Year

1987

Degree Name

Doctor of Philosophy

Department

Department of Electrical and Computer Engineering

Abstract

Single-Frame Cascaded Induction Machine (SFCIM) is a brushless single-unit version of the well known cascade connected two-machine system. Applications of the SFCIM include, slip-power recovery for speed control; operation as a high power stepping motor; and generation of constant frequency in both aircraft and wind energy applications.

A new steady-state circuit theory for the SFCIM having a 'multicircuit single-layer bar rotor winding' is described. Assumptions made in the development of the new model are no different to those made in the analysis previously existing and yet the new theoretical model is relatively simple in comparison. It is based on the analysis of coupled electric circuits having time-varying coefficients and the complete machine is represented by a coupling impedance matrix having elements which are simple to calculate. The experimental results are compared with the theoretical results calculated using the new model. It is shown that this model also can be applied to SFCIMs having phase-wound rotors.

It is hypothesised that an SFCIM having a phase-wound rotor winding can be represented by the series-connected equivalent circuits of two conventional induction machines. The no-load and locked-rotor tests performed on conventional induction machines cannot be applied in this case because of the extra loop present in the equivalent circuit. Techniques suitable for the experimental determination of its parameters are described and the experimentally determined equivalent circuit is compared with the theoretically calculated equivalent circuit. Performance characteristics predicted using these two circuits are compared with the experimentally observed characteristics.

Some theoretical aspects of iron losses in an SFCIM are considered which are supplemented by experimental results. The component of iron losses considered in the theoretical work is only due to the two fundamental flux density waves. In the asynchronous operation of the SFCIM as a motor, it is shown that the supply of the large pole pair stator side at constant frequency, while allowing the small pole pair stator side to carry the slip frequency currents, leads to lower iron losses.

The wide ratio pole combination of 1 and 3 gives the highest possible cascade synchronous speed for an SFCIM. In this case the 'multicircuit single-layer bar rotor winding' is unsatisfactory. The phase-wound rotor windings are better suited as their design is more flexible. Some design aspects of these windings are considered, giving emphasis to optimisation of certain performance characteristics. It is shown that the minimisation of harmonic leakage reactance of the phase-wound rotor winding plays an important role in the design.

As a brushless generator the SFCIM is suitable for constant-frequency variable-speed generation in aircraft and wind energy applications. Steady-state, experimental and theoretical characteristics of operation, of the SFCIM, as a variable-speed constant-frequency generator are presented. The theoretical analysis of this mode of operation is carried out using the new steady-state circuit theory presented.

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