The operating characteristics and basis of control of a variable-speed induction-motor drive with load-commutated inverter

The authors present an operating chart for the drive which shows the electrical conditions over the full range of operation expressed in terms of speed and torque and describe the control of the drive in the light of the chart and governing equations. Operation over the full range of speed involves both natural and forced commutation of the inverter. Natural commutation can only be used when the load supplied by the inverter has a sufficiently leading power factor. This changes towards a lagging value as speed is reduced and at some point, dependent on load conditions, a switch to forced commutation has to be made. The development of the operating chart is based on a consideration of the fundamental components of the voltage and current at the output terminals of the inverter