Page 135 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

This would have been the actual value of the induced voltage if all the coils in a phase were (i)full pitched and (ii)
concentrated or bunched in one slot. (In actual practice, the coils of each phase are distributed in several slots
under all the poles.) This not being so, the actually available voltage is reduced in the ratio of these two factors
which are explained below.
Pitch factor (K or K .): The voltage generated in a fractional pitch winding is less than the full pitch winding.
c
p
The factor by which the full pitch voltage is multiplied to get voltage generated in fractional pitch is called pitch
factor, and it is always less than one; and denoted as Kp or Kc. Normally this value is given in problems directly;
occasionally this value needs to be calculated by a formula Kp = Kc = Cos a/2
where a is the electrical angle by which the coil span falls short of full pitch.
Distribution factor (K ): It is imperative that the conductors of the same phase need to be distributed in the slots
d
instead of being concentrated at one slot. Because of this, the emf generated in different conductors will not be
in phase with each other, and hence, cannot be added together to get the total induced emf per phase but to be
added vectorially. This has to be taken into account while determining the induced voltage per phase.

Therefore, the factor by which the generated voltage must be multiplied to obtain the correct value is called a
distribution factor, denoted by Kd and the value is always less than one. The formula for finding the value of Kd
is given below. Sin m /2
K =
d
m Sin /2

where m is the number of slots per phase per pole
180 o
=
No.of slots per pole
Characteristic and voltage regulation of the alternator

Objectives: At the end of this lesson you shall be able to:
• explain the load characteristic of an alternator and the effect of the P.F. on terminal voltage
• explain the regulation of alternators and solve problems therein.

Load characteristic of an alternator: As the load on the alternator is changed, its terminal voltage is also found
to change. The reason for this change is due to the voltage drop in the alternator because of
• armature resistance Ra
• armature leakage reactance XL
• armature reaction which, in turn, depends upon the power factor of the load.
Voltage drop in armature resistance: Resistance of each phase winding of the alternator causes a voltage drop
in the alternator, and it is equal to IpRa where Ip is the phase current and Ra is the resistance per phase.
Voltage drop in armature leakage reactance: When the flux is set up in the alternator due to the current flow in
the armature conductors, some amount of flux strays out rather than crossing the air gap. These fluxes are known
as leakage fluxes. Two types of leakage fluxes are shown in Figs 1a and b.

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CITS : Power - Electrician & Wireman - Lesson 76-85 CITS : Power - Electrician & Wireman - Lesson 76-85