In the paper, a novel technique for the direct torque
control (DTC) of an induction motor is proposed, which overcomes
the trouble of high torque ripple afflicting the conventional DTC
technique. With the novel technique, the inverter voltage vector
selected from the switching table is applied for the time interval
needed by the torque to reach the upper (or the lower) limit of the
band, where the time interval is calculated from a suitable modeling
of the torque dynamics. By this approach, the control system
emulates the operation of a torque hysteresis controller of analog
type since the application time of the inverter voltage vector is dictated
by the allowed torque excursion and not by the sampling period.
It is shown by experimental results that the technique yields a
considerable reduction of the torque ripple. A further and ultimate
reduction is obtained by compensating for the delay inherent in the
discrete-time operation of the control system. The outcome is that
the torque ripple of the motor is constrained within the hysteresis
band of the torque controller, for a band of customary value. An
ancillary merit of the technique is the almost full elimination of the
average torque error inherent in the conventional technique. If the
hysteresis band is shrunk, the torque ripple is bound to swing out
the band limits. Under this circumstance, an extension of the technique
is developed, which helps keep the torque ripple at minimum.
To assess the characteristics of the proposed DTC technique, the
following quantities: average torque error, rms value of the torque
ripple, and inverter switching frequency are measured for different
stator flux angular speeds and hysteresis bands of the torque and
flux controllers. As a comparison, the same quantities are given for
the conventional DTC technique.
