POLE CHANGING (Induction Motor)

3.4
A formula for speed, nM, of the induction motor as a function of the supply frequency, /, number of pole pairs, pp, of the magnetic field, and slip, s, of the motor can be obtained from Eqs. (2.4) and (2.6) as

On the other hand, with a fixed output power, the speed is inversely proportional to the developed torque [see Eq. (2.9)]. Therefore, observing two motors of the same power, frequency, and voltage ratings, of which one has a two-pole stator winding and the other a four-pole winding, and which drive identical loads, the four-pole machine would rotate with half of the speed of the two-pole one but with twice as high a torque. Thus, a motor with pp pole pairs is equivalent to a two-pole machine connected to the load through gearing whose gear ratio, N, as defined by Eq. (1.4), is 1/pp. The gear-ratio property of the number of poles is utilized in certain motors for speed control. Such motors have stator windings so constructed that they can be connected in various arrangements, in order to produce magnetic fields of an adjustable pole number, for instance two, four, and six. In this way, the synchronous speed can assume several distinct values, such as 3600 r/min, 1800 r/min, and 1200 r/min.

The topic of stator windings in ac machines is vast, and it exceeds the scope of this topic.

Interested readers are referred to relevant sources, for instance the excellent manual by Rosenberg and Hand, 1986, which can be found in the Literature section at the end of this topic. Here, only
one example of pole changing is illustrated in Figure 3.6. It shows a four-coil winding of phase A, which can be connected to produce a four- or eight-pole magnetic field. In the four-pole arrangement seen in Figure 3.6(a), terminals x and y are shorted forming one end of the winding, while terminal z makes up the other end. When, as in Figure 3.6(b), x and y are disconnected from each other and used as ends of the winding, an eight-pole field is generated.

Arrangement of stator windings affects the developed torque, because the torque is dependent on stator current,

which, in turn, depends on the stator impedance. These dependencies allow better matching of a motor to the load. For instance, when a two-pole stator is reconnected to four-pole operation, the resulting pull-out torque can be the same as before (constant torque connection), half of its previous value (square-law torque connection), or twice its previous value (constant power connection). Clearly, these three types of torque-speed relationship are most suitable for loads with the constant, positive, and negative coefficient k in Eq. (1.12), respectively (see Figure 1.1).