2 The present invention provides an embodiment of an energy-saving method for automatic switching between variable frequency and mains frequency, the method comprising S1 to S4, including the following steps:

[0053] S1: The main circuit contactor is closed and the power frequency bypass contactor is open, controlling the inverter to soft start and driving the motor to run in the variable frequency speed regulation operation state;

[0054] The closing of the main circuit contactor and the opening of the power frequency bypass contactor are controlled by the controller outputting high and low level signals. The controller first outputs a high level to the main circuit contactor coil to make it engage, and then outputs a low level to the power frequency bypass contactor coil to make it disengage. After the contactor status feedback signal is sent back to the controller, the controller starts the inverter soft start program.

[0055] It is important to emphasize that in this method, the closing of the main circuit contactor and the opening of the power frequency bypass contactor are achieved through high and low level signals output by the controller. The operation strictly follows the logic of first engaging the main circuit contactor and then disconnecting the power frequency bypass contactor. The inverter soft-start program is only initiated after the contactor status feedback signal is transmitted back to the controller. This design is not a simple arrangement of action sequences, but rather a deep consideration of electrical circuit safety and starting stability, possessing irreplaceable functional value. From a safety perspective, this sequential action logic fundamentally avoids the risk of a power short circuit caused by both contactors conducting simultaneously. In industrial electrical systems, the main circuit contactor connects the inverter to the motor, while the power frequency bypass contactor directly connects the power grid to the motor. If both are closed simultaneously, a short circuit will be formed directly connecting the inverter output to the power grid. The resulting massive short-circuit current could burn out core equipment such as the inverter, contactor, and motor, and may even cause a power grid failure. This application ensures that the main circuit contactor is fully engaged and the motor is reliably connected to the frequency converter drive circuit through precise timing control of the controller before disconnecting the power frequency bypass contactor, completely cutting off the direct connection between the power grid and the motor, forming a single drive circuit. This eliminates the risk of short circuits from the perspective of action logic. Compared with the sequential control that relies on physical structure for traditional mechanical interlocking, the response speed of electrical signal control is faster and the action consistency is higher. It can complete the contactor state switching in milliseconds, which is suitable for the high-frequency start and stop requirements in industrial production.

[0056] From the perspective of the status verification mechanism, the closed-loop design of the contactor status feedback signal returning to the controller constructs a status confirmation barrier before startup. By receiving the feedback signal from the contactor, the controller can determine in real time whether the main circuit contactor is indeed engaged and whether the power frequency bypass contactor is indeed disengaged, avoiding drive abnormalities caused by mechanical faults of the contactor, such as contact sticking or coil burnout. For example, if the main circuit contactor fails to engage normally due to contact sticking, the feedback signal will promptly inform the controller. At this time, the controller will pause the inverter startup program and issue a fault alarm, preventing equipment damage caused by starting the inverter under no-load conditions or starting the motor without connecting it to the drive circuit. Compared with traditional open-loop control, this improves the reliability of the startup process and ensures that every startup is based on the normal circuit status.

[0057] From the perspective of startup performance optimization, the timing setting of the inverter's soft-start program directly determines the smoothness of motor startup and grid compatibility. In traditional direct-start methods using industrial frequency, the motor starting current can reach 3-7 times the rated current. This huge inrush current can cause a sudden drop in grid voltage, affecting the normal operation of other electrical equipment on the same grid. Simultaneously, it generates a strong thermal shock to the motor windings, accelerating insulation aging and shortening the motor's lifespan. In this application, the inverter's soft-start program starts after the contactor status is confirmed. The inverter's output voltage gradually increases from low to high, and the motor speed rises smoothly with the voltage increase. The starting current is strictly controlled within 1.5 times the rated current. This soft-start method not only reduces the impact on the grid and ensures grid voltage stability but also reduces electromagnetic losses and mechanical shocks during motor startup, extending the motor's lifespan. This advantage is particularly prominent in high-power motor applications, such as water pumps and fans exceeding 500kW. Traditional industrial frequency startup can cause grid voltage fluctuations exceeding 10%, triggering shutdowns of other precision equipment. The soft-start function of this application can control voltage fluctuations within 3%, ensuring the stable operation of the entire production system.

[0058] Furthermore, the controller employs a high- and low-level signal control contactor design, possessing strong adaptability and scalability. Whether it's a programmable logic controller, an embedded microprocessor, or a control unit integrated within the frequency converter, it can output high and low level signals through a standard digital output interface, eliminating the need for additional dedicated control modules and reducing system integration costs. Simultaneously, the high and low level signals exhibit strong anti-interference capabilities. Even in complex electromagnetic environments in industrial settings, such as harmonic interference generated by the frequency converter and electromagnetic pulses from the start-up and shutdown of high-power equipment, the signal remains stable, preventing erroneous triggering of control commands. Compared to analog signal control, its reliability and environmental adaptability are superior.

[0059] In summary, the precise timing control of the main circuit contactor and the power frequency bypass contactor not only solves the impact problem of traditional starting methods but also avoids the risk of short circuits, making it irreplaceable by traditional starting control methods.

[0060] S2: In the variable frequency speed regulation operation state, the control device monitors the motor operating frequency and actual power. When the motor operating frequency reaches the set full frequency value, the actual power meets the rated power threshold and continues for a preset time, it is determined that the variable frequency to power frequency switching condition is met.

[0061] It should be noted that the core value of variable frequency to mains frequency switching lies in on-demand switching. That is, energy-saving benefits can only be achieved by switching to mains frequency operation when the motor is running at full frequency and high power stability. If the switching timing is misjudged, such as switching under light load or during a momentary full frequency surge, not only will energy savings not be achieved, but it will also lead to decreased operating efficiency or equipment shock. This application achieves accurate identification of switching conditions through a multi-stage closed-loop judgment logic involving preset key parameters, real-time signal acquisition, logic AND judgment, and timing verification. Its functional design possesses irreplaceable scientific rigor and reliability. The three key judgment parameters preset within the control device are not fixed standard values, but can be flexibly adjusted according to the rated parameters of the motor, load characteristics, and production needs. For example, the rated power of a fan-type load may correspond to an operating frequency close to 50Hz, while a compressor-type load may reach its rated power at 48Hz. This application allows users to set the full-frequency value, rated power threshold, and preset duration according to actual operating conditions, enabling the switching judgment logic to adapt to the operating characteristics of various motors. Compared with the fixed parameter judgment scheme in the prior art, it has stronger adaptability and a wider range of applications. At the same time, the control device supports multiple options such as programmable logic controllers, embedded microprocessors, or control units integrated within the frequency converter. These control units all have high-speed computing capabilities and anti-interference capabilities, and can operate stably in complex industrial environments, ensuring accurate storage and retrieval of parameter settings. For example, the programmable logic controller has a modular design and can add signal acquisition channels through expansion modules to adapt to multi-motor linkage control scenarios; the control unit integrated within the frequency converter can directly read the frequency and power signals output by the frequency converter, reducing signal transmission losses and interference, and improving the real-time performance of parameter comparison.

[0062] It should also be noted that the control device performs a logical AND judgment on the frequency and power compliance flags. The timer is only activated when both flags are true simultaneously; if either flag becomes false, the timer is immediately reset. This design eliminates invalid switching caused by a single parameter meeting the standard. In industrial production, motors may operate at full frequency but under light load, or at high power but not at full frequency. For example, a fan may reach full frequency in the later stages of startup, but the load has not yet reached its rated value. Switching to mains frequency operation at this time would cause the motor to operate inefficiently under light load conditions, increasing energy consumption. Similarly, a compressor may briefly reach its rated power during a sudden load change, but the frequency has not yet reached full frequency. Switching at this time would cause a shock due to the mismatch between the speed and the mains frequency voltage. The logical AND judgment design of this application ensures that switching is only possible when the motor simultaneously meets the two core conditions of full-frequency operation and high-power load, avoiding the aforementioned invalid switching scenarios. The control device generates a switching enable signal only when the timer's accumulated time reaches the preset duration. This delay design is the final verification step for determining the switching conditions. This application uses a preset delay to allow the control device sufficient time to confirm the stability of the motor's operating state, ensuring that the motor has entered a continuous and stable full-load operation phase. For example, high-power motors have significant inertia and may require 8 seconds to stabilize power after reaching full frequency. A preset 8-second delay effectively avoids switching during power fluctuations, ensuring a smooth switching process. Compared to existing technologies with no or short delays in decision-making, this application's delay design better aligns with the dynamic operating characteristics of motors, further verifying the sufficiency of switching conditions from a time perspective. This ensures that each switch is a rational decision based on the motor's stable operating state, rather than a misjudgment of instantaneous operating conditions.

[0063] In summary, the closed-loop design with multi-parameter presets, high-precision signal conversion, dual-standard logic, and delay judgment can accurately identify the optimal switching conditions of the motor, avoiding invalid and erroneous switching. Its judgment accuracy and reliability are irreplaceable by existing single-parameter judgment or simple logic judgment schemes.

[0064] S3: Based on the judgment result, the control device sends a synchronization command to the frequency converter, the frequency converter enters the tracking mode, and the control device compares the phase difference between the output voltage of the frequency converter and the grid voltage in real time. When the phase difference is less than the set safety angle threshold, the main circuit contactor is disconnected, and after a preset dead time delay, the power frequency bypass contactor is closed to switch the motor to the power frequency grid for direct operation.

[0065] Furthermore, when the phase difference is less than the set safe angle threshold, the main circuit contactor is disconnected, and after a preset dead time delay, the power frequency bypass contactor is closed to switch the motor to direct operation on the power frequency grid, including:

[0066] (1) After confirming that the phase difference between the inverter output voltage and the grid voltage is stable and less than the preset safe angle threshold, the control device generates a switching trigger signal;

[0067] Furthermore, the process of setting the safety angle threshold includes: measuring the total time required from the control device issuing the disconnection command to the complete disconnection of the main circuit contactor contacts, converting this time into the corresponding power frequency electrical angle, which is the minimum response angle of the system; determining the maximum allowable phase difference that does not generate harmful inrush current at the moment of switching through electrical analysis or simulation, which is the maximum allowable safe angle; selecting a specific value between the minimum response angle of the system and the maximum allowable safe angle as the theoretical safe angle threshold, which must be greater than the minimum response angle and less than the maximum allowable safe angle; applying the theoretical safe angle threshold to the actual system for switching tests, fine-tuning the theoretical safe angle threshold based on the measured voltage and current waveforms, and finally determining the optimized preset safety angle threshold.

[0068] (2) Based on the switching trigger signal, the control device sends a disconnection command to the main circuit contactor and monitors the status of its auxiliary contacts to confirm that it has been reliably disconnected;

[0069] Furthermore, by switching the trigger signal, the electrical connection between the frequency converter and the motor is severed, and the motor is temporarily disconnected from all power sources. The control device confirms successful disconnection by monitoring the state changes of the auxiliary normally closed contacts of the main circuit contactor.

[0070] (3) After the main circuit contactor is confirmed to be disconnected, the control program executes the preset dead time delay; the dead time delay has three functions: the primary purpose is to ensure that the arc that may be generated between the main contacts of the main circuit contactor due to the disconnection current has enough time to be completely extinguished and the insulation strength restored; the secondary purpose is to provide a buffer to ensure that the main circuit contactor is completely in a stable state of mechanical disconnection; the ultimate purpose is to establish an absolute electrical isolation period to ensure that the power frequency bypass contactor is closed in a safe state where the main circuit contactor is completely de-energized and disconnected.

[0071] (4) After the dead zone delay ends, the control device immediately sends a closing command to the power frequency bypass contactor to put it into operation, thereby connecting the motor to the power frequency grid. At this time, the motor is directly powered by the power frequency grid.

[0072] (5) After the control device confirms that the auxiliary normally open contact of the power frequency bypass contactor has been reliably closed by detecting the contact, the control device controls the frequency converter to enter the standby state and updates the system operation mode to power frequency operation.

[0073] It should be explained that when the switching conditions are met, the control device sends a synchronization command to the frequency converter, triggering the frequency converter to switch from the conventional control mode to the tracking mode. This step is the core technological breakthrough for achieving smooth switching between frequency converter and power frequency. Its functional design directly determines the impact and stability of the switching process. Compared with the existing technology that only supports a single command form, the dual command design method of this application can seamlessly connect with different types of frequency converters and control systems without the need for large-scale modification of existing equipment, thus reducing system upgrade costs. For example, the control system of an old production line may only support digital signal interaction and can directly use digital synchronization commands; the control system of a newly built smart factory has complete communication functions and can achieve more refined synchronization control through communication messages. The flexible selection of the two modes greatly expands the application scope of this application. During variable frequency speed regulation operation, the V / F control mode has the advantages of simple structure, high reliability, and low cost, and can meet the speed regulation requirements of most general-purpose motors. By setting the ratio between voltage and frequency, linear adjustment of motor speed can be achieved. The vector control mode can achieve decoupled control of speed and torque, with higher speed regulation accuracy and dynamic response speed. It is suitable for scenarios with strict speed control requirements, such as precision machine tools and synchronous drive of conveyor belts. The compatibility of the two conventional control modes ensures that the motor can achieve accurate and stable speed regulation according to load requirements during variable frequency speed regulation operation, giving full play to the energy-saving advantages of variable frequency technology. Upon receiving a synchronization command, the inverter switches to tracking mode. This mode is specifically designed for inverter-to-power frequency switching. Its core function is to ensure that the phase and frequency of the inverter's output voltage are precisely synchronized with the grid voltage. In traditional switching schemes, the inverter directly stops outputting and switches to power frequency. Due to the deviation between the motor speed and the power frequency voltage frequency, huge current and torque surges are generated, leading to contactor contact erosion, motor insulation damage, and grid voltage fluctuations. However, the tracking mode of this application achieves synchronous phase and frequency between the output voltage and the grid voltage through precise internal adjustment of the inverter, eliminating the switching shock at its source. This design of conventional speed regulation mode and tracking switching mode allows the inverter to perform optimally in different operating stages. Compared with the existing single-mode operation scheme of the inverter, it ensures both the accuracy of the inverter speed regulation stage and the smoothness of the switching stage, achieving seamless connection between the two operating states.

[0074] In summary, the flexible design of the synchronization command and the precise switching of the control mode provide adaptability and synchronization performance that cannot be replaced by the simple switching mode in the existing technology, ensuring that the switching process is fast, accurate and stable.

[0075] The synchronization command is a digital signal or a communication message containing a mode switching command. The synchronization command directly triggers the inverter's operating mode to change from conventional V / F control or vector control to the tracking mode.

[0076] S4: When the motor is running under the drive of the power frequency grid and the control device determines that the speed needs to be reduced according to the load demand, the control device starts the frequency converter and makes its output voltage track the back electromotive force of the motor under the current power frequency grid drive. After the output voltage of the frequency converter is synchronized with the back electromotive force of the motor, the power frequency bypass contactor is disconnected and the main circuit contactor is closed. The frequency converter captures the rotating motor and smoothly reduces the speed, and re-enters the variable frequency speed regulation operation state.

[0077] The main circuit contactor and the power frequency bypass contactor are equipped with a double interlock, including an electrical soft interlock implemented by the control device program and a hardware hard interlock composed of the contactor auxiliary contacts.

[0078] It is important to understand that in industrial production, the motor, as the core power source, directly determines the energy consumption level and operational stability of the entire production system. Traditional motor operation modes have inherent drawbacks. In pure power frequency operation, the motor speed is fixed and cannot be flexibly adjusted according to load requirements. Energy consumption is extremely high under light load conditions, and the huge inrush current generated during startup can affect grid stability and motor lifespan. In pure variable frequency operation, although precise speed regulation can be achieved, the power loss of the inverter itself during the stable operation of the motor at full load leads to a decrease in overall operating efficiency, failing to fully utilize the motor's rated performance. To resolve this contradiction, variable frequency to power frequency switching technology has emerged. However, existing switching schemes generally suffer from problems such as large switching impact, low synchronization accuracy, inaccurate switching condition determination, high risk of short circuits, and uneven speed reduction, which restrict energy-saving effects and equipment safety. The variable frequency to power frequency automatic switching energy-saving method proposed in this application, through precise and systematic design, forms a complete closed loop from startup control, switching determination, synchronization adjustment, circuit switching, and safety protection, achieving a dual breakthrough in energy-saving benefits and operational safety.

[0079] The specific logic for determining whether the frequency conversion to power frequency switching condition is met is as follows:

[0080] S2.1: Three key judgment parameters are preset inside the control device; the key judgment parameters include the full-frequency value of the motor operating frequency, the rated power threshold of the actual power of the motor, and the preset duration; the control device is a programmable logic controller, an embedded microprocessor, or a control unit integrated inside the frequency converter;

[0081] S2.2: The control device collects the motor operating frequency signal and the actual power signal from the frequency converter in real time, and converts them into operating frequency value and actual power value through the analog-to-digital converter inside the control device. The analog-to-digital conversion process is the prior art in this field and is not an inventive solution of this application, so it will not be described in detail here.

[0082] Furthermore, the specific steps of S2.2 include:

[0083] (1) The frequency converter converts the motor operating frequency and actual power obtained by its internal monitoring into a standard analog signal and transmits it to the signal input port of the control device through a cable;

[0084] (2) The control device conditions the received analog signal, including amplitude adjustment and noise filtering, so that it meets the input requirements of the analog-to-digital converter;

[0085] (3) Under the unified clock control, the sampling and holding circuit of the control device simultaneously samples and holds the instantaneous values ​​of the two analog signals, namely the conditioned motor operating frequency and the actual power.

[0086] (4) The analog-to-digital converter converts the instantaneous value of the analog signal locked by the sample-and-hold circuit into the corresponding original digital value;

[0087] (5) The central processing unit of the control device calculates and restores the original digital quantity into a physically meaningful engineering value based on the preset conversion relationship, namely the final operating frequency value and actual power value of the motor.

[0088] S2.3: The control device compares the converted operating frequency value with the set full frequency value. If the operating frequency value is greater than or equal to the full frequency value, a signal indicating that the frequency meets the standard is generated. At the same time, the actual power value is compared with the rated power threshold. If the actual power value is greater than or equal to the rated power threshold, a signal indicating that the power meets the standard is generated.

[0089] Furthermore, the specific steps in S2.3 include:

[0090] (1) The processor of the control device reads the real-time operating frequency value, the set full frequency value, the real-time actual power value and the rated power threshold from the memory;

[0091] (2) The processor compares the real-time operating frequency value with the set full frequency value; if the operating frequency value is greater than or equal to the full frequency value, the internal frequency compliance flag is set to the valid state; otherwise, it is set to the invalid state.

[0092] (3) The processor compares the real-time actual power value with the rated power threshold; if the actual power value is greater than or equal to the rated power threshold, the internal power compliance flag is set to the valid state; otherwise, it is set to the invalid state.

[0093] (4) After completing (1)-(3), the current valid or invalid status of the frequency compliance flag and the power compliance flag is output as the result of the logic judgment.

[0094] S2.4: The control device performs a logical AND judgment on the frequency compliance flag and the power compliance flag. Only when both the frequency compliance flag and the power compliance flag are true will the control device start an internal timer to begin accumulating time. If either compliance flag becomes false, the internal timer will be immediately reset to zero.

[0095] Furthermore, the specific steps in S2.4 include:

[0096] (1) During each control cycle, the control device reads the current status of the frequency compliance flag and the power compliance flag;

[0097] (2) The control device performs a logical AND operation on the two flag states read. The result of the operation is valid only when both flag states are valid at the same time.

[0098] (3) The control device controls the internal timer based on the result of the logical AND operation:

[0099] When the calculation result is valid, the control device enables the internal timer to perform cumulative counting.

[0100] When the calculation result is invalid, the control device immediately resets the internal timer to zero;

[0101] (4) Repeated execution is performed to achieve continuous monitoring of the flag status and timing process.

[0102] S2.5: The control device compares the accumulated time of the internal timer with the preset duration in real time. When the accumulated time of the internal timer is greater than or equal to the preset duration, it is determined that the motor is running stably at full frequency and at a state greater than or equal to a preset percentage of the rated power. In this invention, the preset percentage is set to 90% of the rated power.

[0103] Furthermore, the specific steps of S2.5 include:

[0104] (1) The control device reads the current cumulative time value and preset duration of the internal timer;

[0105] (2) The control device compares the current cumulative time value with the preset duration to determine whether the cumulative time value has reached or exceeded the preset duration;

[0106] (3) When the comparison result is that the cumulative time value is greater than or equal to the preset duration, the control device determines that the stable operation conditions are met and generates a switching enable signal;

[0107] (4) The switching enable signal will trigger the control device to execute the subsequent frequency converter synchronization and switching process.

[0108] S2.6: Based on the positive result of the duration determination, the control device generates a switching enable signal internally, indicating that the switching conditions from frequency conversion to power frequency have been fully met.

[0109] Furthermore, the specific steps in S2.6 include:

[0110] (1) When the control device completes the comparison between the accumulated time of the internal timer and the preset duration, and determines that the accumulated time has met the requirements, this determination result generates an internal switching permission in the control device.

[0111] (2) After obtaining internal switching permission, the control device immediately performs system safety interlock verification. System safety interlock verification includes: confirming that there are no high-level fault alarms in the system, such as overcurrent or overheating, and confirming that the system is in automatic operation mode.

[0112] (3) If and only if the system safety interlock verification passes, the control device sets one of its internal specific status bits, namely the switching enable flag, to the valid state. The setting of the switching enable flag is the result of the combined effect of internal switching permission and system safety interlock verification, indicating that the conditions for switching from frequency converter to power frequency have been fully met.

[0113] (4) When the switching enable flag is set to valid, the control device immediately activates the process execution interlock. This interlock logic enables the control device to temporarily ignore the fluctuations of the front-end monitoring parameters such as motor operating frequency and power during the entire subsequent switching process execution, thereby ensuring that the switching operation that has been started can be executed continuously and without interference.

[0114] The frequency converter enters tracking mode, and the control device compares the phase difference between the frequency converter output voltage and the grid voltage in real time, including:

[0115] S3.1: When the control device sends a synchronization command to the frequency converter, the frequency converter enters the tracking mode and then activates its internal integrated grid voltage sampling circuit to start sampling the grid voltage in real time.

[0116] Furthermore, the specific steps of S3.1 include:

[0117] (1) The control device sends the synchronization command to the frequency converter through the communication bus or hard-wiring method. The main control board of the frequency converter receives the synchronization command through its communication interface or digital input channel. After the synchronization command arrives, the central processing unit inside the frequency converter will decode the synchronization command, identify that it is a command that requires it to enter the tracking mode, and confirm the integrity and validity of the command.

[0118] (2) After the central processing unit confirms that the synchronization instruction is valid, it immediately executes the switching of the operating mode. This means that the frequency converter will exit the current control logic from the normal frequency conversion speed regulation control mode and call the control program prepared for the tracking mode. At the same time, the processor will send an instruction to the system resource manager to allocate the necessary computing resources and hardware resources for the tracking mode, especially to prepare to start the grid voltage sampling function.

[0119] (3) After the mode switching and resource allocation are completed, the central processing unit will send an enable signal to the grid voltage sampling circuit. The function of this enable signal is to activate the circuit and switch it from the low-power standby state to the normal working state. Specifically, this enable signal will connect the power supply of the grid voltage sampling circuit, start the internal signal conditioning chip and the reference voltage source of the analog-to-digital converter, and allow the entire grid voltage sampling circuit to warm up and enter the ready state.

[0120] (4) The central processing unit starts a high-precision synchronous sampling timer. This timer generates sampling pulses at a fixed frequency based on the expected period of the power grid frequency. The arrival of each sampling pulse is a command to collect the instantaneous value of the power grid voltage.

[0121] (5) When the first sampling pulse arrives, the grid voltage sampling circuit immediately operates. Its internal sample-and-hold circuit will instantly capture the instantaneous analog value of the grid voltage and hold it. Then, the analog-to-digital converter starts working and converts this instantaneous analog value into the corresponding digital value. After the conversion is completed, the analog-to-digital converter will issue a conversion completion interrupt signal. The central processing unit responds to this interrupt and reads the digital value of the grid voltage at the first point from the data register of the grid voltage sampling circuit. Thus, the first sampling is completed, and the real-time sampling process of the grid voltage is officially started and enters the continuous loop working state.

[0122] S3.2: The inverter uses the grid voltage it samples as a reference signal to start the internal phase-locked loop control unit. The phase-locked loop control unit dynamically adjusts the phase and frequency of the drive carrier signal of the inverter's own power devices through feedback regulation to synchronize it with the grid voltage.

[0123] Furthermore, the specific steps in S3.2 include:

[0124] (1) The phase-locked loop control unit of the frequency converter receives real-time voltage data from the grid voltage sampling circuit. The phase detector inside the phase-locked loop control unit starts to work. The phase detector compares the sampled grid voltage signal with a feedback signal currently generated by the voltage-controlled oscillator inside the phase-locked loop control unit. That is, it calculates the instantaneous phase difference between the two signals and outputs an error voltage signal that is proportional to the magnitude and direction of the phase difference. When the two signals are completely in phase, the error voltage is zero.

[0125] (2) The error voltage signal is immediately fed into the loop filter. The function of the loop filter is to filter out high-frequency noise, and at the same time amplify and shape the effective error voltage signal, converting it into a smooth and stable DC control voltage, and obtaining the filtered and amplified DC control voltage.

[0126] (3) The filtered and amplified DC control voltage is applied to the control terminal of the voltage-controlled oscillator of the phase-locked loop control unit. The characteristic of the voltage-controlled oscillator is that its output frequency is proportional to the input control voltage. When the control voltage is positive, it means that the output phase of the frequency converter is lagging, and the voltage-controlled oscillator will increase its output frequency. When the control voltage is negative, it means that the output phase of the frequency converter is leading, and the voltage-controlled oscillator will decrease its output frequency. In this way, the phase error is converted into a frequency adjustment command.

[0127] (4) The frequency signal output by the voltage-controlled oscillator, which has been precisely calibrated, is sent to the pulse width modulation generator of the frequency converter as a carrier signal. The pulse width modulation generator uses this carrier signal as a reference to generate pulse waveforms with a specific duty cycle. After being amplified by the drive circuit, these pulse waveforms directly control the power devices, such as the insulated gate bipolar transistor, to turn on and off. Finally, the power devices chop the DC bus voltage into an AC voltage that is synchronized with the carrier signal and output it to the motor. The phase and frequency of this output voltage are determined by the output signal of the voltage-controlled oscillator.

[0128] (5) The phase information of the voltage finally output by the frequency converter will be sent back to the phase detector in the first step through the feedback loop, and compared with the grid voltage reference signal in a new round to form a complete closed-loop feedback control system, thereby realizing the precise synchronization between the output voltage and the grid voltage.

[0129] S3.3: The control device calculates the phase difference between the inverter output voltage and the grid voltage by reading the real-time phase data calculated by the phase-locked loop control unit or by obtaining the zero-crossing time difference between the inverter output voltage and the grid voltage through an independent phase detection circuit.

[0130] Furthermore, if an independent phase detection circuit is used to obtain the phase difference, the specific steps include:

[0131] (1) The collected output voltage and grid voltage are first amplified or attenuated by a signal conditioning circuit and filtered to eliminate high-frequency noise glitches. Then, the voltage signal is sent to a zero-crossing comparator. The zero-crossing comparator converts the smooth sine wave voltage signal into a square wave signal. The moment when its level changes from high to low or from low to high corresponds precisely to the moment when the original sine wave voltage crosses zero from the negative half-cycle and enters the positive half-cycle.

[0132] (2) A high-precision timer inside the control device starts working. When the first voltage signal, such as the zero-crossing edge of the grid voltage, is detected, the timer starts counting. When the second voltage signal, such as the zero-crossing edge of the inverter output voltage, is detected, the timer stops counting. In this way, an accurate time difference value is obtained. This time difference value is latched and converted into a digital value, that is, the digital value of the time difference.

[0133] (3) For the acquired digital time difference, the central processing unit of the control device divides it by the period of the grid voltage and then multiplies it by 360 degrees, thereby converting the time difference into a phase difference in degrees.

[0134] Furthermore, if a direct reading method is used, the read phase register value is directly used, and the processor directly uses it as the phase difference, or performs a simple linear conversion based on the scale set by the phase-locked loop control unit to obtain the final phase difference in degrees.

[0135] When the motor is running under the drive of the power frequency grid and the control device determines that a speed reduction is needed based on the load demand, the control device starts the frequency converter and causes its output voltage to track the back electromotive force of the motor under the current power frequency grid drive, including:

[0136] S4.1: When the control device receives an external speed reduction command through its communication interface or determines through its internal logic that the actual power of the motor is continuously lower than the preset cut-off power threshold and reaches the preset cut-off judgment time, it generates an internal speed reduction command.

[0137] Furthermore, the specific steps of S4.1 include:

[0138] S4.1.1: The control device simultaneously activates two independent command monitoring paths. The first path is the external command path: the control device's communication interface, such as Ethernet, fieldbus, or hardwired input point, continuously listens for external speed reduction command signals from the operation interface. The second path is the internal judgment path: the control device continuously acquires the real-time actual power value of the motor under the drive of the power frequency grid through its data acquisition system and compares it with the internally preset back-cut power threshold.

[0139] S4.1.2: When the communication interface detects a valid level transition or an external speed reduction command signal that conforms to the communication protocol, a hardware interrupt is generated or a status flag is set. The central processing unit of the control device responds to this event and first performs a validity check on the external speed reduction command signal. The check includes: verifying whether the communication checksum is correct, whether the command source address is valid, and whether the current system is in a mode that allows external command operation. Only if all checks are passed can the external speed reduction command signal be confirmed as a valid speed reduction request.

[0140] S4.1.3: In the internal judgment path, the control device compares the real-time actual power value with the cut-off power threshold. When the real-time actual power value is continuously lower than the cut-off power threshold, an internal condition satisfaction flag is set. At the same time, the control device starts a dedicated timer to accumulate the duration of the condition satisfaction. The control device compares the accumulated duration of the timer with the preset cut-off judgment duration in real time. Only when the accumulated duration is greater than or equal to the preset cut-off judgment duration does it mean that the low power load state is stable, rather than a momentary fluctuation. At this time, the internal condition satisfaction flag is finally confirmed.

[0141] S4.1.4: The control device performs a logical OR operation on the outputs of the above two paths. That is, as long as either the external speed reduction command signal is valid or the internal condition is confirmed, the result of the logical operation is true. When the result is true, the control device writes a value into one of its internal storage units, that is, writes a value into the speed reduction command register, or sets a status flag bit, that is, the internal speed reduction command flag, to the valid state. This operation marks the formal generation of the internal speed reduction command.

[0142] S4.1.5: After the internal speed reduction command is generated, the main control loop of the control device detects the valid status of the flag bit. Subsequently, the control system performs two key actions: First, it puts the internal speed reduction command into the task queue as a highest priority task event, triggering the subsequent switchback process; Second, it updates the internal state variables of the system, changing them from the power frequency steady-state operation to the waiting switchback to the variable frequency speed regulation operation state, and prepares resources for the execution of the switchback operation.

[0143] S4.2: Based on the internal speed reduction command, the control device starts the frequency converter, and the frequency converter outputs a pre-excitation voltage with an amplitude lower than the rated voltage to the motor terminals;

[0144] Furthermore, the specific steps in S4.2 include:

[0145] (1) After the control device generates the internal speed reduction command, its main control program immediately parses the internal speed reduction command and confirms that the operation of switching from power frequency to frequency conversion needs to be performed. Subsequently, the control device sends a start command to the frequency converter in standby state through the communication bus or hard-wired control channel. The start command contains a clear mode setting, that is, the frequency converter is required not to directly drive the motor to rotate after starting, but to prepare to enter a pre-excitation voltage output mode.

[0146] (2) After receiving the start command, the inverter first executes the internal power-on self-test process. After the self-test is passed, the main control unit of the inverter begins to pre-charge its DC bus capacitor to establish a stable DC bus voltage. At the same time, it calls the special control parameters preset for the pre-excitation voltage output mode from the memory to complete the initialization of the control system.

[0147] (3) After initialization, the inverter main control unit calculates a safe voltage value, namely the pre-excitation voltage command value, according to the preset strategy. The pre-excitation voltage command value is lower than the rated voltage of the motor, for example, it is set to 10% to 20% of the rated voltage. Its core purpose is to establish a stable electrical reference for voltage detection without generating excessive inrush current. After the calculation is completed, the pre-excitation voltage command value is sent to the pulse width modulation controller.

[0148] (4) After receiving the pre-excitation voltage command value, the pulse width modulation controller generates a set of pulse waveforms with corresponding duty cycles according to the amplitude of the pre-excitation voltage command value. After being amplified by the drive circuit, the pulse waveforms control the on and off of the power devices inside the frequency converter, and convert the DC bus voltage into a three-phase AC voltage with a lower amplitude and the same frequency as the power grid. This precisely controlled low amplitude AC voltage is the pre-excitation voltage, which is applied to the motor terminals through the closed main circuit contactor.

[0149] (5) After the pre-excitation voltage is output, the grid voltage sampling circuit inside the inverter detects its actual value in real time and feeds it back to the phase-locked loop control unit. The phase-locked loop control unit compares the actual voltage value with the pre-excitation voltage command value. Once a deviation occurs, it immediately adjusts the pulse width modulation signal to correct it, forming a closed-loop control to ensure that the voltage output to the motor terminal is stable and accurately maintained at the preset safe amplitude. Thus, a stable pre-excitation voltage has been established at the motor terminal.

[0150] S4.3: The frequency converter uses its output pre-excitation voltage as a reference, and detects the back electromotive force generated by the motor when it rotates under the drive of the power grid through its internal detection circuit. Based on the frequency and phase of the back electromotive force, the frequency, phase and amplitude of its output voltage are automatically adjusted until they are synchronized with the back electromotive force of the motor. The back electromotive force of the motor is the residual voltage on the motor terminals.

[0151] Furthermore, the specific steps of S4.3 include:

[0152] (1) The inverter first outputs a pre-excitation voltage with known amplitude and phase to the motor terminal. At this time, because the motor is driven to rotate by the power frequency grid, its rotor magnetic field cuts the stator winding and generates a motor back electromotive force at the terminal, i.e. residual voltage. The two signals of the pre-excitation voltage and the motor back electromotive force are superimposed at the motor terminal to form a total voltage signal. The high-precision voltage detection circuit inside the inverter continuously collects this total voltage signal.

[0153] (2) Since all parameters of the pre-excitation voltage, including amplitude, phase and frequency, are known controlled quantities, the control algorithm of the frequency converter subtracts the known pre-excitation voltage component from the total voltage signal collected by vector operation, thereby separating and extracting the pure back electromotive force signal generated only by the rotation of the motor.

[0154] (3) The phase-locked loop control unit of the inverter uses the real-time phase and frequency of the back electromotive force signal extracted in the second step as a reference, and uses the phase and frequency of the current output voltage of the inverter as feedback. The phase comparator in the phase-locked loop control unit continuously calculates the instantaneous phase difference and frequency difference between the two, and outputs an error voltage signal that is proportional to the difference.

[0155] (4) The error voltage signal is filtered and then sent to the voltage-controlled oscillator. The voltage-controlled oscillator dynamically adjusts its output frequency according to the polarity and magnitude of the error voltage signal: if the inverter output phase lags behind, the frequency is increased to catch up; if it leads, the frequency is decreased to wait. The generated new frequency signal directly controls the pulse width modulation generator, thereby adjusting the phase and frequency of the inverter output voltage so that it approaches the phase and frequency of the motor back electromotive force.

[0156] (5) After multiple cycles of dynamic adjustment, the phase difference and frequency difference between the inverter output voltage and the motor back electromotive force tend to zero, and a stable synchronization state is reached. At this time, the inverter adjusts the amplitude of the output voltage to match it. When the frequency, phase and amplitude are consistent, the synchronization is determined to be completed.

[0157] S4.4: The control device or frequency converter monitors the phase difference between its output voltage and the back electromotive force of the motor in real time. When the phase difference is less than the set synchronization tolerance angle threshold, it is determined that synchronization is completed.

[0158] Furthermore, the specific steps of S4.4 include:

[0159] (1) While the inverter is performing synchronous adjustment, the control device continuously acquires the instantaneous phase of the inverter output voltage and the instantaneous phase of the motor back electromotive force, and calculates the phase difference between the two in real time.

[0160] (2) The control device instantaneously compares the real-time phase difference obtained by monitoring with a preset synchronization tolerance angle threshold. If the phase difference is less than or equal to the synchronization tolerance angle threshold, the output condition satisfies the signal; otherwise, the output condition does not satisfy the signal.

[0161] Furthermore, the process of setting the synchronization tolerance angle threshold includes: accurately measuring the total time required from issuing the switching command to the contactor completely disconnecting, and converting it into the corresponding power frequency electrical angle. This power frequency electrical angle is the shortest time required for the system to respond and execute the operation, constituting the technical lower limit for setting the threshold; through simulation analysis of the inrush current generated by switching under different phase differences, determining that the inrush current does not exceed the maximum phase difference corresponding to the safe tolerance range of the system components, this angle is the safe upper limit to ensure that the equipment is not damaged; within the range formed by the obtained technical lower limit and safe upper limit, according to the specific requirements of the application for switching speed, impact magnitude and reliability, a specific synchronization tolerance angle threshold is selected. A smaller threshold reduces the impact but increases the risk of switching failure, while a larger threshold increases the switching success rate but withstands a larger impact; the selected threshold is applied to the actual system for switching tests, key electrical parameters are measured, and the effectiveness of the synchronization tolerance angle threshold is verified based on the test results. If the effect is not ideal, fine-tuning is performed until a synchronization tolerance angle threshold that achieves the best balance between safety and reliability is obtained, and then finally confirmed.

[0162] (3) The control device determines the duration of the output condition signal. When the condition-satisfied signal continues to appear and accumulates to a preset stable time, it determines that the synchronization state has reached stability. If a condition-unsatisfied signal appears during the period, the timing is immediately reset and the determination is repeated.

[0163] (4) Once the stable duration determination is passed, the control device generates a synchronization completion flag inside its internal system. The generation of this flag signifies that the synchronization operation has been officially determined to be completed.

[0164] (5) After the flag is generated synchronously, a latching logic is immediately activated. This latching logic keeps the flag valid during the switching process and will not reset due to small fluctuations in the phase difference. At the same time, the flag triggers the subsequent switching operation instruction sequence.

[0165] After the output voltage of the inverter is synchronized with the back electromotive force of the motor, the power frequency bypass contactor is disconnected, and then the main circuit contactor is closed, so that the inverter can capture the rotating motor and smoothly reduce its speed, including:

[0166] S4.5: After the synchronization is completed, the control device disconnects the power frequency bypass contactor, and after a preset second dead time delay, closes the main circuit contactor to complete the switching of the motor power supply circuit from the power frequency grid to the frequency converter.

[0167] Furthermore, the specific steps of S4.5 include:

[0168] (1) After the control device completes the internal synchronization of the flag validity, it first verifies the final safety interlocking conditions of the system, confirms that the power frequency bypass contactor is in the closed state, the main circuit contactor is in the open state and the system is fault-free. After the verification is passed, the execution permission of the switching operation is unlocked.

[0169] (2) After the safety interlock verification is passed, the control device sends a disconnection command to the power frequency bypass contactor and confirms that it is completely disconnected, thereby disconnecting the motor from the power frequency grid;

[0170] (3) After confirming that the power frequency bypass contactor has been disconnected, the control device starts the second dead zone delay timer and waits for it to finish counting. This delay is used to ensure that the arc is extinguished and to establish a safe electrical isolation period.

[0171] Furthermore, the process of setting the second dead zone delay is as follows: Based on the technical data of the power frequency bypass contactor, obtain the typical arc extinction time when it interrupts the current and the time required for the dielectric between the contacts to recover its insulation strength. The sum of the typical extinction time and the time required to recover the insulation strength constitutes the core electrical safety reference time for setting the delay. Based on the technical data, confirm the mechanical breaking time required for the contactor to reach a stable state from coil de-energization to complete separation of the main contacts. Compare the obtained core electrical safety reference time with the confirmed mechanical breaking time, and take the larger one as the basis. On this basis, add an engineering safety margin to cope with fluctuations and aging. After adding them, obtain the theoretical initial value of the second dead zone delay. Round the theoretical initial value up to an integer value that conforms to the timer resolution of the control system to obtain a standardized theoretical delay value. Apply the standardized theoretical delay value to the actual system for testing and verification. Check its effectiveness based on the measured electrical waveform. If the effect is ideal, confirm it as the final value. If it is not ideal, fine-tune the theoretical initial value until the final value of the second dead zone delay that achieves the best balance between safety and efficiency is obtained.

[0172] (4) After the second dead zone delay ends, the control device immediately sends a closing command to the main circuit contactor and confirms that it is reliably closed, thereby connecting the motor to the frequency converter;

[0173] (5) After the main circuit contactor is closed, the control device instructs the frequency converter to switch from synchronous tracking mode to torque control mode, starts to control the motor to reduce speed, and updates the system status to frequency conversion speed regulation operation status.

[0174] S4.6: After the main circuit contactor is closed, the frequency converter automatically performs frequency tracking based on the detected back electromotive force of the motor, so that its output frequency is synchronized with the actual rotation frequency of the motor.

[0175] Furthermore, the specific steps of S4.6 include:

[0176] (1) When the main circuit contactor is closed, the output terminal of the inverter establishes a direct electrical connection with the stator winding of the rotating motor through the cable. At this time, the motor is still rotating at high speed due to inertia. Its rotor magnetic field cuts the stator winding and generates a three-phase sinusoidal back electromotive force with a frequency proportional to the speed at the motor terminal. This back electromotive force signal is directly applied to the output power module of the inverter through the closed main circuit.

[0177] (2) The inverter is equipped with a high-precision voltage detection circuit. These voltage detection circuits immediately begin to sample the three-phase output voltage connected to the motor terminals at high speed. Since the power devices of the inverter have not yet started to output drive current at this time, their output terminals are in a high impedance state. Therefore, the voltage signal measured by the sampling circuit is actually the real waveform of the back electromotive force generated by the motor rotation. These analog voltage signals are converted into digital quantities in real time.

[0178] (3) The digital signal processor of the inverter continuously analyzes the acquired back EMF digital waveform and calculates the period of the back EMF waveform in real time through the zero-crossing detection method. Specifically, the zero-crossing detection method accurately captures the time interval between the waveform from one zero-crossing point to the next zero-crossing point. The reciprocal of this time interval is the frequency of the back EMF. Since there is a strict linear relationship between the rotational speed of the motor and the frequency of the back EMF, the calculated frequency of the back EMF directly corresponds to the actual mechanical rotational frequency of the motor rotor. The strict linear relationship between the rotational speed of the motor and the frequency of the back EMF is determined by the number of pole pairs of the motor. The zero-crossing detection method is the prior art in this field and is not an inventive solution of this application. It will not be elaborated here.

[0179] (4) The calculated actual rotational frequency of the motor is immediately set as the given value of the current output frequency of the inverter. The control core of the inverter sends an instruction to its pulse width modulation generator to make its carrier signal frequency strictly follow this given rotational frequency. This means that the inverter no longer runs according to an externally set speed command, but locks its output frequency to the actual speed of the motor.

[0180] (5) While adjusting the output frequency synchronously, the inverter also needs to ensure that the phase of its output voltage is aligned with the phase of the back electromotive force. The position of the magnetic pole of the motor rotor at the current moment is determined by calculation, that is, the direction of the back electromotive force vector. Based on this direction, the phase of the inverter output voltage vector is controlled to coincide with it, so that the voltage waveform output by the inverter overlaps with the back electromotive force waveform of the motor in both frequency and phase.

[0181] (6) When the output voltage of the inverter is consistent with the back electromotive force of the motor in terms of frequency, phase and amplitude, the voltage difference between the two is zero. At this time, the inverter begins to output a small torque current that is in the same direction as the motor. Since the voltage difference is zero, the establishment of this current will be smooth and without impact. The motor will not produce any torque jitter. In this way, the inverter smoothly takes over the rotating motor and realizes the seamless transition from inertial coasting to being driven, that is, it completes the shockless restart.

[0182] S4.7: After completing frequency tracking, the inverter smoothly reduces its output frequency according to a preset deceleration curve, thereby driving the motor speed to decrease synchronously.

[0183] Furthermore, the specific steps in S4.7 include:

[0184] (1) After the inverter successfully tracks and receives the motor speed, the control device sends a start deceleration command to the inverter. The start deceleration command contains target information, namely the target operating frequency to be reached. After receiving the start deceleration command, the main control unit of the inverter immediately obtains the preset deceleration curve parameters from its internal memory or from the control device through communication, including the final target operating frequency of deceleration, the total time required for the deceleration process, and the deceleration mode.

[0185] (2) The inverter uses the motor running frequency at the instant after successfully completing frequency tracking as the starting frequency of the deceleration process. At the same time, a curve generator inside the inverter for managing speed changes is activated. The generator initializes its internal state, sets the current time to zero, sets the starting frequency to the current output frequency, and loads parameters such as the target running frequency and the total deceleration time. Thus, a deceleration reference coordinate system with time as the horizontal axis and frequency as the vertical axis is established.

[0186] (3) The central processing unit of the inverter will perform a speed reduction calculation once in each control cycle. Based on the loaded speed reduction curve parameters and the current time accumulated since the start of speed reduction, the theoretical output frequency setpoint corresponding to the current moment is calculated. For example, for a linear speed reduction curve, the processor calculates the speed reduction slope by dividing the difference between the starting frequency and the target operating frequency by the total speed reduction time. Then, it calculates the frequency drop by multiplying the current time and the speed reduction slope. Finally, it obtains the frequency setpoint at the current moment by calculating the difference between the starting frequency and the frequency drop.

[0187] (4) The calculated real-time frequency setpoint is sent to the speed regulator of the inverter. The speed regulator compares the frequency setpoint with the actual operating frequency of the motor and generates an error signal. Based on the magnitude and trend of the error signal, the speed regulator calculates the torque current command to be applied so as to drive the motor speed to accurately follow the frequency setpoint decrease, ensuring that the motor speed can smoothly and without overshoot track the preset deceleration trajectory.

[0188] (5) While reducing the output frequency, the voltage control unit inside the inverter calculates and outputs a voltage value that matches the current frequency set value in real time according to the motor load characteristics, ensuring that the motor magnetic flux remains constant, thereby generating a smooth braking torque during the deceleration process, avoiding current surges or speed fluctuations caused by sudden changes in magnetic flux, and achieving smooth electrical braking.

[0189] (6) The inverter compares the current output frequency setpoint with the preset target operating frequency. When the output frequency setpoint drops to the target operating frequency and the actual speed of the motor is also stabilized at the speed corresponding to the target operating frequency through closed-loop regulation, the inverter determines that the speed reduction process is complete. Subsequently, the system exits the speed reduction mode, enters the stable target frequency operating state, and performs normal speed regulation operation according to the load change. The speed reduction process ends.

[0190] S4.8: When the output frequency of the frequency converter drops to the preset target operating frequency, the speed reduction process ends and the motor enters a stable speed regulation operation state driven by the frequency converter.

[0191] Example 2:

[0192] Please see Figure 3 Another embodiment of the present invention provides: an energy-saving system for automatic switching between variable frequency and mains frequency, comprising:

[0193] Control module, power switching module, frequency converter drive module, signal detection module;

[0194] The control module is used to execute control logic, process sensor signals, and make switching decisions;

[0195] The power switching module is used to physically change the power supply path of the motor under the command of the control module, so as to achieve safe switching between the frequency conversion circuit and the power frequency circuit.

[0196] The variable frequency drive module, as the core actuator of the system, is used for soft starting and speed regulation of the motor in variable frequency mode, and can also work in tracking mode during switching.

[0197] The signal detection module is used to monitor the motor's operating frequency and actual power.

[0198] The control module includes: a central processing unit and a communication interface unit;

[0199] The central processing unit, including a programmable logic controller or an embedded microprocessor, is used to run the built-in switching control algorithm, make conditional judgments based on preset parameters, and generate control commands for the contactor and frequency converter.

[0200] The communication interface unit is used for communication, receiving external speed reduction commands, and uploading system status and fault information.

[0201] The power switching module includes: a main circuit contactor, a power frequency bypass contactor, and an interlocking unit;

[0202] The main circuit contactor is used to connect the circuit between the motor and the frequency converter during frequency conversion operation.

[0203] A power frequency bypass contactor is used to connect the direct circuit between the motor and the power frequency grid when the motor is running at power frequency.

[0204] Interlocking units are designed with electrical or mechanical interlocking to ensure that the main circuit contactor and the power frequency bypass contactor can never close simultaneously, thus preventing power short circuits.

[0205] The variable frequency drive module includes: a power conversion unit and a synchronization control unit;

[0206] The power conversion unit is used to convert the mains frequency power supply into a variable frequency power supply with adjustable frequency and voltage according to the control signal.

[0207] The synchronization control unit is used to activate phase-locked loop technology after receiving a synchronization command, so that its output voltage can accurately track the frequency, phase and amplitude of the grid voltage or the motor back electromotive force.

[0208] The signal detection module includes: a power grid voltage sampling unit, an operating parameter detection unit, and a phase difference detection unit;

[0209] The power grid voltage sampling unit is used to detect the voltage signal of the power frequency power grid in real time and to compare the phase during the synchronization phase.

[0210] The operating parameter detection unit is used to detect parameters such as the motor's operating frequency, output current, and actual power in real time, providing data for determining switching conditions.

[0211] The phase difference detection unit is used to accurately calculate the real-time phase difference between the inverter output voltage and the grid voltage.

[0212] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments under the guidance of the present invention without departing from the spirit and scope of the present invention. All of these variations are within the protection scope of the present invention.

Claims

1. An energy-saving method for automatic switching between variable frequency and mains frequency, characterized in that, include: When the main circuit contactor is closed and the power frequency bypass contactor is open, the frequency converter is soft-started and the motor is driven to run in the frequency conversion speed regulation operation state. In the variable frequency speed regulation operation state, the control device monitors the motor operating frequency and actual power. When the motor operating frequency reaches the set full frequency value, the actual power meets the rated power threshold and continues for a preset time, it is determined that the variable frequency to power frequency switching condition is met. Based on the judgment result, the control device sends a synchronization command to the frequency converter, the frequency converter enters the tracking mode, and the control device compares the phase difference between the output voltage of the frequency converter and the grid voltage in real time. When the phase difference is less than the set safety angle threshold, the main circuit contactor is disconnected, and after a preset dead time delay, the power frequency bypass contactor is closed to switch the motor to direct operation on the power frequency grid. When the motor is running under the drive of the power frequency grid and the control device determines that the speed needs to be reduced according to the load demand, the control device starts the frequency converter and makes its output voltage track the back electromotive force of the motor under the current power frequency grid drive. After the output voltage of the frequency converter is synchronized with the back electromotive force of the motor, the power frequency bypass contactor is disconnected and the main circuit contactor is closed. The frequency converter captures the rotating motor and smoothly reduces the speed, and re-enters the frequency conversion speed regulation operation state. The specific logic for determining whether the frequency conversion to power frequency switching condition is met is as follows: Three key judgment parameters are preset inside the control device; the key judgment parameters include the full frequency value of the motor operating frequency, the rated power threshold of the actual power of the motor, and the preset duration; the control device is a programmable logic controller, an embedded microprocessor, or a control unit integrated inside the frequency converter. The control device collects the motor operating frequency signal and the actual power signal from the frequency converter in real time, and converts them into operating frequency value and actual power value through the analog-to-digital converter inside the control device; The control device compares the converted operating frequency value with the set full frequency value. If the operating frequency value is greater than or equal to the full frequency value, a signal indicating that the frequency meets the standard is generated. At the same time, the actual power value is compared with the rated power threshold. If the actual power value is greater than or equal to the rated power threshold, a signal indicating that the power meets the standard is generated. The control device performs a logical AND judgment on the frequency compliance flag and the power compliance flag. The control device starts the internal timer to start accumulating time only when both the frequency compliance flag and the power compliance flag are true. If either compliance flag becomes false, the internal timer is reset to zero. The control device compares the accumulated time of the internal timer with the preset duration in real time. When the accumulated time of the internal timer is greater than or equal to the preset duration, it determines that the motor is running stably at full frequency and at a state greater than or equal to the preset percentage of rated power. Based on the positive result of the duration determination, the control device generates a switching enable signal internally, indicating that the variable frequency to power frequency switching condition has been met.

2. The energy-saving method for automatic switching between variable frequency and power frequency as described in claim 1, characterized in that, The closing of the main circuit contactor and the opening of the power frequency bypass contactor are controlled by the controller outputting high and low level signals. The controller first outputs a high level to the main circuit contactor coil to make it engage, and then outputs a low level to the power frequency bypass contactor coil to make it disengage. After the contactor status feedback signal is sent back to the controller, the controller starts the inverter soft start program.

3. The energy-saving method for automatic switching between variable frequency and power frequency as described in claim 2, characterized in that, The synchronization command is a digital signal or a communication message containing a mode switching command. The synchronization command directly triggers the inverter's operating mode to change from V / F control or vector control to the tracking mode.

4. The energy-saving method for automatic switching between variable frequency and power frequency as described in claim 3, characterized in that, The frequency converter enters tracking mode, and the control device compares the phase difference between the frequency converter output voltage and the grid voltage in real time, including: When the control device sends a synchronization command to the frequency converter, the frequency converter enters the tracking mode and activates its internal integrated grid voltage sampling circuit to sample the grid voltage in real time. The inverter uses the grid voltage it samples as a reference signal to start the internal phase-locked loop control unit. The phase-locked loop control unit dynamically adjusts the phase and frequency of the drive carrier signal of the inverter's own power devices through feedback regulation to synchronize it with the grid voltage. The control device calculates the phase difference between the inverter output voltage and the grid voltage by reading the real-time phase data calculated by the phase-locked loop control unit or by obtaining the zero-crossing time difference between the inverter output voltage and the grid voltage through an independent phase detection circuit.

5. The energy-saving method for automatic switching between variable frequency and power frequency as described in claim 4, characterized in that, When the motor is running under the drive of the power frequency grid and the control device determines that a speed reduction is needed based on the load demand, the control device starts the frequency converter and causes its output voltage to track the back electromotive force of the motor under the current power frequency grid drive, including: The control device receives an external speed reduction command through its communication interface or determines through its internal logic that when the actual power of the motor is continuously lower than the preset cut-off power threshold and the preset cut-off judgment time is reached, it generates an internal speed reduction command. Based on the internal deceleration command, the control device starts the frequency converter, and the frequency converter outputs a pre-excitation voltage with an amplitude lower than the rated voltage to the motor terminals. The inverter uses its output pre-excitation voltage as a reference and detects the back electromotive force generated by the motor when it rotates under the drive of the power grid through its internal detection circuit. It then automatically adjusts the frequency, phase, and amplitude of its output voltage based on the frequency and phase of the back electromotive force until it is synchronized with the back electromotive force of the motor. The back electromotive force of the motor is the residual voltage on the motor terminals. The control device or frequency converter monitors the phase difference between its output voltage and the back electromotive force of the motor in real time. When the phase difference is less than the set synchronization tolerance angle threshold, synchronization is determined to be complete.

6. The energy-saving method for automatic switching between variable frequency and power frequency as described in claim 5, characterized in that, The process of generating the internal deceleration command includes: The control device simultaneously activates the external command path and the internal judgment path; the external command path listens for external deceleration command signals from the operation interface through the communication interface of the control device; the internal judgment path obtains the real-time actual power value of the motor under the drive of the power frequency grid through the data acquisition system of the control device, and compares it with the internally preset back-cut power threshold. In the external command path, when the communication interface detects a level transition or an external speed reduction command signal, a hardware interrupt is generated or a status flag is set. The central processing unit of the control device verifies the validity of the external speed reduction command signal to confirm whether the external speed reduction command signal is valid. In the internal judgment path, the control device compares the real-time actual power value with the back-off power threshold. When the real-time actual power value is lower than the back-off power threshold, the internal condition satisfaction flag is set. At the same time, the control device starts a timer to accumulate the duration of condition satisfaction. The control device compares the accumulated duration of the timer with the preset back-off judgment duration in real time. Only when the accumulated duration is greater than or equal to the preset back-off judgment duration is the internal condition satisfaction flag finally confirmed. The control device performs a logical OR operation on the outputs of the external command path and the internal judgment path to generate an internal deceleration command.

7. The energy-saving method for automatic switching between variable frequency and power frequency as described in claim 6, characterized in that, After the output voltage of the inverter is synchronized with the back electromotive force of the motor, the power frequency bypass contactor is disconnected, and then the main circuit contactor is closed, so that the inverter can capture the rotating motor and smoothly reduce its speed, including: After the synchronization is completed, the control device disconnects the power frequency bypass contactor, and after a preset second dead time delay, closes the main circuit contactor to complete the switching of the motor power supply circuit from the power frequency grid to the frequency converter. After the main circuit contactor is closed, the frequency converter automatically performs frequency tracking based on the detected back electromotive force of the motor, so that its output frequency is synchronized with the actual rotation frequency of the motor. After completing frequency tracking, the inverter reduces its output frequency according to the preset deceleration curve, thereby causing the motor speed to decrease synchronously. When the output frequency of the frequency converter drops to the preset target operating frequency, the speed reduction process ends, and the motor enters the speed regulation operation state driven by the frequency converter.

8. The energy-saving method for automatic switching between variable frequency and power frequency as described in claim 7, characterized in that, The main circuit contactor and the power frequency bypass contactor are equipped with a double interlock, including an electrical soft interlock implemented by the control device program and a hardware hard interlock composed of the contactor auxiliary contacts.

9. An energy-saving system for automatic switching between variable frequency and mains frequency, used to implement the energy-saving method for automatic switching between variable frequency and mains frequency as described in any one of claims 1-8, characterized in that, include: Control module, power switching module, frequency converter drive module, signal detection module; The control module is used to execute control logic, process sensor signals, and make switching decisions; The power switching module is used to change the power supply path of the motor under the instruction of the control module, and to complete the switching between the frequency conversion circuit and the power frequency circuit. The variable frequency drive module is used for soft start and speed regulation of the motor in variable frequency mode, and to operate in tracking mode during switching. The signal detection module is used to monitor the motor's operating frequency and actual power.

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