Direct Torque Control (DTC) is the latest AC motor control method developed by ABB. In Direct Torque Control all switch changes are based directly on the electromagnetic state of the motor. Optimum switching is determined for every control cycle on a timelevel of 25 microseconds. That is also the main difference between Direct Torque Control and the traditional AC drive control methods. In DTC, there is no seperate voltage- and frequency controlled PWM modulator. The switch positions of the power module are determined by a calculated stator flux and motor torque. The used selection of the switches forces the stator flux in the direction where the reference values of the torque and the stator flux are achieved. The calculation of the stator flux is based on an accurate motor model and the measurement of the motor input voltage and current. The measurement of the shaft speed is not needed. The reference value of the torque controller either comes from speed controller or is an external reference. The torque reference is modified internally in order to keep the dc intermediate voltage limited and the motor torqie and the inverter current in the allowed range. Direct Torque Control offers excellent dynamic performance, with torque step rise timetypically better than 2 ms. Accurate torque control can also be provided at low frequencies, including zero speed. DTC is realized by using Digital Signal Processors and ASIC technology.
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