Page 761 - Basic Electrical Engineering

180° electrical which is equal to 90° mechanical. If the winding is made for
six-poles, the coil span will be counted as 180° electrical but will be equal to
60° mechanical.

In general, the relationship between electrical degrees and mechanical
degrees is expressed as

When P = 2, 1° mechanical is equal to 1° electrical or 180° mechanical is

equal to 180° electrical.

10.7 TYPES OF WINDINGS
Three-phase windings are made for a different number of poles. Each phase

winding generally will have a number of coils connected together. The three-
phase windings are displaced in space by 120°. All the coils are placed inside

the slots made in the stator periphery and are secured such that they do not
come out easily. The whole winding is distributed uniformely throughout the

periphery instead of making big slots and placing a large number of coils
together in two slots for each phase.
The advantages of distributed winding over concentrated winding are

better dissipation of heat generated due to current flow through the windings
and better wave form of the generated EMF (better EMF generated means

that the shape of the voltage wave should be sinusoidal).
All the coils forming a winding for each phase can be connected in a

number of ways. These are called types of windings. They are

i. lap winding;
ii. wave winding; and
iii. spiral winding.

The windings are also made in single-layer type or double-layer type. In

single-layer type each slot will contain one coil side only. However, each coil
will have a large number of turns. In double-layer type, as the name indicates,
two coil sides will occupy one slot in the whole of the armature winding. The