VOLTAGE REGULATION OF THREE PHASE ALTERNATOR

The voltage regulation of an alternator is defined as the rise in voltage when full load is removed divided by the rated terminal voltage.

% Regulation = (E0 – Vrated) / Vrated × 100 %

METHODSEMF MethodMMF MethodZPF MethodASA Method

EMF METHOD(OPEN CIRCUIT AND SHORT CIRCUIT TESTS)

PROCEDUREOPEN CIRCUIT TEST

Initially TPST Switch is kept open.Supply is given by closing the DPST Switch in the

Motor side.Using the DC three point starter, start the motor.The field rheostat of motor is adjusted to run the

motor at rated speed and note down the corresponding alternator field current.

Increase the field current in steps by varying the resistance of the alternator field and note down the corresponding open circuit voltage.

 

PROCEDURE SHORT CIRCUIT TEST

Supply is given by closing the DPST Switch in the motor side.

Using the DC three point starter, start the motor.

The field rheostat of motor is adjusted to run the motor at rated speed.

The TPST Switch is closed.The alternator field rheostat is adjusted for the

rated short circuit armature current and note down the corresponding field current.

OCC and SCC Curves

CALCULATION OF REGULATIONArmature resistance, Ra = 1.6 Rdc in Ohms

where, Rdc = Resistance in DC supply in Ohms

Synchronous Impedance, Zs = E0/ Isc in Ohms

where, E0= Open circuit voltage in Volts

Isc = Short circuit current in Amps

Synchronous Reactance, Xs = (Zs 2 – Ra 2 )^1/2 in Ohms

Open circuit voltage (For lagging power factor), E0

E0 = [(Vrated cos Φ + Iasc Ra )2 + ( Vrated sin Φ + Iasc Xs )2 ]1/2 in Volts

where, Iasc = Rated short circuit armature current in Amps

CALCULATION OF REGULATIONOpen circuit voltage (For leading power

factor), E0

E0 = [(Vrated cos Φ + Iasc Ra )2 + ( Vrated sin Φ - Iasc Xs )2 ]1/2

in Volts

Open circuit voltage (Unity power factor), E0

E0 = [(Vrated + Iasc Ra )2 + ( Iasc Xs )2 ]1/2 in Volts

% Regulation = (E0 – Vrated) / Vrated × 100 %

 % REGULATION Vs POWER FACTOR

MMF METHOD (OPEN CIRCUIT AND SHORT CIRCUIT

TESTS)

PROCEDUREOPEN CIRCUIT TEST

Initially TPST Switch is kept open.Supply is given by closing the DPST Switch in the

Motor side.Using the DC three point starter, start the motor.The field rheostat of motor is adjusted to run the

motor at rated speed and note down the corresponding alternator field current.

Increase the field current in steps by varying the resistance of the alternator field and note down the corresponding open circuit voltage.

 

PROCEDURE SHORT CIRCUIT TEST

Supply is given by closing the DPST Switch in the motor side.

Using the DC three point starter, start the motor.

The field rheostat of motor is adjusted to run the motor at rated speed.

The TPST Switch is closed.The alternator field rheostat is adjusted for the

rated short circuit armature current and note down the corresponding field current.

OCC and SCC Curves

CALCULATION OF REGULATIONFor lagging power factor Ifr

= [(If12+If2

2 - 2If1If2Cos (90+Φ)] 1/2 in AmpsFor leading power factor Ifr

= [(If12+If2

2 - 2If1If2Cos (90-Φ)] 1/2 in Amps

where,If1 = Field current required to give open circuit emf, E in

AmpsIf2 =Field current corresponding to short circuit current in

Amps E = [ Vph + IaRaCos ] on open circuit in Volts

% Regulation = (E0 – Vrated) / Vrated × 100 %

% REGULATION Vs POWER FACTOR