18 Electric Motors The magnetic fi eld in the stator core has a fi xed location, but its direction varies, as shown in Fig. 1.7 One phase produces an alternating fi eld. The speed of rotation is determined by the supply frequency. At a frequency of 50 Hz, the fi eld changes direction 50 times per second. If two phase windings are connected to the respective supply phases, two magnetic fi elds are induced in the stator core. In a two-pole motor, one fi eld is displaced by 120 degrees relative to the other. The maximum fi eld values are also displaced in time, as shown in Fig. 1.8 Two phases produce an asymmetrical rotating fi eld. Fig.
1.8 Two phases produce an asymmetrical rotating fi eld This produces a rotating magnetic fi eld in the stator which is highly asymmetrical until the third phase is connected. When the third phase is connected, there are three magnetic fi elds in the stator core. There is a 120° displacement between the three phases, as shown in Fig. 1.9 Three phases produce a symmetrical rotating fi eld. Fig. 1.9 Three phases produce a symmetrical rotating fi eld The stator is now connected to the three-phase supply. The magnetic fi elds of the individual phase windings form a symmetrical rotating magnetic fi eld. This magnetic fi eld is called the rotating fi eld of the motor.