Page 673 - Basic Electrical Engineering

Let us now understand how windings can be made for different number of
poles. Fig. 8.4 shows a simple two-pole and four-pole stator winding. For
simplicity, winding for only one phase has been shown.

In Fig. 8.4 (a) a single coil has been used. The distance between the coil
sides is 180° mechanical. This distance is called the span of the coil or simply

coil span. When dc supply with fixed polarity is applied, current will flow
through the coil and a magnetic field with two poles will be formed as has

been shown. If ac supply is given, the polarities will change, the direction of
current through the coil will change, and hence the positions of North and

South poles will change continuously in every half cycle of power supply.
That is to say, when ac supply is applied, a two-pole alternating magnetic
field will be produced whose axis will lie along the horizontal direction as

shown. Since for a sinusoidal ac supply both magnitude as well as direction
of current will change, the magnetic fields produced will have its magnitude

as well as direction changing continuously along a fixed axis.