Problem solving

Brushless DC motor, the same number of poles, rotating by hand without power
Release Date : 2019-03-08 09:08:32

First, caused by cogging torque, the magnetic circuit is well done, as follows:

Cogging torque is an inherent phenomenon of a permanent magnet motor. It is a torque generated by a magnetic field generated by a permanent magnet and a tooth groove of an armature core acting in a circumferential direction in a state where the armature winding is not energized. It arises from 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, and thus the trend An oscillating torque produced.

The armature core of the brushless DC motor must have teeth and slots for the stator windings. Due to the existence of the slots, the air gap is not uniform. The magnetic flux in one pitch is relatively concentrated on the teeth, so that the air gap is not constant. . When the rotor rotates, the energy storage of the air gap magnetic field changes, and the cogging torque is generated. This torque is constant, which is related to the rotor position, and thus causes torque ripple as the rotor position changes [2] ]. It is related to the structural size of the rotor, the structure of the stator slot, the size of the air gap, the shape of the magnetic pole and the magnetic field distribution, etc., regardless of how the windings are placed in the slots and how much current is fed into the windings of the phases.

The cogging torque causes the motor torque to fluctuate, generating vibration and noise, and the speed fluctuation occurs, so that the motor cannot run smoothly and affects the performance of the motor. At the same time, the motor produces undesired vibration and noise. 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.

Second, different methods of weakening and comparative analysis

Oblique or inclined pole: Stator chute or rotor ramp is one of the most effective and widely used methods for suppressing cogging torque ripple. This method is mainly used for motors with a large number of stator slots and long axial length [3] . Practice has proved that the chute makes the amplitude of each harmonic of the electromagnetic torque of the motor reduced. The chordalization of the winding back electromotive force caused by the chute or the oblique pole will increase the electromagnetic torque ripple. The oblique pole is rarely used in engineering due to complicated processing and high material cost.

Magnetic pole block shifting: Since the rotor pole pole will greatly increase the cost and the processing process will become complicated, the magnetic pole block shifting method is often used in the application, and the magnetic pole arc coefficient is obtained by calculation, and then it is taken. Optimization, finally, several sections of the magnetic steel are placed at a certain angle in the circumferential direction to approximate the equivalent of a continuous magnetic pole [4]. There are usually two kinds of displacement methods: continuous displacement and cross shift, the former eliminates All the cogging torque harmonic components except the integer multiple of the number of magnetic steel blocks, the latter can only eliminate the odd harmonics of the cogging torque, and have no effect on the even harmonics.

Fractional groove method: This method can increase the frequency of the cogging torque fundamental wave and significantly reduce the cogging torque ripple. However, after the fractional slot is used, the distribution of the windings of the poles is asymmetrical, so that the effective torque component of the motor is partially cancelled, and the average torque of the motor is correspondingly reduced [5].

Magnetic groove wedge method: The magnetic groove wedge method is to apply a layer of magnetic groove mud on the stator slot of the motor, and form a groove wedge with certain magnetic permeability after solidification. The magnetic wedge reduces the influence of the opening of the stator slot, making the air gap flux between the stator and the rotor more uniform, thus reducing the torque ripple caused by the cogging effect [6]. Since the magnetic permeability of the magnetic wedge material is not very good, the degree of weakening of the torque ripple is limited.

Closed groove method: the stator slot is not open, the notch material is the same as the tooth material, and the magnetic permeability of the notch is better, so the closed groove can eliminate the torque ripple more effectively than the magnetic groove wedge [7]. However, the use of closed slots brings great inconvenience to the winding rules, and also greatly increases the slot leakage resistance and increases the time constant of the circuit, thereby affecting the dynamic characteristics of the motor control system. It is also possible to reduce the cogging torque by reducing the slot width, but the reduction of the slot width can weaken the cogging torque, but it brings difficulties to the winding down process, and also increases the leakage flux, which ultimately affects the motor output. 

Optimized magnetic steel design: the motor air gap magnetic field and back EMF waveform are closer to sine wave in parallel magnetization, parallel magnetization has less influence on torque ripple; the larger the motor pole pair, the larger the torque ripple; the motor pole arc The larger the coefficient, the smaller the torque ripple [8].

Slotless winding: Cogging torque is essentially the interaction of the magnetomotive force generated by the permanent magnet with the magnetoresistance change due to the slotting of the stator, so the most thorough and simple method is to use a slotless Winding structure. The slotless structure was applied to DC motors as early as the mid-1970s. The armature windings were attached to the smooth rotor surface, and were also made into moving coils or printed windings of disc motors (printed). Circuit winding), no matter what form the thickness of the armature winding is always a part of the actual air gap, the actual equivalent air gap of the slotless motor is much larger than that of the slotted motor, and the required field magnetism is also To be much larger, this limited the capacity and development of slotless motors in the early days. In recent years, with the rapid development of high magnetic energy products such as NeFeB, the rapid development of the non-grooved permanent magnet Rl machine provides an opportunity. The slotless windings currently used in permanent magnet brushless DC motors can be classified into three main categories: ring windings, non-overlapping concentrated windings, and cup windings.

Auxiliary groove method: The purpose of adding an auxiliary groove is to reduce the main harmonic component, and at the same time, the auxiliary groove itself generates harmonics. When the harmonic generated by the auxiliary groove changes in phase with the harmonic generated by the original stator, The positioning torque will be increased; otherwise, the positioning torque will be lowered, and the angle between the center line of the auxiliary groove and the center line of the stator punch determines whether the two are in phase or in opposite phase. The harmonic generated by the auxiliary groove will cancel the P harmonic of the original harmful harmonic component. The same punch adds two auxiliary grooves at the symmetrical position to cancel the harmonic component, and the angle is appropriate. The choice, the reduction of the opening position of the punching hole can reduce the energy change. On the same punch, the auxiliary grooves can be arranged in a symmetrical position to achieve better results.