Motors Pole changing

Speed control in slip-ring motors has been discussed in the previous chapter. Squirrel cage motors have limitations ill their speed control in view of their fixed rotor parameters. Speed variation, in fixed steps, however, is possible in such motors if the stator is wound for multipolcs and such motors arc known as pole changing motors. Up to four different speeds can be achieved in such motors economically, in combinations of 2/4, 4/6. 4/8, 6/8, 6/12, 2/4/6. 4/6/8. 2/4/6/12 and 4/6/8/12 poles etc, or any other similar combination. For limitation in the motor size and tlux distribution, winding sets of more than two are not recommended. I he two windings can be arranged for two. three oi (maximum) four different speeds. 

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

In a mnlti-pole motor for eight-pole operation, the adjacent poles change in polarity from North to South around the air gap. 

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. 

Occasionally in refineries there is a need for large gas compressors to operate at two different speeds for long periods of time. If these two speeds can be matched to the pole arrangements of a multi-pole motor, then pole changing can be used satisfactorily. 

Auger shaft speeds in general Constant up to max. 3 different with pole-changing motors or with switchgear Infinitely variable by motors with speed control... 

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). 

The speed of an electric motor can be changed by altering the frequency of the electric current. This is because the ratio is the same as 60 or 50 f/p (f = the frequency of the current, p = the number of poles in the stator). Frequency converters are built of electronic components, frequently combined with microprocessors. They provide good motor protection and are superior to the traditional bimetal protection. The characteristic curve for a pump and fan motor is also quadratic, making lower demands to the frequency converters When the frequency of the electrical current is changed in the frequency converter, the main AC supply is transformed into DC. The DC is then treated... 

When a universal motor is run on direct current, the magnetic poles in the armature change while those of the field magnet remain constant. 

The experimental results for the modified seesaw appear in Figs. 5.10, 5.11, and 5.12 for pole loeations X = 1, 2 and 3, respectively. When X = 1, the system was stable but experiment and simulation were quite different. When X = 2 and 3, the system was stable with a good response to setpoint changes with experimen-tal/simulation discrepancies relatively smaller than in the unmodified case. The closed-loop system was unstable for both smaller values of X larger values of X were not possible due to excessive high-frequency motor movement caused by seesaw encoder quantisation which led to slippage of the cart driving wheel. 

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