In recent years, with the development of motor control technology, the direct torque control technology of the motor with the stator flux linkage as the control object is receiving wide attention. The basic idea of ​​direct torque control (DTC) is to simultaneously control the stator flux linkage and electromagnetic torque of the motor. Unlike normal vector control, there is no current loop in the direct torque control loop. Since direct torque control does not require a rotary 3/2 transformation like vector control, the control algorithm is greatly simplified compared to vector control. For general direct torque control, the inverter switch state is selected by checking the switch table, so it does not need pulse width modulation to ensure fast torque response, and it can be easily obtained. Output voltage of each phase. Moreover, for direct torque control, in the high-speed running section, it is not necessary to know other parameters of the motor except the stator resistance of the motor, so the direct torque control is less dependent on the motor parameters than the vector control.
This paper discusses the direct torque control system of asynchronous motor, and proposes corresponding solutions to several problems encountered.
2 Direct Torque Control Principle In the static two-phase coordinate system (the straight axis a axis is on the stator A phase axis), the calculation of the stator flux linkage and electromagnetic torque of the asynchronous motor is power electronics and electric drive.
As follows: us, is stator voltage, stator current S, Rr stator, rotor resistance S, wr - stator flux and rotor flux r, Lm stator, rotor self-inductance and mutual inductance - leakage inductance, = ur motor speed Tr motor Rotor time constant, Tr = It can be seen from equation (2) that there is an inertia between the stator flux and the rotor flux, which makes the rotor flux vector remain essentially unchanged when the stator flux changes. As long as the spatial position of the stator flux vector is changed, the angle S between the stator and the rotor flux can be easily changed. According to equation (4), the electromagnetic torque of the motor can be easily changed. It can be seen from equation (1) that the stator flux linkage moves in the direction of the voltage vector if the voltage drop of the stator resistance is ignored. Therefore, by reasonably controlling the stator voltage vector, not only the magnitude of the stator flux linkage can be controlled, but also the angle between the stator and rotor flux linkages can be controlled to directly control the torque without controlling the stator as in vector control. Current to indirectly control torque.
3 Direct Torque Control System The schematic diagram of the direct torque control asynchronous motor speed control system is shown. Comparing the detected motor speed n with a given value n*, generating a torque command signal Te* via the PID regulator. The DC bus voltage Udc and the phase current ia, ib are detected and then subjected to a static 3/2 transformation to obtain two phases. The direct-axis component and the cross-axis component in the stationary coordinate system are ua, ia and u (3, ie. The stator flux linkage and electromagnetic torque are obtained by equations (1) and (3), respectively, and the position angle of the stator flux linkage is obtained. Then, it is obtained by the formula (5). The obtained stator flux linkage and the estimated value of the torque are compared with the corresponding given value via the hysteresis loop HCi, HC2, and the corresponding logic signal is output together with the stator flux linkage position angle to the switch table. To determine the switching state of the switching device on the corresponding bridge arm.
For the direct torque control system with the stator flux linkage as the control object, the correctness of the stator flux linkage calculation is directly related to the performance of the system. For the calculation formula shown in equation (1), since it has a large gain for the DC offset, it is very sensitive to the DC component in the integrated signal. In the actual system, it is easy to achieve integral saturation due to the influence of DC offset and thus lose its effect. Therefore, in the actual system, the equation (1) changes as follows: the cutoff frequency q: can not be obtained too little, otherwise the suppression effect on the DC offset is too weak. Too large will cause too much phase deviation and amplitude deviation, generally taken as 50 ~ 70rad / s. So at the higher frequency, the phase shift caused by the two is not much difference. However, at low speeds, the estimation of the stator flux linkage is not accurate due to the large phase error.
In order to solve the calculation problem of the stator flux linkage at low speed, a magnetic flux model switching method is adopted in the system, that is, the stator flux linkage U"/model shown in equation (6) is used at high speed, and at low speed, Stator flux linkage /-N model shown in equation (7)... Beijing: National Defense Industry Press, 1989. Zhou Wei, Chen Hong, Wan Shuzhen. Model switching in direct torque control. Ordnance Automation, 1998 ( 3): 1~4.
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