Page 195 - Electrician - TT (Volume 2)

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4 The main winding is kept first in the stator slots and the starting winding is kept over the main windings.

5 Normally, the main winding consists of thick winding wire, and the starting winding of thin winding wire. In
certain motors both the windings may have same size of winding wire.
6 The number of turns in the main and starting windings may or may not have the same number of turns.

7 In concentric coil winding, the coils in the same group may or may not have the same number of turns.
8 Each slot may contain one or two coil sides.
9 The overhang of the coils should be of exact in size. If it is less, the insertion of the coils will be difficult and if
the size is more, the coils may not allow the end covers to be fitted.

10 While inserting concentric coils, start with the smaller pitched coil set.
11 There may be empty slots in the stator. Note their position.
Concentric winding: Concentric type of winding is probably the most common type of winding used in fractional
horsepower single phase motors. The winding may be hand wound or may be form wound.
As the starting winding is designed to split the phase and is used to start the motor, it may have less slots (coils)
allotted when compared to the main winding. For example there may be 8 coils for main winding and 4 coils only
for the starting winding.
Further it is a standard practice to wind only about 70% of the slots of a single phase motor, as owing to the effect
of the distribution or spread factor, no advantage is gained by making a single phase winding any wider. Even
if the whole of the slots were to be wound, the extra winding would be useless for producing the useful torque.
Similarly it has been found that in single phase motors, no extra loss takes place if all the slots of each pole face
are not wound. Thus the running winding looses nothing in efficiency, because some of the slots of each pole are
taken for the starting winding.
Winding calculation and diagrams for concentric type winding : Let us discuss the following examples.

Example 1
Prepare the winding table, draw the connection and developed diagrams for a single phase, 4 pole, whole coil
connected capacitor motor having 24 slots, 12 coils (8 coils for main and 4 coils for starting winding) with pitches
5, 3 for the main and 5 for the starting winding.

Number of coils per pole in main winding
Total number of main winding coils 8 2 coils/pole
Number of poles 4

In other words, there will be 8 coils in the main winding forming 4 pole groups. Each group will have two coils
under each pole. Pitches assigned will be 5 and 3 for each coil group.

Number of coils per pole in starting winding = 4/4 =1 coil /pole.
There will be 4 groups in starting winding having one coil per group. Pitch assigned will be 5 for the coil.
Summarising the results we have the coil group as given below in Table 1.

Table 1
Winding Groups Coil per pole Pitches Coil throw Connection
Main 4 2 5, 3 1-6, 2-5 Whole coil-end to end and start
to start
Starting 4 1 5 1-6 Whole coil-end to end and start
to start.

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CITS : Power - Electrician & Wireman - Lesson 86-92 CITS : Power - Electrician & Wireman - Lesson 86-92