Abstract: According to the DC brushless motor torque characteristics, analysis of the pulse generated in the motor torque causes, and for the commutation torque ripple caused by current design has been optimized, in practical applications achieved good the effect.
Keywords: permanent magnet brushless DC motor torque ripple current compensation

1 Introduction

Permanent magnet brushless DC motor due to its simple structure, high reliability, low speed high torque characteristics have been more widely used, especially in recent years in the electric bicycle has been widely applied. As the mini electric scooter is that people's daily means of transport, so people start the vehicle stationary, noise and other indicators of the high demand. Most of the existing electric car motors permanent magnet brushless DC motor, the motor core structure for the straight groove, winding three-phase star connection, the inverter usually operates in twenty-two conduction state. As the straight-slot motor torque fluctuations at work, so we have optimized the structure of the motor and controller with optimized design in order to obtain satisfactory results. In this paper, how to design ultra-quiet DC brushless motor controllers are discussed.

2 DC brushless motor torque ripple analysis

As the electromagnetic brushless motor factors, the impact of alveolar current commutation, armature reaction will have a strong pulse of torque. In the design of the motor and the corresponding control system should be seriously considered, to take measures to avoid excessive torque ripple.

2.1 Electromagnetic torque ripple caused by factors

Electromagnetic torque ripple is due to interaction of the stator current and rotor magnetic field generated torque ripple, it is with the current waveform, back-EMF waveform, air-gap flux density distribution has a direct relationship. Ideally, the stator current of square wave, trapezoidal wave back-EMF waveform, flat-top width of 120 ° electrical angle, the electromagnetic torque is constant. The actual motor, the design and manufacture of reasons, may make back-EMF waveform is not a trapezoidal wave, or wave the top width of 120 ° electrical angle, this will cause the motor torque ripple.

2.2 cogging torque ripple caused by

As the teeth of the stator core slots there, making the permanent magnet armature and the corresponding air-gap permeance uneven surface, when the rotor rotates, making the state a magnet, the magnetic circuit reluctance to change, causing torque ripple. Cogging torque ripple is caused by the interaction of the magnetic field generated by the rotor, stator current has nothing to do with. Therefore, the inhibition caused by the cogging torque ripple focused on the optimization of motor design, such as the chute method.

2.3 Current commutation torque ripple caused by

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Figure 1: Brushless Motor Control System
Figure 1 shows the Utility Vehicle Manufacturer motor control system block diagram, the controller operates in twenty-two conduction state. Every 60 ° electrical angle for a phase MOSFET. If the current for Q1 and Q5 turns on, then after a 60 ° electrical angle, Q1 and Q6 are turned on. In Q1, Q5 conduction period, the current flowing through the coil AB, after commutation current flows through the AC coil. As the motor coil inductance in the switching process, B-phase current will decrease exponentially, C phase current will rise exponentially. When Q5 turns off, AB-phase coil of current through the coil Q1 → AB → Q2 relative to the body diode of freewheeling, AB-phase coil current decay to zero very quickly, but the AC phase current requires a relatively long time to rise to the change relative to the size of the former. Therefore, a larger electrical current pulse. Shown in Figure 2a in the CH3. One of the current pulse has reached 12A. Electromagnetic torque during commutation is:

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Where Te is the motor torque, ea, eb, ec for the phase winding electromotive force, ia, ib, ic for the phase winding current.

As the commutation time is very short, can be approximated that eba, eca in commutation region does not change, so the torque is proportional to the current relationship between the current fluctuations led directly to the motor torque fluctuations. In the case of low speed high load operation, the motor torque ripple is particularly evident.

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CH1: A phase voltage CH3: A phase current
Figure 2a: motor phase current pulse waveform

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CH1: A phase voltage CH3: A phase current
Figure 2b: After the motor phase current compensated
In permanent magnet brushless DC motor torque ripple causes, the first two mainly by optimizing the design of the motor to achieve the purpose, for the first three kinds of torque ripple, we can current compensation method to reduce motor commutation process the torque ripple. This article will focus on this approach.

3. Current compensation method reduces the motor torque ripple

As the low-speed high load operation, the rotation of the motor back EMF is very small, in order to make the motor phase current does not exceed the maximum allowed, PWM duty cycle is usually small, which makes for a new phase after phase winding current rise slowly. Figure 3 is a commutation of the motor current simulation waveforms. Motor inductance between the two phases Lm = 0.4mH, resistance Rm = 0.28 ohms (actual motor parameters). Using AOS (Bandai semiconductor) production AOT460 MOSFET simulation. MOSFET PSPICE model using level3 level. As can be seen from Figure 3, if the PWM duty cycle of 30%, the current from zero up to 30A takes about 1.3ms. This is the measured waveform in Figure 2a is similar. To make a rapid increase in current after the commutation, we can make after the commutation to 100% PWM duty cycle to compensate for the current until the current rises to the current value before commutation, this could make possible commutation of current fluctuations small, the time as short as possible. I2 from the simulation waveforms can be seen in the current up to 30A in less than 300us.

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Figure 3: After the motor commutation phase current waveform simulation
4. Control system design

How precise control for compensation after the current phase, that is, how precise control PWM100% duty cycle time is the key to controller design ultra-quiet! This requires the control of the MCU has the following characteristics:

1) fast A / D conversion rate could change in quick succession after the sampling phase;

2) PWM can trigger a specific time during the opening of A / D sampling; because the PWM inverter with inductive load control system, due to some parasitic systems in the PWM motor on and off during the emergence of the phase line ring (Figure 4), the ringing can couple to the A / D sampling of the loop, so we should avoid PWM switching process in the A / D sampling. If we opened during the mid-point of the PWM trigger current sample, we will get the average current, which will benefit our current compensation control.

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Figure 4: Current waveform sampling
In fact, the MCU is not difficult to find such, for example, dedicated motor control chip Infineon XC866, CYPRESS-chip programmable controller chip CY8C24533 and so on. XC866's A / D conversion rate can be achieved within 2us, coupled with the program execution time, an A / D conversion requires a total time 8us within such a time interval to determine the current compensation is complete enough. CYPRESS CY8C24533 is developed specifically for motor control with high-speed A / D chip, the switching speed can reach 3us SAR8 within, coupled with the automatic alignment of trigger A / D mode, you can trigger at any time in the PWM current A / D sampling We are also very easy to achieve precise control of current.

Figure 2b is the use of the system XC866 control chip after the above method optimized for the measured phase current waveforms can be seen from Figure 2b, for the basic phase of the current pulse to eliminate the motor phase current is close to a square wave. The control system with the electric car, start the motor speed and run-time shock has been basically eliminated, to achieve ultra-quiet design of the controller.

5 Conclusion

By appropriate choice of MCU can be triggered by high-speed A / D sampling of the permanent magnet brushless motor commutation current for accurate compensation, to eliminate the commutation current ripple, the effect of reducing the vibration motor.