Starting method of permanent magnet synchronous motor

Abstract

The invention relates to the technical field of permanent magnet synchronous motors, in particular to a starting method of a permanent magnet synchronous motor, which comprises the following steps of: S1, driving a rotor of the permanent magnet synchronous motor to rotate through a frequency converter, so that the rotor starts to rotate from a static state and is gradually accelerated to a first preset rotating speed; s2, after the rotating speed of the rotor reaches a first preset rotating speed, electric connection between the frequency converter and the permanent magnet synchronous motor is disconnected; s3, after the frequency converter is disconnected, when the rotating speed of the permanent magnet synchronous motor is reduced to a second preset rotating speed, the frequency conversion control mode of the frequency converter is switched to a power frequency control mode; and S4, after switching to the power frequency control mode, continuously accelerating the rotor by means of a starting copper bar arranged on the periphery of the rotor core until the rotating speed of the rotor is drawn to be synchronous with the rated rotating speed of the rotating magnetic field of the stator. According to the scheme, smooth starting and power frequency efficient operation of the permanent magnet synchronous motor under a large load can be realized, the energy consumption and the cost are effectively reduced, and the operation stability is improved.

Description

Technical Field

[0001] This invention relates to the technical field of permanent magnet synchronous motors, specifically to a starting method for a permanent magnet synchronous motor. Background Technology

[0002] Permanent magnet synchronous motors, with their significant advantages such as high efficiency, high power density, small size and rapid dynamic response, have been widely used in many core fields such as new energy vehicles, industrial servo systems, home appliances, and aerospace, becoming key actuators in modern power control and precision transmission systems.

[0003] The starting principle of a permanent magnet synchronous motor is based on the generation of a rotating magnetic field by passing alternating current through the stator windings. A permanent magnet is fixed on the rotor, and the rotating magnetic field drives the rotor to rotate. However, because the rotating magnetic field rotates very fast and the rotor has a large inertia, coupled with a connected load, if the motor is directly connected to the mains frequency power supply during startup, the stationary rotor cannot instantly start rotating following the magnetic field. Therefore, permanent magnet synchronous motors require a frequency converter to assist in starting. During startup, the frequency converter's output frequency continuously increases from zero to the required frequency until the rotor speed and the rotating magnetic field speed are synchronized, completing the startup. This is the most common starting method for conventional permanent magnet synchronous motors, but it still has the following technical problems: Because permanent magnet synchronous motors require the rotor speed to be synchronized with the rotating magnetic field on the stator side after startup and normal operation to ensure smooth motor operation, and the motor requires a large torque when driving a heavy load, if the motor is directly connected to the mains frequency power supply, it cannot generate enough torque to drive the rotor to gradually reach the speed of the rotating magnetic field. Therefore, a frequency converter is needed to adjust the power frequency to control the motor speed. The frequency converter drives the motor throughout the entire operation, and the frequency converter cannot stop running, in order to ensure that the rotor speed and the speed of the rotating magnetic field on the stator side remain synchronized. Once the frequency converter stops running, the motor will instantly lose controllable torque, resulting in the rotor speed being out of sync with the rotating magnetic field of the stator, i.e., the "loss of synchronization" phenomenon. Once the motor loses synchronization, the rotor will be out of the synchronous traction of the stator magnetic field, the motor noise and vibration will increase, and the motor's operating stability will be reduced. However, keeping the frequency converter running continuously will greatly increase energy consumption and operating costs.

[0004] Therefore, there is an urgent need for a starting method that can both improve the operational stability of permanent magnet synchronous motors and save energy, in order to solve the starting defects of existing motors. Summary of the Invention

[0005] This invention provides a starting method for a permanent magnet synchronous motor, which can solve the problem that in the prior art, when a permanent magnet synchronous motor drives a large load to start, the frequency converter needs to be continuously activated, resulting in high energy consumption and operating costs.

[0006] This application provides the following technical solution: a starting method for a permanent magnet synchronous motor, comprising the following steps: S1: Drive the rotor of the permanent magnet synchronous motor to rotate through the frequency converter, so that the rotor starts to rotate from rest and gradually accelerates to the first predetermined speed; S2: After the rotor speed reaches the first predetermined speed, disconnect the electrical connection between the frequency converter and the permanent magnet synchronous motor; S3: After disconnecting the inverter, when the speed of the permanent magnet synchronous motor drops to the second predetermined speed, switch the inverter's frequency conversion control mode to the power frequency control mode. S4: After switching to the power frequency control mode, the rotor is accelerated by the starting copper bar set on the outer periphery of the rotor core until the rotor speed is brought into synchronization with the rated speed of the rotating magnetic field of the stator.

[0007] Beneficial effects: This invention saves energy, reduces operating costs, and improves the stability and reliability of motor operation. It creatively utilizes the starting copper bars on the rotor, combined with a switching control strategy between variable frequency and mains frequency, to achieve asynchronous starting of a permanent magnet synchronous motor. In step S1, the frequency converter gradually accelerates the rotor to a first predetermined speed, allowing the permanent magnet synchronous motor to start smoothly and normally, avoiding insufficient speed to drive the rotor when directly switching to mains frequency power. Subsequently, in steps S3 and S4, the frequency converter is disconnected, and once the speed of the permanent magnet synchronous motor drops to a second predetermined speed, the frequency converter is switched back to mains frequency. The rotating magnetic field on the stator induces a current in the starting copper bars on the rotor, generating asynchronous torque. Under mains frequency power, this asynchronous torque assists the rotor in continuing to accelerate, ultimately synchronizing the rotor speed with the rotating magnetic field of the stator to complete the start-up. After speed synchronization, the motor can operate stably under mains frequency power without the need for a frequency converter. This approach leverages both the starting advantages of frequency converters and the low cost and high reliability of power supply. Under heavy load starting and operation conditions, it can significantly reduce motor energy consumption and effectively save operating costs.

[0008] Furthermore, in step S3, the frequency converter's frequency conversion control mode is switched to the power frequency control mode within a time of 0.05 seconds to 0.1 seconds.

[0009] Beneficial effects: Prevents excessive speed drop. Setting a fast switching time of 0.05s-0.1s ensures that the rotor speed drop due to load resistance is kept to a minimum after the inverter is disconnected. If the switching time is too long, the rotor speed may drop outside the optimal speed range, making it impossible to re-pull back to synchronous speed after switching to the mains frequency, thus ensuring a smooth switching process.

[0010] Furthermore, in step S3, the second predetermined speed is 75% to 90% of the first predetermined speed.

[0011] Beneficial effects: This speed range is the optimal range for permanent magnet synchronous motors to achieve asynchronous acceleration and eventually be pulled into synchronous speed using the starting copper bar. At this speed, the induced current generated by the starting copper bar cutting the magnetic field lines is at a relatively high level, which can provide sufficient asynchronous torque to overcome load resistance, and can pull the rotor into synchronous speed most stably and quickly.

[0012] Furthermore, in step S4, when the rotor speed reaches the rated speed, there is a corresponding pull torque, which is more than 1.3 times the rated torque of the motor.

[0013] Beneficial effects: A pull-in torque of over 1.3 times provides ample torque margin. Under heavy load conditions, the motor needs to overcome maximum static friction and dynamic inertia when approaching synchronous speed. If the pull-in torque is insufficient, the motor may be slowed down by the load when approaching synchronous speed due to insufficient torque, causing the motor to "lose sync". The high torque of over 1.3 times ensures that the electromagnetic force can lock the rotor instantly, making the rotor strictly synchronized with the stator magnetic field, completely solving the technical problem of high-power permanent magnet motors being difficult to pull into synchronization under heavy loads.

[0014] Furthermore, in step S4, the rotor is further accelerated by the torque generated by electromagnetic induction through the starting copper bar.

[0015] Beneficial effects: Compared with complex vector control algorithms, the acceleration is achieved by utilizing the physical induction characteristics of copper bars. The control logic is simple, the physical structure is robust, the anti-interference ability is strong, and the motor start-up process is smooth and effective.

[0016] Furthermore, in step S4, multiple opening slots are evenly distributed on the outer circumference of the rotor core, and the starting copper strip is fixed in the opening slots.

[0017] Beneficial effects: The open slot structure facilitates the generation of reluctance torque. Simultaneously, the open slot structure promotes heat dissipation of the starting copper bar under high-frequency induced current, preventing overheating and burn-out. Furthermore, compared to closed slots, open slots make the insertion and fixing of the starting copper bar easier and faster, reducing the processing difficulty and manufacturing cost of the motor rotor. Attached Figure Description

[0018] Figure 1 This is a front view of the rotor structure of the present invention.

[0019] Figure 2 This is a graph showing the variation of the generator braking torque with rotational speed.

[0020] Figure 3 This is a graph showing the change in starting torque as a function of speed.

[0021] Figure 4 This is a graph showing the changes in the combined braking torque curve and starting torque curve. Detailed Implementation

[0022] The following detailed description illustrates the specific implementation method: The markings in the accompanying drawings include: rotor core 1, starting copper bar 2, and magnet 3.

[0023] Example 1 This embodiment describes the method of the present invention in detail by using a permanent magnet synchronous motor to drive a ball mill for startup. The rated speed of the permanent magnet synchronous motor is 750 r / min, and the frequency of the power supply is 50 Hz. Figure 1 As shown, in this scheme, the rotor core 1 of the permanent magnet synchronous motor has multiple opening slots evenly distributed on its outer periphery. The starting copper bar 2 is fixed in the opening slot, and the magnet 3 is fixed in the corresponding mounting hole of the rotor core 1.

[0024] A starting method for a permanent magnet synchronous motor includes the following steps: S1: The rotor of the permanent magnet synchronous motor is driven to rotate by the frequency converter, causing the rotor to start rotating from rest and gradually accelerate to a first predetermined speed. The purpose of this step is to allow the frequency converter to start the motor first, so that the rotor speed approaches the rated speed of the motor. In this example, considering the load characteristics of the ball mill, the first predetermined speed range is 710 r / min to 720 r / min. This speed can both allow the rotor to get rid of the maximum static friction force in the stationary state and reserve sufficient inertial margin to avoid a rapid drop in speed after the frequency converter is disconnected, ensuring a smooth start-up process and avoiding mechanical and current shocks.

[0025] S2: After the rotor speed reaches the first predetermined speed, the electrical connection between the frequency converter and the permanent magnet synchronous motor is disconnected; the frequency converter stops outputting power to the motor stator winding; at this time, the rotor begins to decelerate slowly under its own rotational inertia and the load of the ball mill.

[0026] S3: After disconnecting the frequency converter, when the speed of the permanent magnet synchronous motor drops to the second predetermined speed, switch the frequency converter's variable frequency control mode to the power frequency control mode. The switching time range is 0.05 seconds to 0.1 seconds. The second predetermined speed is 75% to 90% of the first predetermined speed. In this example, taking the first predetermined speed of 720 r / min as an example, the second predetermined speed is approximately 540 r / min to 650 r / min. That is, when the rotor reaches the first predetermined speed of 720 r / min, the frequency converter is disconnected, and the rotor speed will gradually drop to the range of the second predetermined speed of 540 r / min to 650 r / min. At this time, the motor is switched to the power frequency power supply. The motor is no longer driven by the frequency converter, but directly driven by the power frequency power supply. The frequency of the power frequency power supply is 50Hz, which is the unified standard of AC power in my country.

[0027] S4: After switching to the power frequency control mode, the motor is connected to the power frequency power supply. The rotational magnetic field speed on the stator side is the rated speed of 750 r / min. Therefore, there is a speed difference between the rotor speed and the rotational magnetic field speed. At this time, the starting copper bar 2 on the outer periphery of the rotor core 1 will generate an induced current due to electromagnetic induction. Since the rotating magnetic field on the stator side is rotating, it is equivalent to the starting copper bar 2 cutting the magnetic field lines. The starting copper bar 2 will generate force, causing the rotor to generate torque. At this time, the motor is equivalent to the starting mode of an asynchronous motor. The rotor continues to accelerate by relying on the torque until the rotor speed is gradually pulled from 540 r / min to 650 r / min and synchronized with the rated speed of the rotating magnetic field of the stator at 750 r / min. After the rotor speed is synchronized with the rotating magnetic field speed on the stator side, the starting copper bar 2 and the rotating magnetic field on the stator side remain relatively stationary and no longer cut the magnetic field lines, so no torque is generated. The torque becomes zero, and the motor can maintain stable operation under the drive of the power frequency power supply.

[0028] It should be noted that when the rotor speed reaches the rated speed, there is a corresponding pull torque. This pull torque is to ensure that the rotor speed can be smoothly pulled into synchronization with the rated speed when it approaches the rated speed. Since the rotor needs to overcome the maximum static friction resistance and dynamic inertia at the moment it approaches the synchronous speed, the pull torque is more than 1.3 times the rated torque of the motor.

[0029] like Figures 2 to 4 As shown, the curve obtained by starting the permanent magnet synchronous motor using the method of the present invention is... Figure 2 The graph shows the change of the generator braking torque with the rotational speed. When the motor starts under load, the initial resistance torque is the maximum of 35000 N·m. As the rotational speed increases, the rotor speed gradually synchronizes with the rotational speed of the rotating magnetic field, and the resistance torque becomes smaller and smaller and approaches 0. Figure 3 This is a graph showing the change in starting torque as a function of speed. Figure 4 for Figure 2 Braking torque curve and Figure 3 The synthesized starting torque curve shows that when the motor starts using a frequency converter, the torque gradually increases with the speed. After reaching the first predetermined speed of 720 r / min, the frequency converter is disconnected, allowing the speed to gradually decrease to the second predetermined speed of approximately 540 to 650 r / min. Then, the power supply is switched back to the mains frequency, and the asynchronous electromagnetic torque generated by the starting copper bars 2 on the rotor further accelerates the rotor speed. Figure 3 and Figure 4 As can be seen, when the rotor speed is pulled up to the rated speed of 750 r / min to achieve speed synchronization, the corresponding torque drops to 0, indicating that the method of the present invention can achieve safe and reliable asynchronous start-up of permanent magnet synchronous motor.

[0030] The starting method of this embodiment uses a frequency converter to achieve asynchronous starting of the permanent magnet synchronous motor only during the starting phase, avoiding the starting shock caused by direct connection to the power frequency power supply. After starting, the motor relies on the power frequency power supply to maintain synchronous operation, without the need for a frequency converter to drive it throughout the process. This greatly reduces energy consumption and operating costs, improves the stability and reliability of motor operation, and can adapt to the starting and operation requirements of equipment with large inertia and large load.

[0031] The above are merely embodiments of the present invention, and the invention is not limited to the fields covered by these embodiments. Commonly known structures and characteristics in the solutions are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the structure of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A starting method for a permanent magnet synchronous motor, characterized in that: Includes the following steps: S1: Drive the rotor of the permanent magnet synchronous motor to rotate through the frequency converter, so that the rotor starts to rotate from rest and gradually accelerates to the first predetermined speed; S2: After the rotor speed reaches the first predetermined speed, disconnect the electrical connection between the frequency converter and the permanent magnet synchronous motor; S3: After disconnecting the inverter, when the speed of the permanent magnet synchronous motor drops to the second predetermined speed, switch the inverter's frequency conversion control mode to the power frequency control mode. S4: After switching to the power frequency control mode, the rotor is accelerated by the starting copper bar set on the outer periphery of the rotor core until the rotor speed is brought into synchronization with the rated speed of the rotating magnetic field of the stator.

2. The starting method for a permanent magnet synchronous motor according to claim 1, characterized in that: In step S3, the frequency converter switches from frequency conversion control mode to power frequency control mode within 0.05 to 0.1 seconds.

3. The starting method for a permanent magnet synchronous motor according to claim 2, characterized in that: In step S3, the second predetermined speed is 75% to 90% of the first predetermined speed.

4. The starting method for a permanent magnet synchronous motor according to claim 3, characterized in that: In step S4, when the rotor speed reaches the rated speed, there is a corresponding pull torque, which is more than 1.3 times the rated torque of the motor.

5. The starting method for a permanent magnet synchronous motor according to claim 4, characterized in that: In step S4, the rotor is further accelerated by the torque generated by electromagnetic induction through the starting copper bar.

6. The starting method for a permanent magnet synchronous motor according to claim 5, characterized in that: In step S4, multiple slots are evenly distributed on the outer circumference of the rotor core, and the starting copper strip is fixed in the slots.

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