Variable-Speed a.c. Drives with Slip-Ring Induction Machines and a Resistively Loaded Force Commutated Rotor Chopper

The paper deals with variable-speed drives using an induction machine with wound rotor, controlled by variation of an external rotor resistor by a parallel electronic chopper. After tracing the historical development of this type of controlled drive, the power-flow relationships are investigated to arrive at the fundamental characteristics of this type of system. Considerations leading to the selection of a variable-slip system with rotor control rather than stator control are covered, leading to a discussion of the structure for a simple variable-speed drive using a force-commutated rotor chopper. The system is modelled by idealising the power switches and analysing their behaviour as function of slip, and the switched induction machine is modelled by a torque directly proportional to chopper input current. A second-order total system model is arrived at by taking only a speed-loop time constant and a mechanical time constant into account. Particular attention is given to establishing design relationships for this kind of drive. Component ratings, rotor resistor value and commutation requirements are related to operational requirements such as maximum slip, minimum torque, rated current and minimum expected current. No detail of power-chopper design is given. Some practical characteristics of typical variable-speed drives in the 11 kW to 22 kW range arc presented to illustrate the validity of the approach taken in analysis and design. These results indicate that the simple analysis presented is adequate for understanding and predicting the steady-state behaviour of the system. The second-order model gives sufficient accuracy in predicting dynamic behaviour to be useful for the design of systems in the response range normally applicable to this type of drive in practice. When designing systems for fast dynamic response, the modelling will have to be improved.