A novel direct torque control method for three-phase asynchronous motor

Introduction: The traditional direct torque control method of 6-sector voltage vector selection will lead to problems such as asymmetric flux control and large torque pulsation. This issue introduces a new three-phase asynchronous motor direct torque control method that subdivides the sector into 12 sectors. The simulation results show that the flux trajectory, speed and torque pulsation are significantly reduced, and the three-phase stator current waveform of the asynchronous motor is smoother and closer to a sine waveform.

1. Traditional direct torque control (two levels and six sectors)

Figure 1-1 Block diagram of asynchronous motor direct torque control

In actual operation of the motor, the stator flux amplitude is kept at the rated value to fully utilize the motor core; the rotor flux amplitude is determined by the load. The torque of the motor can be controlled by controlling the angle between the stator flux and the rotor flux, that is, the torque angle. In direct torque control, the basic control method is to control the rotation speed of the stator flux by selecting the voltage space vector, controlling the stator flux to start and stop, so as to change the average rotation speed of the stator flux, thereby changing the torque angle, so as to achieve the purpose of controlling the motor torque.

2. Improved Direct Torque Control

The traditional direct torque control method 6-sector voltage vector selection will lead to problems such as asymmetric flux control and large torque pulsation. This paper proposes a new three-phase asynchronous motor direct torque control method that subdivides the sector into 12 sectors. The vector selection table is shown in Table 1.

The sector subdivision method is one sector every 30°, with a total of 12 sectors. The flux angle of the first sector is -15°～15°, the second sector is 15°～45°, and each subsequent sector increases by 30°. This division method effectively avoids control failure caused by some voltage vectors being at the sector boundary. However, in order to reduce excessive current during the starting process and improve the flux trajectory, the flux and current are limited during the starting phase. When the motor just starts, in order to reduce the starting current, the current is limited, and the initial given speed is made zero, and the voltage vectors Us1 and Us7 are used to continuously increase the flux amplitude until the expected amplitude. After that, it operates normally according to direct torque control. Once the current reaches the upper limit of the current amplitude, Us7 is selected, and below the specified amplitude, the voltage vector is selected using the vector table.

3. Simulation model construction and waveform analysis

Figure 3

-1 Simulation of asynchronous motor direct torque control system (two-level 12 sectors)

Figure 3-2 Sector division

As can be seen from Figure (3-2), the sector is subdivided into 12 sectors in the simulation.

(a) 6 sectors

(b) 12 sectors

Figure 3-3 Simulation waveform changes of asynchronous motor direct torque control system (800r/min)

It can be seen from Figures (3-2) and (3-3) that the pulsation of the stator flux and torque based on the 12-sector DTC is relatively reduced.