A method and system for variable frequency soft start and soft stop control of an alternating current motor

Abstract

This invention relates to the field of high-voltage motor frequency conversion control technology, and discloses a method and system for soft start and soft stop control of AC motor frequency conversion. During the start-up phase, after pre-charging, self-testing, minimum magnetization, and low-frequency trial operation, the frequency conversion main circuit adaptively adjusts the frequency ramp-up slope and small-step voltage compensation based on the output current, reference current, and continuous acceleration state. This is combined with phase, frequency, voltage, and current fluctuations, as well as bypass contactor operation delay compensation, to achieve hot switching of the power frequency bypass. During the stop phase, the circuit first switches back to the frequency conversion main circuit, rapidly reducing the voltage to a low flux level. Then, the frequency is slowly reduced according to the stop time. The frequency reduction trajectory is corrected based on speed estimation, slip estimation, DC bus voltage, and its ramp-up slope to maintain positive slip margin and suppress regenerative backflow and bus overvoltage.

Description

Technical Field

[0001] This invention relates to the field of high-voltage motor frequency conversion control technology, specifically to an AC motor frequency conversion soft start and soft stop control method and system. Background Technology

[0002] For high-voltage, high-power motors, such as 10kV high-voltage motors, in situations where the motor operates at the power frequency for a long time and requires controlled shutdown, if only free-slip stopping is used, the shutdown time is uncontrollable and cannot meet the process requirements. If the motor is simply stopped by rapidly reducing the frequency, it is easy for the motor to enter the regenerative power generation state because the actual speed of the motor is higher than the synchronous speed, which will cause the DC bus voltage to rise or even the overvoltage protection to activate.

[0003] Currently, soft stop control of high-voltage motors is often simplified to ordinary deceleration control, without fully considering the regenerative suppression problem of high-voltage AC motors. Especially under no-load conditions, it is very easy for the synchronous speed to drop rapidly below the actual speed of the motor, resulting in energy being fed back into the DC bus and causing the DC bus voltage to rise. Summary of the Invention

[0004] This invention provides a method and system for controlling the soft start and soft stop of an AC motor via frequency conversion, so as to at least solve the problem of DC bus overvoltage caused by the AC motor entering regenerative power generation state during the soft stop phase.

[0005] In a first aspect, the present invention provides a method for controlling the soft start and soft stop of an AC motor via frequency conversion, the method comprising: In response to the received start command, the AC motor is connected to the inverter main circuit, the inverter main circuit performs a soft start on the AC motor, and after successful start, the AC motor is switched to the power frequency bypass. In response to the received shutdown command, the AC motor is switched to the frequency converter main circuit, the output voltage of the frequency converter main circuit is reduced to the preset low magnetic flux voltage at the first step-down rate, and the output frequency of the frequency converter main circuit is reduced according to the frequency deceleration trajectory corresponding to the preset stop time. Obtain the estimated actual speed and synchronous speed of the AC motor, and correct the deceleration trajectory of the output frequency based on the estimated actual speed and synchronous speed to ensure that the synchronous speed is not lower than the estimated actual speed or maintains a preset positive slip margin. The system monitors preset shutdown parameters in real time, and when the preset shutdown parameters meet the shutdown conditions, it controls the main circuit of the frequency converter to stop operating.

[0006] The above solution rapidly reduces the output voltage to a low flux level and gradually decreases the output frequency along a slower frequency deceleration trajectory, making the synchronous speed decrease slower than the actual speed of the motor. Then, the deceleration trajectory of the output frequency is corrected based on the estimated actual speed and synchronous speed to maintain positive slip margin and reduce the risk of energy backflow into the DC bus.

[0007] Optionally, the deceleration trajectory of the output frequency is corrected based on the estimated actual speed and the synchronous speed to ensure that the synchronous speed is not lower than the estimated actual speed or to maintain a preset positive slip margin, including: The slip estimate is calculated based on the actual speed estimate and the synchronous speed estimate. When the actual speed estimate is greater than or equal to the synchronous speed, or the slip estimate is less than the preset positive slip margin, reduce the attenuation rate of the output frequency or stop reducing the output frequency.

[0008] The above solution quantifies the slip boundary conditions, accurately identifies the critical operating condition when the motor is about to enter the power generation feedback phase, intervenes in the deceleration curve in advance, and suppresses feedback energy from the source.

[0009] Optionally, after controlling the output voltage of the frequency converter main circuit to decrease to a preset low flux voltage at a first step-down rate, and controlling the output frequency of the frequency converter main circuit to decrease according to a frequency deceleration trajectory corresponding to a preset stop time, the method further includes: The DC bus voltage of the inverter main circuit is detected. When the DC bus voltage meets the preset overvoltage condition, the attenuation rate of the output frequency is reduced or the reduction of the output frequency is stopped.

[0010] The above solution incorporates both slip condition and bus voltage constraints to correct the deceleration curve, taking into account both motor operating conditions and inverter hardware safety, providing dual protection against overvoltage faults. By slowing the frequency descent rate during deceleration, reducing the motor deceleration slope, decreasing braking feedback power, and minimizing energy accumulation in the bus capacitor, the risk of overvoltage can be reduced without the need for additional large-capacity braking resistors.

[0011] Optionally, the preset overvoltage condition is that the DC bus voltage is greater than a preset bus voltage threshold or the rise slope of the DC bus voltage is greater than a preset positive threshold for a continuous first duration.

[0012] The above solution can take into account both instantaneous high voltage surge and continuous voltage rise fault conditions. Compared with a single voltage threshold judgment, the fault identification coverage is more comprehensive and is conducive to identifying the device withstand voltage risk caused by slow voltage rise.

[0013] Optionally, after detecting the DC bus voltage of the inverter main circuit, the method further includes: When the DC bus voltage exceeds the preset bus voltage threshold and the duration exceeds the second duration, free shutdown protection is executed.

[0014] The above scheme employs a tiered protection mechanism. For minor overvoltage, only the deceleration curve is corrected and soft intervention is used, while for severe and continuous overvoltage, the system is shut down directly. This layered protection balances operational continuity and equipment safety. Furthermore, the timing delay avoids accidental shutdowns due to single voltage spikes, reducing unnecessary equipment downtime and improving system operational continuity.

[0015] Optionally, when correcting the deceleration trajectory of the output frequency, the relationship between the output frequency before correction and the output frequency after correction is as follows:

[0016] In the formula, fout(k) and fout(k+1) are the output frequencies before and after correction, respectively; Δf_stop is the initial frequency drop step; Kslip is the slip correction coefficient; and Kudc is the bus voltage correction coefficient. Both Kslip and Kudc take values ​​from 0 to 1. Kslip is determined by the remaining amount of the slip estimate relative to the preset positive slip margin. The smaller the remaining amount, the smaller Kslip. Kudc is determined by the DC bus voltage and its rising slope. The higher the DC bus voltage and / or the greater the rising slope, the smaller Kudc.

[0017] The degree of correction of the output frequency is quantified by the above mathematical model. The correction intensity changes continuously and smoothly with the working conditions without segmental abrupt changes. The motor deceleration torque is continuous and without impact. Furthermore, through the dual-coefficient coupling control of slip and bus voltage, the coordinated adjustment of the motor operating state and the hardware state of the frequency converter main circuit is realized simultaneously, resulting in high control accuracy.

[0018] Optionally, the main circuit of the frequency converter is controlled to perform a soft start on the AC motor, including: The main circuit of the frequency converter is used to pre-start the AC motor and to determine whether the AC motor forms a continuous rotation trend during the pre-start process. If a continuous rotation trend is formed, the output frequency of the inverter main circuit is increased by the first frequency step control. The output current of the inverter main circuit and the reference current are compared. If the output current is greater than or equal to the reference current, the output frequency of the inverter main circuit is increased by the second frequency step control. The first frequency step is greater than the second frequency step. Determine whether the AC motor is continuously accelerating. If it cannot continue to accelerate, increase the output voltage of the inverter main circuit by the first voltage step control. The synchronous switching conditions are determined based on the output frequency, output voltage, and output current. If the synchronous switching conditions are met, the AC motor is controlled to switch to the power frequency bypass.

[0019] The above scheme adopts a segmented frequency conversion starting strategy. When the current does not reach the reference current, the speed is increased by a large frequency ramp. After the current reaches the reference current, the frequency ramp is reduced or maintained. When the motor does not achieve continuous acceleration, voltage small-step compensation is performed to balance starting current constraints and starting efficiency.

[0020] Optionally, the main circuit of the frequency converter is controlled to pre-start the AC motor, and it is determined whether the AC motor forms a continuous rotation trend during the pre-start process, including: Control the main circuit of the frequency converter to perform pre-charge and self-test; Minimum magnetization and low-frequency test run of AC motor are performed using the frequency converter main circuit; Obtain the estimated actual speed during the soft start process, and determine whether the AC motor has formed a continuous rotation trend based on the estimated actual speed.

[0021] The above solution avoids hardware failures in advance by pre-charging upon power-on, self-testing, minimum magnetization, and low-frequency trial operation, thus preventing the components from burning out due to faulty startup.

[0022] Optionally, the synchronous switching conditions are that the phase difference between the output voltage and the bypass voltage is less than 5°, the frequency difference between the output frequency and the bypass frequency is not greater than 1% of the rated frequency, the voltage difference between the output voltage and the bypass voltage is less than 5% of the rated voltage, and the fluctuation rate of the output current is less than 0.1. Controlling the switching of the AC motor to the power frequency bypass includes: Based on the historical or online measured operating time of the bypass contactor, a closing command is issued in advance to control the bypass contactor to close, and after feedback from the bypass contactor, the input and output contactors are controlled to open.

[0023] The above solution significantly reduces the circulating current and impact torque during switching by using strict multi-dimensional synchronization constraints, thus avoiding contactor arcing and motor shaft impact damage.

[0024] Secondly, the present invention provides an AC motor variable frequency soft start and soft stop control system, including a variable frequency main circuit, a power frequency bypass, a sampling unit, and a control unit; the variable frequency main circuit includes an input contactor K1, a precharge contactor K2, a three-level INPC inverter circuit, and an output contactor K4; the power frequency bypass includes a bypass contactor K3; the sampling unit is used to collect DC bus voltage, midpoint voltage, output voltage, output current, and bypass side synchronization signal; the control unit is used to control the conduction or disconnection of K1, K2, K3, and K4, and to perform synchronous switching control based on the estimated operating time of the bypass contactor K3. Attached Figure Description

[0025] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram of the AC motor frequency conversion soft start and soft stop control system according to an embodiment of the present invention; Figure 2 This is a flowchart illustrating the AC motor frequency conversion soft start and soft stop control method according to an embodiment of the present invention; Figure 3 This is a schematic diagram of a soft-start process according to an embodiment of the present invention; Figure 4 This is a schematic diagram of another soft-start process according to an embodiment of the present invention; Figure 5 This is a schematic diagram of a soft stop process according to an embodiment of the present invention; Figure 6 This is a timing control diagram of the AC motor variable frequency soft start and soft stop control method according to an embodiment of the present invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] In situations where high-voltage, high-power motors require controlled shutdown, if only free-slip stopping is used, the shutdown time is uncontrollable and cannot meet process requirements. If the motor is simply stopped by rapidly reducing the frequency, it is easy for the motor to enter regenerative power generation mode because the actual speed of the motor is higher than the synchronous speed, which will cause the DC bus voltage to rise and, in severe cases, cause system overvoltage faults.

[0029] Furthermore, when high-voltage, high-power motors use variable frequency soft starters, it is necessary to balance starting current limiting, smooth torque establishment, and impact control during the final switch to mains frequency operation. In existing technologies, if only conventional proportional voltage and frequency boosting methods are used, mismatches in phase, frequency, or voltage between the inverter output and the mains frequency side can easily occur near the bypass switching stage, resulting in large inrush currents and mechanical shocks when the bypass contactor is engaged.

[0030] In view of this, embodiments of the present invention provide a method for controlling the soft start and soft stop of an AC motor via frequency conversion. During the start-up phase, the starting current of the high-voltage AC motor can be reduced and the hot switching between power supply and frequency conversion can be smoothly achieved. During the stop phase, a strategy of first rapidly reducing the voltage and then slowly reducing the frequency can be used to suppress the motor from entering the regenerative power generation state, thereby reducing the risk of DC bus overvoltage and improving the controllability of the soft stop process.

[0031] The AC motor variable frequency soft start and soft stop control method of this invention is implemented through an AC motor variable frequency soft start and soft stop control system. The AC motor is generally a 10kV AC asynchronous motor.

[0032] like Figure 1 As shown, the AC motor frequency conversion soft start and soft stop control system of this embodiment includes a frequency conversion main circuit, a power frequency bypass, a sampling unit and a control unit.

[0033] The input power supply is connected to the AC motor through a parallel variable frequency main circuit and a power frequency bypass. The sampling unit is used to collect electrical parameters in the variable frequency main circuit and the power frequency bypass, and transmit the electrical parameters to the control unit. The control unit is used to execute the AC motor variable frequency soft start and soft stop control method.

[0034] Specifically, the input power supply is used to input 3.3kV-10kV three-phase AC power (R, S, T).

[0035] The main frequency converter circuit includes, in sequence, an input contactor K1, an input reactor, a rectifier circuit, a precharge circuit, a DC bus circuit, a three-level INPC inverter circuit, an output reactor, a filter circuit, and an output contactor K4.

[0036] The rectifier circuit includes 6 diodes, and the precharge circuit includes a precharge contactor K2 and a precharge resistor R1. During precharge, the precharge contactor K2 remains open, and the DC bus capacitor is charged through the precharge resistor R1. After precharge is completed, K2 is closed to bypass the precharge resistor R1.

[0037] The DC bus circuit includes a first bus capacitor C1a and a second bus capacitor C1b connected in series between the positive and negative terminals of the DC bus.

[0038] The three-level INPC inverter circuit comprises three inverter branches connected in parallel between the positive and negative terminals of the DC bus. Each inverter branch includes a first switch S1a, a second switch S2a, a third switch S3a, and a fourth switch S4a connected in series, as well as an upper clamping diode D1a and a lower clamping diode D1b. All switches are IGBTs. The upper clamping diode D1a is connected between the node between the first and second switches S1a and the midpoint D of the bus, and the lower clamping diode D1b is connected between the node between the third and fourth switches S3a and the midpoint D of the bus. For each inverter branch, both the upper and lower arms include two switches connected in series to accommodate high-voltage operating conditions and improve circuit reliability. In the three-level INPC inverter circuit, each diode, switch, resistor, and capacitor is not limited to one; two or more can be used in series as required.

[0039] The filtering circuit includes a first filter resistor R2a, a second filter resistor R2b, a third filter resistor R2c, a fourth filter resistor R3, a first filter capacitor C2a, a second filter capacitor C2b, a third filter capacitor C2c, and a fourth filter capacitor C3. The first filter resistor R2a and the first filter capacitor C2a are connected in series and then connected in series through the fourth filter resistor R3 between the U-phase output terminal and the bus midpoint D. The second filter resistor R2b and the second filter capacitor C2b are connected in series between the V-phase output terminal and the fourth filter resistor R3. The third filter resistor R2c and the third filter capacitor C2c are connected in series between the W-phase output terminal and the fourth filter resistor R3. The fourth filter capacitor C3 is connected in parallel with the fourth filter resistor R3. The filtering circuit suppresses the DV / DT surge caused by excessively fast inverter output edges and improves the insulation protection effect of the high-voltage motor.

[0040] The main frequency converter circuit is connected to the AC motor via output contactor K4. The AC motor is a 10kV high-voltage motor M.

[0041] The power frequency bypass includes bypass contactor K3, which connects the input power supply and the AC motor.

[0042] The sampling unit includes a current detection unit (CT) and a voltage and current sampling board. The output current is detected by the current detection unit (CT), and information such as DC bus voltage, midpoint voltage, input power supply voltage and current, output voltage and output current are collected by the voltage and current sampling board.

[0043] The control unit uses a DSP processor, which is connected to an operation panel and a communication interface. Users can input commands to the DSP processor through the operation panel, and the communication interface is used to communicate with external devices.

[0044] The DSP processor also samples the synchronization signal of the input power supply through an electronic voltage transformer (EVT), and controls the conduction and disconnection of each contactor through relay output, as well as controls the on / off of each switching transistor through output IGBT signal.

[0045] According to an embodiment of the present invention, an embodiment of a method for controlling the soft start and soft stop of an AC motor via frequency conversion is provided. It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions. Furthermore, although a logical order is shown in the flowchart, in some cases, the steps shown or described may be executed in a different order than that shown here.

[0046] This embodiment provides a method for controlling the soft start and soft stop of an AC motor via frequency conversion, such as... Figure 2 , Figure 3 , Figure 4 , Figure 5 and Figure 6 As shown, the method includes the following steps: Step S201: In response to the received start command, control the AC motor to connect to the inverter main circuit, control the inverter main circuit to perform soft start on the AC motor, and switch the AC motor to the power frequency bypass after successful start.

[0047] Specifically, the start command is the command that controls the start of the AC motor, and it can be input through the operation panel.

[0048] The AC motor is a high-voltage asynchronous motor with a rated power of 1600kW, a rated voltage of 10kV, a rated current of 108A, 4 poles, and a rated speed of 1493r / min. The load is no-load.

[0049] Upon receiving the start command, the main frequency converter circuit is controlled to perform a soft start on the AC motor by closing the input contactor K1 and the output contactor K4 and opening the bypass contactor K3. During the soft start process, the voltage and frequency are gradually increased in a low-frequency, low-voltage manner through the main frequency converter circuit (i.e., the frequency converter) to limit the starting inrush current and smoothly drive the AC motor to speed up.

[0050] After successful startup, a bypass closing command is issued in advance based on the historical operating time or the estimated operating time of the bypass contactor K3. After the inverter output and the bypass side meet the synchronous switching conditions and the K3 closing feedback is detected, the input contactor K1 and the output contactor K4 are disconnected to achieve hot switching between industrial and frequency converters.

[0051] In step S202, in response to the received stop command, the AC motor is switched to the frequency converter main circuit, the output voltage of the frequency converter main circuit is controlled to decrease to the preset low magnetic flux voltage at a first step-down rate, and the output frequency of the frequency converter main circuit is controlled to decrease according to the frequency deceleration trajectory corresponding to the preset stop time.

[0052] Specifically, the stop command is a command to control the AC motor to stop, which can be input through the operation panel.

[0053] Upon receiving a shutdown command, it determines whether the AC motor is in bypass operation. If it is in bypass operation, it closes the input contactor K1 and keeps the pre-charge contactor K2 open, allowing the DC bus to complete pre-charge through the pre-charge resistor. After pre-charge is completed, it closes K2. Then, it synchronizes the inverter output with the bypass side voltage. After the synchronization condition is met, it closes the output contactor K4. After the status of K4 is confirmed, it disconnects the bypass contactor K3, and the inverter main circuit takes over the AC motor again.

[0054] Simultaneously, a stop time Tstop is set, and the stop time Tstop is generally not less than the free coasting time Tcoast. The deceleration trajectory of the output frequency can be set according to the stop time Tstop; under normal soft-stop conditions without triggering overvoltage suppression or free stop protection, the output frequency is reduced to the set cutoff frequency within Tstop. If trajectory correction or free stop protection is triggered, the safety of the DC bus and power devices is prioritized.

[0055] Specifically, during the initial soft stop phase, the output voltage of the inverter main circuit is reduced at a relatively fast initial voltage reduction rate, causing the effective value of the output voltage to drop to the low flux voltage value corresponding to the low flux level, thereby reducing excitation energy and recovery capability. The low flux voltage value is preset, and once the output voltage drops to the low flux voltage value, the rate of decrease in the output voltage can be slowed down.

[0056] While the output voltage decreases rapidly at the first step-down rate, the output frequency of the inverter main circuit decreases according to the frequency deceleration trajectory corresponding to the preset stop time, so that the output frequency drops to the set cutoff frequency within Tstop, and the synchronous speed decreases slower than the actual speed of the motor.

[0057] Step S203: Obtain the estimated actual speed of the AC motor and the synchronous speed of the AC motor. Based on the estimated actual speed and the synchronous speed, correct the deceleration trajectory of the output frequency so that the synchronous speed is not lower than the estimated actual speed or maintains a preset positive slip margin.

[0058] The actual rotational speed is estimated based on the sampled output current, output frequency, and motor parameters. The fundamental frequency of the output voltage and output current is extracted to obtain the effective current value I1, power factor angle φ, active component, and excitation component. The slip frequency is estimated by combining the motor's rated frequency, number of poles, equivalent rotor time constant, or no-load slip characteristics.

[0059] In one example, the actual rotational speed estimate The estimation formula is:

[0060] in, This represents the number of pole pairs of an AC motor. For output frequency, This is the rotor resistance of the AC motor. For the rotor inductance of an AC motor, The torque current is obtained from the output current via Parker transformation. It is the d-axis air gap flux linkage, estimated by integrating the output voltage / motor model.

[0061] When a speed detection interface is available, the speed detection value can also be used to correct the actual speed estimate. When the d-axis air gap flux linkage is lower than the preset estimation threshold, the estimation weight based on the flux linkage model is reduced, and the previous stable estimate, slip limit, or speed detection value is used for correction. The synchronous speed is the theoretical rotating magnetic field speed of the motor corresponding to the current output frequency.

[0062] Based on the current output frequency Calculate the synchronous speed ns based on the number of pole pairs p of the motor:

[0063] This synchronous speed serves as a higher-level reference for the soft-stop frequency trajectory. Combined with the actual speed estimation value, it can be used to determine whether the AC motor may enter a negative slip or regenerative state after frequency reduction.

[0064] When the actual speed is estimated When the output frequency approaches or exceeds the synchronous speed (ns), the rate of decrease of the output frequency can be paused or reduced, and the effective value of the output voltage can be reduced first to decrease the magnetic flux and backflow capability, thus avoiding the rise of the DC bus voltage.

[0065] Step S204: Real-time detection of preset shutdown parameters; when the preset shutdown parameters meet the shutdown conditions, control the inverter main circuit to stop running.

[0066] Specifically, preset shutdown parameters include output frequency, output voltage, or output current.

[0067] In one example, the preset shutdown parameters include the output frequency. When the output frequency drops to the cutoff frequency, the shutdown condition is considered met, the inverter pulse is blocked, and the shutdown is completed.

[0068] In this embodiment of the invention, the output voltage is rapidly reduced to a low magnetic flux level, and the output frequency is reduced along a slower frequency deceleration trajectory, so that the synchronous speed decreases slower than the actual speed of the motor. Then, the deceleration trajectory of the output frequency is corrected based on the estimated actual speed and the synchronous speed to maintain positive slip margin and reduce the risk of energy backflow into the DC bus.

[0069] In some embodiments, step S203, correcting the deceleration trajectory of the output frequency based on the estimated actual speed and the synchronous speed to ensure that the synchronous speed is not lower than the estimated actual speed or maintains a preset positive slip margin, includes: Step S2031: Calculate the slip estimate based on the actual speed estimate and the synchronous speed estimate.

[0070] Specifically, slip estimate It is obtained from the difference between the estimated synchronous speed and the actual speed, and the calculation formula is as follows:

[0071] Slip estimate It can also be estimated based on the ratio of active current to excitation current in the output current, the direction of output power, and the equivalent parameters of the motor.

[0072] Step S2032: When the actual speed estimate is greater than or equal to the synchronous speed, or the slip estimate is less than the preset positive slip margin, reduce the attenuation rate of the output frequency or stop reducing the output frequency.

[0073] Specifically, the preset forward slip margin can be set according to the actual working conditions.

[0074] When the actual speed estimate n_est is close to or higher than the synchronous speed ns, or the slip estimate When the frequency reduction is less than the preset positive slip margin s_min, the frequency reduction is paused or the frequency reduction slope is reduced, and the effective value of the output voltage is reduced first to reduce the magnetic flux and the backflow capability; when s_est is greater than the preset positive slip margin s_min, the frequency reduction is allowed to continue slowly according to the target trajectory corresponding to the stop time Tstop.

[0075] The above solution quantifies the slip boundary conditions, accurately identifies the critical operating condition when the motor is about to enter the power generation feedback phase, intervenes in the deceleration curve in advance, and suppresses feedback energy from the source.

[0076] In some embodiments, after controlling the output voltage of the frequency converter main circuit to decrease to the low flux voltage at a first step-down rate and controlling the output frequency of the frequency converter main circuit to decrease according to a frequency deceleration trajectory corresponding to a preset stop time in step S202, the method further includes: Step S2021: Detect the DC bus voltage of the inverter main circuit. When the DC bus voltage meets the preset overvoltage condition, reduce the attenuation rate of the output frequency or stop reducing the output frequency.

[0077] Specifically, the preset overvoltage condition is that the DC bus voltage is greater than the preset bus voltage threshold, or the rise slope of the DC bus voltage dUdc / dt is greater than the preset positive threshold for a continuous first duration.

[0078] The preset bus voltage threshold and the first duration should be determined based on the allowable withstand voltage of the inverter's power devices and DC bus capacitors. In one example, the rated bus voltage is denoted as Udc,rated, and the preset bus voltage threshold is 1.2Udc,rated. When the DC bus voltage Udc is greater than the threshold, or when dUdc / dt is greater than the preset positive threshold for 5 seconds or more, overvoltage suppression is performed, reducing the decay rate of the output frequency or stopping the reduction of the output frequency.

[0079] By simultaneously introducing slip condition and bus voltage dual constraints to correct the deceleration curve, both motor operating conditions and inverter hardware safety are taken into account, providing dual protection against overvoltage faults. During deceleration, the frequency descent rate is slowed, the motor deceleration slope is reduced, braking regenerative power is decreased, and energy accumulation in the bus capacitor is reduced, thus lowering the risk of overvoltage without the need for additional large-capacity braking resistors.

[0080] In some embodiments, after detecting the DC bus voltage of the inverter main circuit, the method further includes: When the DC bus voltage exceeds the preset bus voltage threshold and the duration exceeds the second duration, free shutdown protection is executed.

[0081] Specifically, the second duration is the delay threshold for severe overvoltage faults, which should be determined based on the inverter's power devices, DC bus capacitors, and their allowable withstand voltage, and should not replace the inverter's hardware overvoltage protection.

[0082] Free stop refers to blocking the inverter output, allowing the motor to disengage from the inverter main circuit and naturally decelerate due to load friction.

[0083] If the bus voltage exceeds the threshold and the duration exceeds the second time period, it is determined to be a serious overvoltage fault, and the free stop protection is directly executed to cut off the frequency converter output.

[0084] The above scheme employs a tiered protection mechanism. For minor overvoltage, only the deceleration curve is corrected and soft intervention is used, while for severe and continuous overvoltage, the system is shut down directly. This layered protection balances operational continuity and equipment safety. Furthermore, the timing delay avoids accidental shutdowns due to single voltage spikes, reducing unnecessary equipment downtime and improving system operational continuity.

[0085] In some embodiments, when correcting the deceleration trajectory of the output frequency, the relationship between the output frequency before correction and the output frequency after correction is as follows:

[0086] In the formula, fout(k) and fout(k+1) are the output frequencies before and after correction, respectively; Δf_stop is the initial frequency drop step; Kslip is the slip correction coefficient; and Kudc is the bus voltage correction coefficient. Both Kslip and Kudc take values ​​from 0 to 1. Kslip is determined by the remaining amount of the slip estimate relative to the preset positive slip margin. The smaller the remaining amount, the smaller Kslip. Kudc is determined by the DC bus voltage and its rising slope. The higher the DC bus voltage and / or the greater the rising slope, the smaller Kudc.

[0087] In one example, The speed is set to 3Hz / s or 5Hz / s. A first lookup table is pre-set with the slip estimation value, the deviation value of the preset positive slip margin, and the slip correction coefficient; a second lookup table is set with the DC bus voltage and the bus voltage correction coefficient. The corresponding slip correction coefficient and bus voltage correction coefficient are determined based on the first and second lookup tables. The second lookup table can also be a table showing the correspondence between DC bus voltage, the rise slope of DC bus voltage, and the bus voltage correction coefficient. The corresponding bus voltage correction coefficient is determined by the binary representation of the DC bus voltage and the rise slope of DC bus voltage.

[0088] The degree of correction of the output frequency is quantified by the above mathematical model. The correction intensity changes continuously and smoothly with the working conditions without segmental abrupt changes. The motor deceleration torque is continuous and without impact. Furthermore, through the dual-coefficient coupling control of slip and bus voltage, the coordinated adjustment of the motor operating state and the hardware state of the frequency converter main circuit is realized simultaneously, resulting in high control accuracy.

[0089] In some embodiments, such as Figure 3 and Figure 4 As shown, the control circuit for soft starting of the AC motor includes: Step S301: Control the frequency converter main circuit to pre-start the AC motor and determine whether the AC motor forms a continuous rotation trend during the pre-start process.

[0090] Specifically, pre-start is the initial stage of testing the AC motor at a lower frequency to ensure that the AC motor can operate normally.

[0091] If the AC motor does not show a continuous rotation trend, increase the output voltage in small steps. If it still cannot rotate after the output voltage reaches the upper limit, then issue an output fault alarm or exit the startup.

[0092] In step S302, if a continuous rotation trend is formed, the output frequency of the inverter main circuit is increased by controlling the first frequency step. The output current of the inverter main circuit and the reference current are compared. If the output current is greater than or equal to the reference current, the output frequency of the inverter main circuit is increased by controlling the second frequency step. The first frequency step is greater than the second frequency step.

[0093] Specifically, the first frequency step is 2Hz / s, the second frequency step is 1Hz / s, the motor rated current is 108A, and the reference current is 90A. First, the output frequency of the inverter main circuit is increased by controlling the first frequency step. When the output current Iout is greater than or equal to the reference current Iref, the second frequency step is switched to control the output frequency of the inverter main circuit to increase, so as to reduce the frequency ramp-up slope and avoid further increasing the output current.

[0094] Step S303: Determine whether the AC motor continues to accelerate. If it cannot continue to accelerate, increase the output voltage of the inverter main circuit by the first voltage step control.

[0095] Specifically, the actual speed of the motor is estimated or detected by a speed sensor to determine whether the AC motor is continuously accelerating. If it cannot continue to accelerate, a small step voltage compensation of 1% of the motor's rated voltage Un is performed to avoid a one-time large increase in the output voltage.

[0096] Step S304: Determine whether the synchronous switching conditions are met based on the output frequency, output voltage and output current. If the synchronous switching conditions are met, control the AC motor to switch to the power frequency bypass.

[0097] Specifically, the synchronous switching conditions are: the phase difference between the output voltage and the bypass voltage is less than 5°; the frequency difference between the output frequency and the bypass frequency is not greater than 1% of the rated frequency; the voltage difference between the output voltage and the bypass voltage is less than 5% of the rated voltage; and the fluctuation rate of the output current is less than 0.1.

[0098] The present invention adopts a segmented frequency conversion starting strategy. First, the frequency is controlled to increase with a large frequency ramp. After the current reaches the reference current, the frequency ramp is reduced or maintained. When the motor does not continue to accelerate, voltage small-step compensation is performed to avoid excessive current during the starting process and to take into account the starting speed.

[0099] Furthermore, through multi-dimensional and strict synchronization constraints, the control of the AC motor to switch to the power frequency bypass is only allowed when the synchronization switching conditions are met, which greatly reduces the circulating current and impact torque at the moment of switching and avoids contactor arcing and motor shaft impact damage.

[0100] In some embodiments, in step S301, controlling the inverter main circuit to pre-start the AC motor and determining whether the AC motor forms a continuous rotation trend during the pre-start process includes: Step S3011: Control the main circuit of the frequency converter to perform pre-charge and self-test.

[0101] Specifically, the input contactor K1 is closed, the pre-charge contactor K2 is kept open, and the bypass contactor K3 is disconnected, allowing the bus capacitor to charge through the pre-charge resistor R1. When the DC bus voltage reaches the preset pre-charge completion threshold, the pre-charge contactor K2 is closed to bypass the pre-charge resistor R1, and the bus status, synchronization signal, and contactor status are detected. The purpose of this step is to limit the DC bus charging surge, confirm that the sampling circuit, synchronization signal, contactor feedback, and protection status all meet the start-up conditions, and avoid activating the inverter output when the bus is not stable or the contactor status is uncertain.

[0102] Step S3012: Minimum magnetization and low-frequency test run of AC motor are performed through the frequency converter main circuit.

[0103] Specifically, the minimum magnetic flux establishment process includes: controlling the output voltage to 0.1Un and holding it for 0.5s to allow the motor to establish the minimum necessary magnetic flux.

[0104] The low-frequency test rotation process includes: controlling the output frequency to 1Hz to enter the low-frequency test rotation, and judging whether a continuous rotation trend has formed.

[0105] Step S3013: Obtain the estimated actual speed during the soft start process, and determine whether the AC motor has formed a continuous rotation trend based on the estimated actual speed.

[0106] Specifically, the actual rotational speed is estimated based on the sampled output current, output frequency, and motor parameters. Whether a continuous rotational trend is formed is determined by whether the estimated actual rotational speed remains above a minimum threshold for a period of time.

[0107] The above solution avoids hardware failures in advance by pre-charging upon power-on, self-testing, minimum magnetization, and low-frequency trial operation, thus preventing the components from burning out due to faulty startup.

[0108] like Figure 6 As shown, in the startup phase of this embodiment of the invention, the DC bus is first pre-charged, and after the output contactor K4 is engaged, minimum magnetization and low-frequency trial operation are performed. Subsequently, the frequency ramping slope is dynamically adjusted according to the deviation between the output current and the reference current. Within the synchronization window, the closing command of the bypass contactor K3 is sent in advance based on the estimated closing delay value of the bypass contactor K3. After receiving the closing feedback of K3, the input contactor K1 and the output contactor K4 are disconnected to achieve hot switching between power supply and frequency conversion.

[0109] During the stop phase, the system sets a stop time Tstop, which is not less than the motor's free coasting time Tcoast. When a stop command is received and the system is in bypass operation, the mains frequency to frequency conversion switchover is completed first, allowing the main frequency converter to take over the motor again. Then, the effective value of the output voltage is rapidly reduced to a low flux level, and the output frequency is reduced at a slower slope, so that the synchronous speed decreases slower than the actual speed of the motor. During this process, the deceleration trajectory is corrected by combining the speed estimate, slip estimate, DC bus voltage and its rising slope to maintain positive slip margin and suppress energy backflow. The midpoint voltage detection quantity is used to constrain the midpoint voltage balance during the modulation process of the three-level inverter.

[0110] Compared with the prior art, the embodiments of the present invention dynamically adjust the frequency ramping slope according to the deviation between the output current and the reference current during the startup phase, which can reduce the starting current of the high-voltage motor and smoothly realize the hot switching between power supply and frequency conversion; during the shutdown phase, the strategy of fast voltage reduction and slow frequency reduction can be used to suppress the motor from entering the regenerative power generation state, thereby reducing the risk of DC bus overvoltage and improving the controllability of the soft stop process.

[0111] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A method for controlling the soft start and soft stop of an AC motor using frequency conversion, characterized in that, The method includes: In response to the received start command, the AC motor is controlled to connect to the inverter main circuit, the inverter main circuit is controlled to perform a soft start on the AC motor, and after successful start, the AC motor is switched to the power frequency bypass. In response to the received shutdown command, the AC motor is switched to the frequency converter main circuit, the output voltage of the frequency converter main circuit is controlled to decrease to a preset low magnetic flux voltage at a first step-down rate, and the output frequency of the frequency converter main circuit is controlled to decrease according to a frequency deceleration trajectory corresponding to a preset stop time. Obtain the estimated actual speed of the AC motor and the synchronous speed of the AC motor. Correct the deceleration trajectory of the output frequency based on the estimated actual speed and the synchronous speed to ensure that the synchronous speed is not lower than the estimated actual speed or maintains a preset positive slip margin. The system monitors preset shutdown parameters in real time, and controls the inverter main circuit to stop operating when the preset shutdown parameters meet the shutdown conditions.

2. The method according to claim 1, characterized in that, The step of correcting the deceleration trajectory of the output frequency based on the estimated actual speed and the synchronous speed, so that the synchronous speed is not lower than the estimated actual speed or maintains a preset positive slip margin, includes: The slip estimate is calculated based on the actual speed estimate and the synchronous speed estimate. When the estimated actual rotational speed is greater than or equal to the synchronous rotational speed, or when the estimated slip is less than the preset positive slip margin, the attenuation rate of the output frequency is reduced or the reduction of the output frequency is stopped.

3. The method according to claim 2, characterized in that, After controlling the output voltage of the frequency converter main circuit to decrease to a preset low flux voltage at a first step-down rate, and controlling the output frequency of the frequency converter main circuit to decrease according to a frequency deceleration trajectory corresponding to a preset stop time, the method further includes: The DC bus voltage of the inverter main circuit is detected. When the DC bus voltage meets the preset overvoltage condition, the attenuation rate of the output frequency is reduced or the reduction of the output frequency is stopped.

4. The method according to claim 3, characterized in that, The preset overvoltage condition is that the DC bus voltage is greater than a preset bus voltage threshold or the rise slope of the DC bus voltage is greater than a preset positive threshold for a continuous first duration.

5. The method according to claim 3, characterized in that, After detecting the DC bus voltage of the inverter main circuit, the method further includes: When the DC bus voltage is greater than the preset bus voltage threshold and the duration is greater than the second duration, free shutdown protection is executed.

6. The method according to claim 3, characterized in that, When correcting the deceleration trajectory of the output frequency, the relationship between the output frequency before correction and the output frequency after correction is as follows: In the formula, fout(k) and fout(k+1) are the output frequencies before and after correction, respectively; Δf_stop is the initial frequency drop step; Kslip is the slip correction coefficient; and Kudc is the bus voltage correction coefficient. Both Kslip and Kudc take values ​​from 0 to 1. Kslip is determined by the remaining amount of the slip estimate relative to the preset positive slip margin. The smaller the remaining amount, the smaller Kslip is. Kudc is determined by the DC bus voltage and its rising slope. The higher the DC bus voltage and / or the greater the rising slope, the smaller Kudc is.

7. The method according to claim 1, characterized in that, The control of the frequency converter main circuit to perform soft start on the AC motor includes: The inverter main circuit is controlled to pre-start the AC motor, and it is determined whether the AC motor forms a continuous rotation trend during the pre-start process. If a continuous rotation trend is formed, the output frequency of the inverter main circuit is increased by a first frequency step. The output current of the inverter main circuit and the reference current are compared. If the output current is greater than or equal to the reference current, the output frequency of the inverter main circuit is increased by a second frequency step. The first frequency step is greater than the second frequency step. Determine whether the AC motor is continuously accelerating; if it cannot continuously accelerate, increase the output voltage of the inverter main circuit by the first voltage step control. Based on the output frequency, the output voltage, and the output current, it is determined whether the synchronous switching conditions are met. If the synchronous switching conditions are met, the AC motor is controlled to switch to the power frequency bypass.

8. The method according to claim 7, characterized in that, The control of the frequency converter main circuit to pre-start the AC motor, and the determination of whether the AC motor forms a continuous rotation trend during the pre-start process, includes: The inverter main circuit is controlled to perform pre-charge and self-test; The AC motor is subjected to minimum magnetization and low-frequency test run through the variable frequency main circuit; The estimated actual speed during the soft start process is obtained, and the AC motor is judged to have formed a continuous rotation trend based on the estimated actual speed.

9. The method according to claim 7, characterized in that, The synchronous switching conditions are: the phase difference between the output voltage and the bypass voltage is less than 5°; the frequency difference between the output frequency and the bypass frequency is not greater than 1% of the rated frequency; the voltage difference between the output voltage and the bypass voltage is less than 5% of the rated voltage; and the fluctuation rate of the output current is less than 0.

1. The control of switching the AC motor to power frequency bypass includes: Based on the historical or online measured operating time of the bypass contactor, a closing command is issued in advance to control the bypass contactor to close, and after feedback from the bypass contactor, the input contactor and output contactor are controlled to open.

10. A variable frequency soft start and soft stop control system for an AC motor, characterized in that, It includes a frequency converter main circuit, a power frequency bypass, a sampling unit, and a control unit; the frequency converter main circuit includes an input contactor K1, a precharge contactor K2, a three-level INPC inverter circuit, and an output contactor K4; the power frequency bypass includes a bypass contactor K3. The input power supply is connected to the AC motor through the parallel-connected frequency converter main circuit and the power frequency bypass. The sampling unit is used to collect the DC bus voltage, midpoint voltage, output voltage, output current and bypass side synchronization signal, and transmit the collected electrical parameters to the control unit. The control unit is used to control the conduction or disconnection of K1, K2, K3 and K4, perform synchronous switching control based on the estimated action time of the bypass contactor K3, and execute the AC motor frequency converter soft start and soft stop control method as described in any one of claims 1 to 9.

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