Permanent magnet brushless motor with the electromagnetic factors, the influence of tooth, current commutation, the armature reaction will produce strong pulsating torque. In the design of motor and the corresponding control system should be seriously considered, take measures to avoid excessive torque ripple.
2. 1 electromagnetic torque ripple caused by the factors of
electromagnetic torque ripple is due to the interaction between stator current and rotor magnetic field and the torque ripple, it with the current waveform, the back emf waveform, the air gap magnetic flux density is directly related to the distribution. Ideally, the stator current for the square wave, the back emf waveform as the trapezoidal wave, flat top width is 120 & deg; Electric point of view, the electromagnetic torque is constant value. The actual motor, the design and manufacture of reasons, may make the back emf waveform is not a trapezoidal wave, wave crest width or not for 120 & deg; Electric point of view, this will cause the torque pulsation of the motor.
2. 2 the cogging torque ripple caused by the
as a result of the existence of groove teeth of the stator core, makes the permanent magnet with the corresponding armature surface uneven air gap permeance, when the rotor rotates, made in a state of magnetic, magnetic circuit magnetic resistance changes, that cause torque ripple. The cogging torque ripple caused by the rotor is magnetic fields interact to produce, has nothing to do with the stator current. Thus restrained due to the cogging torque ripple mainly focused on the optimization of motor design, such as chute.
2. 3 current commutation torque ripple caused by the
figure 1 as the block diagram of the control system of electric motor, controller working in two conduction state. Every 60 & deg; Electric Angle MOSFET switch phase at a time. If the current for Q1 and Q5 conduction, after 60 & deg; Electric Angle, Q1 and Q6 conduction. In Q1, Q5 conduction period, the current through the coil of AB, after commutation current flows through the coil of AC. As a result of the motor coil inductance, in the process of switching, B to index the decrease of phase current and phase C current will rise exponentially. When the Q5 clearance patrol, AB phase coil current through Q1 & rarr; AB phase coil & rarr; Q2 body diode of afterflow, AB phase coil current soon decay to zero, but the current of AC phase requires a relatively long time to rise to the front of the phase size. Therefore a larger motor current pulse. As shown in figure 2 a the CH3. The current pulse has reached 12 a. During the phase of the electromagnetic torque is:
with Te for motor electromagnetic torque, ea, eb, ec for phase winding electromotive force, ia, ib, IC for phase winding current.
due to the commutation time is very short, can approximate think ebaeca, don't change, in the area of the commutation torque is proportional to the current relationship, therefore, the current fluctuation directly led to the motor torque fluctuation. In under the condition of low speed high load operation, the motor torque ripple.
in brushless dc permanent magnet motor torque ripple reasons, the former two main by optimizing the design of motor to achieve a goal, for the third kind of torque ripple, we can through the current compensation method to reduce the motor in the process of commutation torque ripple. This article focuses on the method.
