INTRODUCTION

Judged in terms of fitness for purpose coupled with simplicity, the induction motor must rank alongside the screwthread as one of mankind's best inventions. It is not only supremely elegant as an electromechanical energy converter, but is also by far the most important, with something like one-third of all the electricity generated being converted back to mechanical energy in induction motors. Despite playing a key role in industrial society, it remains largely unnoticed because of its workaday role in unglamorous surroundings driving machinery, pumps, fans, compressors, conveyors, hoists and a host of other routine but vital tasks. It will doubtless continue to dominate these Wxed-speed applications, but thanks to the availability of reliable variable-frequency inverters, it is now also the leader in the controlled-speed arena.

Like the d.c. motor, the induction motor develops torque by the interaction of axial currents on the rotor and a radial magnetic Weld produced by the stator. But, whereas, in the d.c. motor the work current has to be fed into the rotor by means of brushes and a commutator, the torque-producing currents in the rotor of the induction motor are induced by electromagnetic action, hence the name induction motor. The stator winding therefore not only produces the magnetic Weld (the excitation ), but also supplies the energy that is converted to mechanical output. The absence of any sliding mechanical contacts and the consequent saving in terms of maintenance is a major advantage of the induction motor over its d.c. rival.

Other differences between the...