In recent years, with the development of permanent magnet materials, permanent magnet motors have become the new favorite of the Motor industry. However, in permanent magnet motors, the presence of cogging torque has a great influence on the control performance of the motor. How does the cogging torque occur? How should we test it?
Friends who have played with permanent magnet motors have had a similar experience: we turned the rotor of the motor in the case of a motor power failure, and found that there is a feeling of stagnation, unlike the traditional DC motor, the rotor can be freehand. Turn it up. This kind of Karton is actually because the permanent magnet motor has cogging torque. The internal structure of the permanent magnet motor is shown in Figure 1. The cogging torque is one of the inherent characteristics of the permanent magnet motor. It is the magnetic field generated by the permanent magnet and the armature core in the state where the armature winding is not energized. The cogging acts on the torque generated in the circumferential direction. It is actually the tangential force between the permanent magnet and the armature tooth, so that the rotor of the permanent magnet motor has a tendency to align with the stator in a certain direction, trying to position the rotor at certain positions. One type of oscillating torque is the cogging torque.
Figure 1 Permanent magnet synchronous motor structure
The cogging torque causes the motor to generate vibration and noise, and the speed fluctuation occurs, so that the motor cannot run smoothly and affect the performance of the motor. In the variable speed drive, when the torque ripple frequency coincides with the mechanical resonance frequency of the stator or the rotor, the vibration and noise generated by the cogging torque are amplified. The presence of cogging torque also affects the low speed performance of the motor in the speed control system and the high precision positioning in the position control system. Therefore, engineers who do permanent magnet motor development want to minimize the cogging torque of their own motor. Engineers using permanent magnet motors want to understand the cogging torque of this motor, so as to optimize his control algorithm. .
In the national standard GBT/ 30549-2014, the cogging torque test has a clear definition: when the motor winding is open, the motor is rotated within one week, the armature core is slotted, and there is a tendency to tend to the minimum reluctance position. Periodic torque. Common methods for measuring cogging torque are: lever measurement method and torque meter method. The lever measurement method is relatively simple and the measurement accuracy is relatively poor, so it is mainly used in occasions where the accuracy is not high. The torque meter method is shown in Figure 2. Since the cogging torque of the servo motor is very small, it is necessary to drive the unpowered motor to test at a very low speed to complete the test. After the prime mover outputs It is necessary to first reduce the speed to about 1 rpm/min through the deceleration system, then drive the motor under test to test, and test the cogging torque with a torque sensor. During the test, it is necessary to deal with the torque fluctuation of the prime mover and the transmission system to make the output speed torque smoother, so as to reduce the influence of the torque fluctuation of the transmission system on the test result.
Figure 2 Cogging torque gantry architecture
Thanks to its in-depth understanding of motor measurement and long-term technical accumulation, Zhiyuan Electronics has introduced a test bench dedicated to motor cogging torque and friction torque testing. After the State Council issued "Made in China 2025", various places have ignited the east wind of intelligent manufacturing of servo robots, aiming to break through the key components such as robot body, reducer, servo motor, controller, sensor and driver, and system integration design and manufacturing technology. bottleneck. Zhiyuan Electronics has long been focusing on dynamic testing of servo motors and has extensive experience in testing time response, step response, and bandwidth test. The introduction of the cogging torque test bench is a more comprehensive test for servo motor testing. era.
Figure 3 Zhiyuan electronic cogging torque test bench
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