In recent years, with the development of motor control technology, the motor direct torque control technology, which uses the stator flux linkage as the control object, has attracted people's attention. The basic idea of ​​direct torque control (DTC) is to control the stator flux and the electromagnetic torque of the motor at the same time. Unlike ordinary vector control, there is no current loop in the closed-loop direct torque control. Since direct torque control does not require rotation 3/2 conversion as does vector control, the control algorithm is greatly simplified compared to vector control. For general direct torque control, the inverter switching state is selected by looking up the switching table, so it does not need to perform pulse width modulation but also can guarantee the rapid response of the torque, and it can also be easily obtained. The output voltage of each phase. Moreover, for the direct torque control, in the high-speed operation stage, other parameters of the motor are not required except for 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 for asynchronous motors, and proposes corresponding solutions for several problems encountered.
2 Direct torque control principle In the stationary two-phase coordinate system (its straight shaft a axis is on the stator A-phase axis), the calculation direction of the stator flux 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 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 link between the stator flux and the rotor flux, which makes the rotor flux vector remain basically unchanged when the stator flux changes. Just changing the spatial position of the stator flux vector can easily change the angle S between stator and rotor flux linkages. It can be seen from equation (4) that the electromagnetic torque of the motor can be easily changed. From equation (1), it can be seen that if the pressure drop of the stator resistance is ignored, the stator flux moves in the direction of the voltage vector. Therefore, by reasonably controlling the stator voltage vector, not only the magnitude of stator flux amplitude can be controlled, but also the angle between stator and rotor flux linkage can be controlled to directly control the torque without the need to control the stator as in vector control. The current controls the torque indirectly.
3 direct torque control system constitutes direct torque control asynchronous motor speed control system as shown. The detected motor speed n is compared with the given value n*, and the torque command signal Te* is generated by the PID regulator. The DC bus voltage Udc and the phase currents ia, ib are detected and after the stationary 3/2 conversion, two phases are obtained. The straight and quadrature components in the stationary coordinate system are ua, ia and u (3, ie. The stator flux and electromagnetic torque are obtained from equations (1) and (3), respectively), and the stator flux position angle The equation (5) is used to obtain the obtained stator flux and torque estimates and corresponding values ​​given by the hysteresis loops HCI and HC2, and the corresponding logic signals are output to the switch table together with the stator flux position angles. To determine the switching state of the switching device on the corresponding bridge arm.
For the direct torque control system with stator flux linkage as control object, whether the stator flux calculation is correct or not is directly related to the performance of the system. For the formula shown in equation (1), since it has a large gain for DC offset, it is very sensitive to the DC component in the integral signal. In the actual system, it is easy to reach the integral saturation due to the influence of the DC bias quantity and lose its effect. Therefore, in the actual system, formula (1) changes as follows: Cutoff frequency q: cannot be obtained too little, otherwise the suppression effect on DC offset quantity is too weak. , but too large will cause too much phase deviation and amplitude deviation, generally taken as 50 ~ 70rad / s. So at high frequencies, the phase shift caused by the two little difference. However, due to the large phase error at low speeds, the estimation of stator flux is not accurate.
In order to solve the problem of stator flux calculation at low speed, a flux linkage model switching method is adopted in this system. That is, the stator flux linkage U"/model shown in equation (6) is used at high speed, and used at low speeds. Stator flux linkage/-N model shown in equation (7). Beijing: National Defense Industry Press, 1989. Zhou Yi, Chen Hong, Wan Shuzheng. Model switching in direct torque control. Ordnance Automation, 1998 ( 3): 1~4.
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