Page 170 - Electrician - TT (Volume 2)

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AC motor startor re-winding

LObjectives: At the end of this lesson you shall be able to:
• tate the terms used in AC winding
• explain the different types of AC winding.

Fundamental terms used in AC Winding: Before taking up AC winding, the trainee should be familiar with the
terms used in AC winding as explained in the following paragraphs.
Coil : A number of turns connected in series is called a coil. A coil has two active sides and two inactive sides.
Turn: It is the closed path of the conductor which is formed by connecting the two inductors under two dissimilar
poles N and S. (Fig 1)
Active side of a coil : It is that part of the coil which lies in the slots of the core. It is also known as an inductor.
(Fig 1)
Inactive side of a coil : It is the portion of the coil which joins the two active sides of a coil. (Fig 1)

Leads of a coil : These are the two ends of a coil which are used for the connection. Leads are also known as
jumpers which may be symmetrical or unsymmetrical as shown in Fig 2.

Fig 1 Fig 2

Pole pitch : The distance between the centre of two adjacent opposite poles is called the pole pitch. Pole pitch is
measured in terms of slots or coil sides.
No. of slots in the stator
Pole pitch
No. of poles
Coil pitch/span and coil throw : The distance between the two active sides of a coil under adjacent dissimilar
poles is called coil pitch/span. Fig 3 shows the coil pitch/span and coil throw (i.e. coil pitch/span = 4 and coil throw
is 1-5).
Pitch factor : Winding pitch need not be equal to the pole pitch. If the pole pitch and winding pitch are equal,
the winding is called full pitched winding. If the winding pitch is less than the pole pitch, the winding is called
fractional pitch winding or short pitch winding. While rewinding, the original winding pitch should not be changed.
The machine designer would have chosen the winding pitch after considering the different factors required for
the better performance of the machine. Any change in the original winding pitch of a machine will affect the
performance of that machine. If the winding pitch is 4, then the coil throw is 1 to 5, and one side of the coil is
placed in slot No.1 and the other side of the coil is inserted in slot No.5 as shown in Fig 3. Then the winding pitch
is 5-1 = 4. The ratio between the winding pitch and pole pitch is called the pitch factor.
Winding pitch
Pitch factor
Pole pitch

PB 157

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