Direct torque control

Direct torque control (DTC) is one method used in variable-frequency drives to control the torque (and thus finally the speed) of three-phase AC electric motors. This involves calculating an estimate of the motor's magnetic flux and torque based on the measured voltage and current of the motor. Stator flux linkage is estimated by integrating the stator voltages. Torque is estimated as a cross product of estimated stator flux linkage vector and measured motor current vector. The estimated flux magnitude and torque are then compared with their reference values. If either the estimated flux or torque deviates too far from the reference tolerance, the transistors of the variable frequency drive are turned off and on in such a way that the flux and torque errors will return in their tolerant bands as fast as possible. Thus direct torque control is one form of the hysteresis or bang-bang control. Overview of key competing VFD control platforms: The properties of DTC can be characterized as follows: Torque and flux can be changed very fast by changing the references High efficiency & low losses - switching losses are minimized because the transistors are switched only when it is needed to keep torque and flux within their hysteresis bands The step response has no overshoot No dynamic coordinate transforms are needed, all calculations are done in stationary coordinate system No separate modulator is needed, the hysteresis control defines the switch control signals directly There are no PI current controllers. Thus no tuning of the control is required The switching frequency of the transistors is not constant. However, by controlling the width of the tolerance bands the average switching frequency can be kept roughly at its reference value. This also keeps the current and torque ripple small. Thus the torque and current ripple are of the same magnitude as with vector controlled drives with the same switching frequency. Due to the hysteresis control the switching process is random by nature. Thus there are no peaks in the current spectrum.