The switching characteristics of an inverter feeding
an induction motor controlled with the direct torque control (DTC)
technique are assessed in steady state. At first, the application share
of the inverter voltage vectors for the stator flux covering half a sextant
is defined and predicted. The prediction indicates that, under
operation at fixed inverter dc link voltage and stator flux magnitude,
the application share depends only on the supply frequency
of the motor and, to a small extent, on the load. Afterwards, the
inverter transitions and the corresponding phase commutations
within a stator flux sextant are analyzed. The outcome of the analysis
permits to compute the commutations of the inverter phases in
one turn of the stator flux and, from them, the inverter switching
frequency is obtained. Its value is influenced by the sampling interval
and the control delay arising from the microprocessor implementation
of DTC. For given sampling interval and control delay,
it is shown that the inverter switching frequency depends on the
same quantities as the application share of the inverter voltage vectors.
Acomparison with the switching characteristics of an inverter
controlled with the space vector modulation technique is carried
out. At last, the paper discusses the sensitivity of the switching frequency
of an inverter for DTC to the following quantities: inverter
dc link voltage, sampling interval and control delay. Throughout
the paper simulation and experimental results are given to confirm
the theoretical findings.
