16 Electric Motors P 1 Copper loss Iron loss Fan loss Friction loss P 2 Shaft output Fig. 1.5 Typical losses in the motor The losses in the motor illustrated in Fig. 1.5 Typical losses in the motor comprise: • Copper losses as a result of the resistances of the stator and rotor windings • Iron losses consisting of hysteresis losses and eddy-current losses Hysteresis losses occur when iron is magnetised by an alternating current (AC). The iron is magnetised and demagnetised repeatedly (that is, 100 times per second with a 50 Hz supply). Magnetising and demagnetising both require energy.
The motor supplies power to cover the hysteresis losses, which increase with frequency and the strength of magnetic induction. Eddy-current losses occur because the magnetic fi elds induce electric voltages in the iron core as in any other conductor (see Fig. 1.6 Eddy-currents are reduced by the laminated form of the motor core). These voltages produce currents that cause heat losses. The currents fl ow in circuits at right angles to the magnetic fi elds. The eddy-current losses are dramatically reduced by dividing the iron core into thin laminations. Fig. 1.6 Eddy-currents are reduced by the laminated form of the motor core