Research on PWM modulation mode of permanent magnet brushless DC motor yuan Fei, realgar Shenghua, Li langru (Huazhong University of science and technology, Wuhan, Hubei Province, the conclusion of minimum current fluctuation and torque ripple. An energy feedback braking method is proposed to realize the energy feedback during motor braking.

1 brushless DC motor model 2 Influence of PWM modulation mode on current when ignoring the salient pole effect, the self inductance and mutual inductance of stator three-phase winding are constant and independent of rotor position. At this time, taking GND as the potential, the model established by the three-phase variables of the stator is as follows: for the permanent magnet brushless DC motor, when the 120 * conduction mode is adopted, each cycle is composed of 6 sectors, each sector accounts for 60 * each switching element is turned on 120 *, that is, it is turned on in two consecutive sectors. There are three different PWM modulation modes: upper half bridge carrier Under the lower half bridge carrier and the full bridge carrier, take the permanent magnet brushless motor with trapezoidal back EMF and flat top width of 120 * as an example to illustrate the influence of different carrier modes on armature current. The back EMF waveform is shown in. Assuming that the motor is just commutated from sector 1-2 to sector 2-3 at this time, the following analysis is carried out in this interval.

The main circuit structure based on the above equivalent model is shown in Figure 2.1. The voltage equation of the upper half bridge carrier mode is: at this time, the neutral point voltage is: UN = K / 2 UD 0, when T3 is on, there is the following voltage equation: / 2 when T3 carrier is on, in Ti ~ T2 time, due to E > > 0, phase a will not continue to flow through D1 at this time; In T2 ~ T3 time, because EA < 0 at this time, when T3 is cut off, UN = 0, UA = e + UN < 0, so when T3 is cut off, phase a will continue to flow through D4. Because the amplitude of EA is larger and larger, the current value is larger and larger, and the frequency of current fluctuation is the same as that of T3 carrier frequency. During the operation of the motor, the cut-off phase is also connected with continuous current, resulting in the three-phase conduction state of the motor at the same time, which will cause large fluctuations in the current in the armature winding of the motor. At this time, the current of phase A and C is consistent with the direction of the marked current, while the current of phase B flows out of phase B and flows into phase A and phase C respectively, Therefore, the current fluctuation of phase B is also large at this time. It can be seen that if the current direction marked in the middle is positive during the upper half bridge carrier, the forward current fluctuation of each winding is large. Similarly, in sector 5-6, when T5 carrier, phase a winding passes through D4 freewheeling in T4 ~ T5 time; During T5 ~ T6 time, there will be no current flowing through phase a winding. In the 2.2 lower half bridge carrier mode, during T2 PWM chopping, the voltage equation of two-phase terminals B and C is as follows: at this time, the neutral point voltage is: UN = m, taking this modulation mode. In T1 ~ T2 time of sector 2-3, because EA > 0, when T2 is cut off, a is connected to D1 freewheeling; In T2 ~ b time, it will not correspond to the upper half bridge carrier through D4 freewheeling, and the negative current fluctuates greatly. 2.3 when PWM chopping is performed simultaneously in T2 and T3 full bridge carrier mode, the voltage equation of two-phase terminals B and C is as follows: the neutral point voltage can be obtained: UN = within 3 sectors, due to 0 summary: the above three PWM modulation methods, whether upper half bridge chopping or lower half bridge chopping, The cut-off phase will generate freewheeling, resulting in the fluctuation of the other two-phase currents. The frequency of current fluctuation is consistent with the chopping frequency, and the higher the motor speed is, the greater the current fluctuation is. If the full bridge chopping is adopted, the two phases are always connected, the cut-off phase will not generate freewheeling, and the neutral point voltage of the motor will not change during the operation of the motor. The current fluctuation is small, and the torque ripple is also small. Although under the same bandwidth and operating conditions, the chopping frequency of the full bridge chopper is much higher than the first two chopping modes, and the loss is also large, the full bridge chopper is still a better chopping mode. 3 feedback braking method and its principle. As a permanent magnet brushless DC motor of the propulsion system, in addition to the four quadrant stable operation, When braking, the energy should be fed back as much as possible to increase the service time of the battery. Different from the electric working condition, among the six switching elements of the inverter bridge, the upper half of the bridge elements are always cut off, and the three elements of the lower half of the bridge are PWM modulated, so as to form a boost chopper circuit. Take the 1-2 sector in the as an example. At this time, T4 is PWM modulated, and all other switching elements are in the cut-off state, For example, in the equivalent circuit diagram of feedback braking, in a chopping cycle, when T4 is on, due to the action of EA and EC, current IA is generated between two-phase windings a and C and T4 and D2. When T4 is off, current ia charges the battery through D1. At this time, the current direction in the armature winding is opposite to that in the electric case. It can be seen that the electromagnetic power is negative, the electromagnetic torque is the braking torque, and the battery is the absorbed power. The essence of the motor converting the mechanical energy into electrical energy and feeding it back to the battery for feedback braking is that when the T4 tube is connected, the mechanical energy of the electric machine is converted into magnetic field energy and stored in the motor winding; When T4 is cut off, the mechanical energy of the motor and the magnetic field energy stored in the motor winding are converted into electrical energy, and the energy is fed back to the battery through the action of inductive boost chopper. Because the armature current direction is opposite to the back EMF direction, the motor sends out power and obtains the braking torque, so as to realize the braking of the motor.

4 computer simulation analysis a permanent magnet brushless DC motor is simulated with different PWM modulation modes. The given current is 20a, power supply voltage UD = 400V, r = 0.02, r = 0.05 IMH under the working conditions of the upper half bridge, the lower half bridge and the full bridge carrier, the current waveforms are as follows. It can be seen from the three figures that under the upper half bridge or the lower half bridge carrier mode, due to the freewheeling effect of the cut-off phase, the current of the normal conduction phase of the motor fluctuates greatly every 60 *. When the upper half bridge carrier wave, the current flowing into the phase winding fluctuates greatly; In the case of the lower half bridge carrier, the current fluctuation is larger, and in the case of the full bridge carrier, the current fluctuation is obviously smaller than the first two methods, so the pulse change fluctuation is much smaller, which proves that the full bridge carrier in front has a smaller fluctuating current in the electric state. It is an ideal carrier method. The braking current is set to 30a, and the motor runs normally when starting After 5 seconds, it will enter the braking state immediately, which is the waveform of phase a current, torque and speed from starting to braking. Among them, the PWM modulation mode is the lower half bridge carrier during electric operation. After 0.5 seconds from static operation, it will turn into braking state.

The simulation waveform of torque and speed can be seen from the figure. When the motor turns into the braking state, the phase a current changes immediately, which is opposite to the current direction of the electric state, so the torque becomes negative, making the motor brake quickly.