Direct Torque Control (DTC) of Frequency Converters (ac drives, frequency changers) (part 2)
Continued from the Direct Torque Control (DTC) of Frequency Converters (ac drives, frequency changers) (part 1) , below we still introduce this advanced control technique of frequency inverters (ac drives, variable speed drives, VSDs).
The heart of direct torque control (DTC) is its adaptive motor model. This model is based on the mathematical expressions of basic motor theory. The adaptive motor model of DTC requires information about the various motor parameters, like stator resistance, mutual inductance, saturation coefficiency, etc.
The algorithm captures all these details at the start from the motor without rotating the motor. But rotating the motor for a few seconds helps in the tuning of the model. The better the tuning, the higher the accuracy of speed and torque control.
With the direct current (DC) bus voltage, the line currents and the present switch position as inputs, the model calculates actual flux and torque of the motor. These values are fed to two-level comparators of the torque and flux, respectively. The output of these comparators is the torque and flux reference signals for the optimal switch selection table. Selected switch position is given to the inverter without any modulation, which means faster response time.
(To be continued)
