Extended data
First, torque control
Direct torque control (DTC) is a method to control the torque of three-phase motor by frequency converter. The method is to calculate the estimated values of motor flux linkage and torque according to the measured motor voltage and current, and then control the motor speed after controlling the torque.
In direct torque control, the stator voltage is integrated. The torque is estimated by the estimated stator flux linkage and the measured current vector inner product. Flux and torque will be compared with reference values. If the error between the magnetic flux or torque and the reference value exceeds the allowable value, the power crystal in the frequency converter will be switched to reduce the error of the magnetic flux or torque as soon as possible. Therefore, direct torque control can also be regarded as a kind of hysteresis or relay control.
Second, the principle of torque control
In direct torque control, the magnitude of the stator flux linkage of the motor is kept at the rated value by vector control of the above voltage, and the torque can be changed by controlling the included angle between the stator flux linkage and the rotor flux linkage. The included angle can be adjusted by controlling the voltage space vector. Because the rotating speed of rotor flux linkage remains unchanged, the angle can be adjusted by adjusting the instantaneous rotating speed of stator flux linkage.
Assuming that the motor rotor rotates counterclockwise, if the actual torque is less than a given value, the voltage vector is selected to make the stator flux rotate counterclockwise, so that the angle increases and the actual torque increases. Once the actual torque is high and reaches a given value, the voltage vector is selected to make the stator flux rotate in the opposite direction. Resulting in a reduction of the angle. By selecting the voltage vector in this way, the stator flux keeps rotating, and its rotating direction is determined by the torque hysteresis controller.
The torque and flux control of direct torque control is realized by hysteresis comparator. The working principle of the hysteresis comparator is that when the error between the current value and the given value is within the tolerance range of the hysteresis comparator, the output of the comparator remains unchanged, and once it exceeds this range, the hysteresis comparator will give the corresponding value.
Comparing the given rotational speed with the estimated rotational speed to obtain a given torque; The torque regulator performs hysteresis processing on the torque difference to obtain a torque control signal; Comparing the estimated flux with a given flux, and obtaining a flux control signal through a hysteresis comparator; Dividing sections according to the calculated rotor displacement; According to the cross section, torque and flux control signals, the space vector is obtained by combining with the look-up table to generate PWM wave; Output to inverter and power supply to motor.