Chapter 9 - Direct Torque and Flux Control (DTFC) of ac Drives

Abstract

This chapter introduces the DTFC of various ac motor drives. Direct torque and stator flux control (through hysteresis controllers and direct triggering of voltage source inverter voltage vectors, for given stator flux position) shows fast response in torque and notable robustness to machine parameter detuning. At low speeds, more elaborate flux and torque SVM controllers provide low torque pulsations and lower current and flux tipples shown through two rather complete case studies on the IM and RSM, respectively. The case studies reveal very good performance over a wide range of speeds and loads. Flux/torque/speed coordination is discussed in some depth according to various optimization criteria both for IMs and various SMs. Notable attentions to flux/torque/speed/position observers led to some unitary configuration similar for both IMs and SMs. The chapter also discusses about large SMs. The DTFC for the CSI-SM drives poses special problems as it handles current vectors instead of voltage vectors, and the sub-transient flux rather than stator flux variable is controlled. As of now DTFC IM drives are on the market from 2–2000 kW units. The DTFC SM drives are still in the laboratory stages, but they might reach the market soon. Given the DTFC definite merits, even when compared with very advanced vector control ac drives, and their high degree of generality, real universal DTFC ac drives are expected to be used for wide ranges of speed and power.