Single-phase motors become impractical in sizes much above 7.5 kW (10 hp) and, for larger power requirements, three-phase motors become the practical choice. Threephase, induction-type motors are available in sizes up to 150 kW (200 hp). A threephase induction motor is illustrated in Figure 3.9a. The rotor has the squirrel-cage design of Figure 3.1. The voltage to the motor (Figure 3.9b) is supplied through the three electrical conductors of a three-phase line. The voltage waveform BB1 lags AA1 by one third of the wavelength or 120° or, for 60 Hz voltage, by 0.0056 seconds; Voltage CC1 lags AA1 by 240° or 0.0111 seconds. As waveform AA1 goes through its sinusoidal variation, electromagnet A in Figure 3.9a builds from zero to a strong north pole, diminishes to zero, builds to a strong south pole, diminishes to zero, and so forth. Electromagnets B and C follow a similar pattern except for their respective phase delays. Thus, the north pole of the magnetic field across the rotor appears to rotate from electromagnet A to B to C to A and so forth, that is, the field appears to rotate clockwise. Thus, as was discussed in Section 3.3, the rotor also rotates clockwise. The synchronous speed, slip, and rotor speed can be calculated by using Equations 3.1, 3.2, and 3.3, respectively. Thus, for 60 Hz current, the synchronous speed would be 3600 rev/min for the two-pole motor shown in Figure 3.9a. If the slip at full load was 4.17%, the rotor would turn at 3450 rev/min at full load. A four-pole motor uses twice as many windings to create two sets of north and south poles simultaneously. As was described in Section 3.3, the magnetic field then rotates halfway around the stator in each cycle of voltage and the synchronous speed becomes 1800 rev/min.
Illustration of a three-phase induction motor
Three-phase induction motor with (a) wye and (b) delta connections
The direction of rotation of the three-phase induction motor can be reversed by reversing any two of the three leads to the motor. Some contemplation will show that reversing any two leads reverses the direction of rotation of the magnetic field and thus reverses the direction of rotation of the rotor.
Three-phase electricity from the electric power grid is provided for either wye or delta connections and motors are available for either type of power. Figure 3.10a shows a wye-type motor while a delta-type motor is shown in Figure.
