17 Electric Motors • Fan losses occur due to the air resistance of the motor fan • Friction losses occur in the ball bearings holding the rotor When determining the effi ciency and motor output power, the losses in the motor are normally subtracted from the supplied power. The supplied power is measured, whereas the losses are often calculated or determined experimentally. 1.3 Asynchronous Motors To understand clearly how an adjustable speed drive system works, it is necessary to understand the principles of operation of this type of motor.
Although the basic design has not changed much in the last decades, modern insulation materials, computer- based design optimisation techniques as well as automated manufacturing methods have resulted in lower cost per kilowatt power and higher effi ciency for a given motor size. The information in this book will apply mainly to the so-called “squirrel-cage“ three-phase asynchronous motor, which is the type commonly used with FC’s. 1.3.1 Rotating Field When applying a multi-phase AC source (typically three-phase) to a suitable winding system, a rotary magnetic fi eld is generated which rotates in the air gap between the stator and the rotor.
If one of the phase windings is connected to a supply phase, a magnetic fi eld is induced. Fig. 1.7 One phase produces an alternating fi eld