Causes of cogging torque & how to reduce cogging torque?（1）
Tianjin Santroll Electric Technology Co., Ltd.
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Cogging torque is the torque generated by the interaction between the permanent magnet and the stator core when the permanent magnet motor winding is not energized. It is caused by the pulsation of the tangential component of the interaction force between the permanent magnet and the stator teeth. When the motor rotor rotates, the magnetic permeance in a small range of the stator slots on both sides of the permanent magnet changes significantly, causing the magnetic field energy storage to change, thereby generating cogging torque.
Cogging torque will cause torque pulsation in the permanent magnet motor, which will lead to speed fluctuations. Torque pulsation will also cause vibration and noise in the motor. When the frequency of the pulsating torque is consistent with the resonant frequency of the armature current, resonance will occur, which will inevitably amplify the vibration and noise of the cogging torque. It seriously affects the positioning accuracy and servo performance of the motor, especially at low speeds.
Mechanism of cogging torque generation:
Cogging torque is the circumferential torque generated by the interaction between the permanent magnet and the armature core when the armature winding is not energized. This torque changes with the change of the rotor position, so it is a pulsating torque. It is generated from the resultant tangential force between the permanent magnet and the stator teeth. The tangential force always tries to align the axis of the permanent magnet magnetic field with the axis of the stator teeth, so that the rotor tends to be positioned at a certain position. It can also be simply understood that when the magnetic resistance of the magnetic circuit is unevenly distributed, the magnetic lines of force will always close along the magnetic circuit with the smallest magnetic resistance, that is, the "principle of minimum magnetic resistance of the magnetic circuit".
From the energy point of view, the cogging torque is caused by the change of magnetic field energy generated by the permanent magnet, and the magnetic field energy is related to the angle of the rotor. Due to the complexity of the magnetic field energy expression, the analytical solution of the cogging torque obtained by the partial differential of the magnetic field energy must be complex. All the methods mentioned in the article to weaken the cogging torque are to reduce the fluctuation of the partial differential of the magnetic field energy.
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Cogging torque reduction method
Scholars have conducted a lot of research on weakening the cogging torque. The methods can be summarized into three categories:
1. Using fractional slot winding
Considering the relationship between the combination of slot number Z and pole number 2p and the cogging torque, it is generally believed that the larger the number of fundamental wave cogging torque cycles, the smaller its amplitude, so the combination of stator slot number Z and rotor pole number 2p with a larger least common multiple should be selected.
The principle that the use of fractional slot winding motor is beneficial to reducing cogging torque is that the magnetic field positions of each slot of its stator are different, so the cogging torque phases generated by each are different. The superposition result not only increases the number of cycles of the fundamental wave cogging torque, but also may produce mutual compensation. The number and position of tooth slots under each magnetic pole of the integer slot winding motor are the same, and the cogging torque generated under all poles is the same phase. The cogging torque of 2p poles is superimposed to greatly increase the total cogging torque.
Santroll has simulated the same 9-slot stator punching, and the rotor is 6-pole and 8-pole. Both schemes are fractional slot motors, but the cogging torque is also very different because their least common multiples of Z and 2p are 18 and 72 respectively. The peak cogging torque of the 9-slot 6-pole motor is 30mNm, while the cogging torque of the 9-slot 8-pole motor is 2mNm.