During the last decade, a lot of modifications in classic Direct Torque Control scheme have been made. The objective of these modifications was to improve the start up of the motor, the operation in overload conditions and low speed region. The modifications also aimed to reduce the torque and current ripple, the noise level and to avoid the variable switching frequency by using switching methods with constant switching frequency.
The basic disadvantages of DTC scheme using hysteresis controllers are the variable switching frequency, the current and torque ripple. The movement of stator flux vector during the changes of cyclic sectors is responsible for creating notable edge oscillations of electromagnetic torque.
Another great issue is the implementation of hysteresis controllers which requires a high sampling frequency. When an hysteresis controller is implemented using a digital signal processor (DSP) its operation is quite different to the analogue one.
In the analogue operation the value of the electromagnetic torque and the magnitude of the stator flux are limited in the exact desirable hysteresis band. That means, the inverter can change state each time the torque or the flux magnitude are throwing the specified limits.
On the other way, the digital implementation uses specific sample time on which the magnitudes of torque and flux are checked to be in the desirable limits. That means, very often, torque and flux can be out of the desirable limits until the next sampling period. For this reason, an undesirable torque and flux ripple is occurred.
Many researchers are oriented to combine the principles of DTC with a constant switching frequency method for driving the inverter by using space vector modulation. This requires the calculation in the control schemes of the reference voltage vector which must be modulated in the inverter output.
Therefore, the Direct Torque Control with Space Vector Modulation method (DTC-SVM) is applied. Since we know the reference voltage vector it is easy to perform the modulation by applying specific switching pattern to the inverter.
In the DTC scheme a speed estimation and a torque control are applied using fuzzy logic. An improvement of DTC with a parallel control FOC is observed. The use of the rotor flux magnitude instead of the stator flux magnitude, improves the overload ability of the motor. This control is sensitive to the machine’s parameters during transient operations.
Source: Democritus University of Thrace
Authors: Adamidis Georgios | Zisis Koutsogiannis