Induction motors Pole changing

In the preceding discussion of multispeed ac motors note that only induction motors are considered. These have no discrete physical rotor poles, so that only the stator-pole configuration need be modified to change speed. To operate multispeed, a synchronous motor would require a distinct rotor structure for each speed. Thus multispeed is practical only for squirrel-cage induction motors. 

For many years it was common practice to give standard open motors a 115% service factor rating that is, the motor would operate at a safe temperature at 15% overload. This has changed for large motors, which are closely tailored to specific applications. Large motors, as used here, include synchronous motors and all induction motors with 16 poles or more (450 rpm at 60Hz). 

Figure 6.91 Longitudinal section drawing of a flameproof cage induction motor, shaft and stator housing, water cooled, with pole changing.

A variation on the theme of pole changing is a particular type of squirrel-cage induction motor, called the Pole Amplitude Modulated (PAM) motor. PAM motors should be used for low speed applications thereby requiring many poles e.g. 10, 12, 16. In addition, the various speeds required should not be widely different. This means that the number of effective poles will not be too dissimilar, e.g. 

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