Page 137 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

The voltage regulation of an alternator: The voltage regulation of an alternator is defined as the rise in voltage
when the load is reduced from the full rated value to zero, with the speed and field current remaining constant. It
is normally expressed as a percentage of the full load voltage.
% of voltage regulation = V NL V FL 100
V FL
where V - no load voltage of the alternator
NL
V - full load voltage of the alternator
FL
The percentage regulation varies considerably, depending on the power factor of the load, and as we have seen
for leading P.F. the terminal voltage increases with load, and for lagging P.F. the terminal voltage falls with the
load.
Example: When the load is removed from an AC generator, its terminal voltage rises from 480V at full load to
660V at no load. Calculate the voltage regulation.
V V
% regulation= NL FL 100
V FL
660 480 100 375%.
480

Parallel operation and synchronisation of three phase
alternators - brushless alternator

Objectives: At the end of this lesson you shall be able to:
• state the necessity and conditions for paralleling of alternators
• explain the methods of paralleling two 3 phase alternators
• state the effect of changes in field excitation and speed on the division of load between parallel operation.

Necessity for paralleling of two alternators : Whenever the power demand of the load circuit is greater than
the power output of a single alternator, the two alternators to be connected in parallel
Conditions for paralleling (synchronising) of two 3 phase alternators
• The phase sequence of both 3 phase alternators must be same. It can be checked by using phase sequence
meters
• The output voltages of the two 3 phase alternators must be same.
• The frequency of both the alternators must be same
Dark lamp method : The following describes the method of synchronizing two alternators using the dark lamp
method.
Fig 1 illustrates a circuit used to parallel two three-phase alternators. Alternator 2 is connected to the load circuit.
Alternator 1 is to be paralleled with alternator 2 Three lamps rated at double the output voltage to the load are
connected between alternator 2 and the load circuit as shown. When both machines are operating, one of two
effects will be observed:

124

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