Voltage Regulation of an Alternator

VOLTAGE REGULATION

The voltage regulation of an alternator is defined as, the increase in terminal voltage when full-load is thrown off, assuming field current and speed remains the same.

The percentage regulation is defined as the ratio of change in terminal voltage from full load to no load to rated terminal voltage.

Percentage regulation = 

where, E₀ = N₀ load terminal voltage

V = Full load rated terminal voltage.

Voltage characteristics of an alternator as shown in Figure 3.51.

To determine the regulation of an alternator, open circuit and short circuit tests are performed, which give open circuit characteristic and short circuit characteristic.

Open Circuit Test:

The machine is run on no load and the induced e.m.f/phase is measured corresponding to various values of field current and the curve is drawn between induced e.m.f per phase E and field current I_f as shown in Figure 4.45. The curve so drawn is known as open circuit characteristics (O.C.C).

Short Circuit Test:

In short circuit test, the armature winding short-circuited through a low resistance ammeter. During this test, the speed is kept constant and short-circuit current is measured corresponding to value of field current. The field current is increased to give short circuit current above twice the full load current.

The short circuit characteristic is drawn by plotting a curve between short circuit current I_sc and field current I_f

If OA is the normal field current, AB gives short circuit current I_sc corresponding to field current OA and AC gives induced e.m.f/phase, for the same excitation. Since on short circuit for excitation OA, the whole of the induced e.m.f. AC is utilized to create a short circuit current I_sc given by AB.

Hence synchronous impedance:

Z_S = AC (in volts) / AB (in amps) ohms