CIRCUIT DIAGRAM (OC & SC TEST)
MACHINE DETAILS
CIRCUIT DIAGRAM (Armature Resistance Test)
EXPERIMENT NO : 6 DATE:
VOLTAGE REGULATION BY MMF METHOD
AIM
a. pre-determine the percentage regulation of three phase alternator by MMF method at various
power factors
b. Plot Power factor Vs % Voltage regulation curves for both full load & ½ full load
APPARATUS REQUIRED
Sl No Name of Apparatus Range & Type Quantity
1 Voltmeter 0-500 V , MI 1 No
2 Ammeter 0-5A, MI 1 No
3 Ammeter 0-5A , PMMC 1 No
4 Rheostat 500 Ω, 4 A 1 No
5 Rheostat 300 Ω, 2 A 1 No
6 Tacho meter Digital 1 No
PRINCIPLE
MMF Method
It is an indirect method of finding voltage regulation in an alternator. It is opposite to EMF method in
the sense that here the armature leakage reactance is treated as additional armature reaction. To find
voltage regulation using MMF method, we need to conduct three tests
1. Open Circuit Test 2. Short Circuit Test 3. Armature Resistance Test .
Since the voltage regulation obtained from MMF method is less than actual voltage regulation, this
method is also called Optimistic method
TABULAR COLUMN (OC TEST)
Sl No Field current, 𝑰𝒇 Open circuit line voltage, 𝑽𝟏 Open circuit phase voltage, 𝑬𝟎
(V) 𝑽
(A) (V), 𝑬𝟎 = 𝟏
√𝟑
TABULAR COLUMN (SC TEST)
Sl No Field current, 𝑰𝒇 (A) Short Circuit Armature Current,
𝑰𝑺𝑪 (A)
TABULAR COLUMN (ARMATURE RESISTANCE TEST)
Sl No Voltage ,V Current,I (A) 𝑉 Armature Resistance
Resistance, R = 𝐼
(Ω)
(V) 𝑅
/Phase, 𝑹𝒂 = (Ω)
2
Mean value of Armature Resistance /Phase, 𝑹𝒂
PROCEDURE
A. OPEN CIRCUIT TEST ( OC TEST)
1. Connections are made as per the circuit diagram.
2. Keep field rheostat of prime mover at minimum resistance position and potential divider of the
alternator field circuit at minimum potential position
3. Keep TPST switch in open position
4. Switch on the supply and start the prime mover by starter
5. Adjust the field rheostat of prime mover until alternator attains rated speed
6. Increase the field current in step by step by adjusting potential divider and note corresponding
voltmeter reading up to 120% of rated voltage
7. Keep speed constant during all the readings with the help of motor field rheostat
8. Bring the potential divider of alternator and field rheostat of prime mover to minimum position
and switch off the supply
9. Plot OCC
B. SHORT CIRCUIT TEST (SC TEST)
1. Connections are made as per the circuit diagram
2. Keep field rheostat of prime mover at minimum resistance position and potential divider of the
alternator field circuit at minimum potential position
3. Keep TPST switch in open position
4. Switch on the supply and start the prime mover by starter
5. Adjust the field rheostat of prime mover until alternator attains rated speed
6. close the TPST switch.
7. Increase the field current of alternator from zero until the short circuit armature current has
reached its maximum safe value.
8. Bring back the potential divider and field rheostat of motor in minimum position and Switch off
the supply
9. Plot SCC
C. ARMATURE RESISTANCE TEST
1. Connections are done as shown the circuit diagram
2. Keep the series rheostat at maximum resistance position
3. Switch on low voltage DC supply
4. Adjust rheostat to a suitable value of current and note the corresponding voltmeter reading
5. Increase the current in step by step using rheostat and note the ammeter & voltmeter readings
in each step
6. Tabulate the readings and calculate the resistance
SAMPLE CALCULATIONS
420
Rated voltage/ phase, V = Volts
√3
Full load current, I = 6.9 A
Armature resistance /phase , 𝑅𝑎 = ……. Ω
REGULATION AT FULL LOAD ………… POWERFACTOR
Cos(ϕ) = ……. , Sin(ϕ) = ……..
E = V + I𝑅𝑎 Cos(ϕ) ( Since the value of resistance is very less, the Armature resistance is neglected)
Hence E = V
Field current corresponding to rated voltage/phase ( V ) from OCC, 𝐼𝑓1 = ………….. A ( From OCC )
Field current corresponding to full load current from SCC , 𝐼𝑓1 = ………….. A ( From SCC )
Resultant field current, 𝐼𝑓 = ……………………
( 𝐼𝑓 = Vector sum of 𝐼𝑓1 & 𝐼𝑓2 , where 𝐼𝑓2 is plotted 90 + ϕ for lagging p.f and 90 − ϕ for leading p.f
or 𝐼𝑓 = √𝐼𝑓1 2 + 𝐼𝑓2 2 + 2𝐼𝑓1 𝐼𝑓2 Cos(180 − (90 ± ϕ) )
Open circuit voltage 𝐸0 corresponding to resultant field current 𝐼𝑓 = …………V (……….. p.f lag )
Open circuit voltage 𝐸0 corresponding to resultant field current 𝐼𝑓 = …………V (……….. p.f lead )
𝐸0 − 𝑉
% Voltage regulation at full load ………. Power factor lag = 𝑉
X 100 =
𝐸0 − 𝑉
% Voltage regulation at full load ………. Power factor lead = X 100 =
𝑉
REGULATION AT HALF LOAD ………… POWERFACTOR
Cos(ϕ) = ……. , Sin(ϕ) = ……..
Field current corresponding to rated voltage/phase ( V ) from OCC, 𝐼𝑓1 = ………….. A ( From OCC )
Field current corresponding to full load current from SCC , 𝐼𝑓1 = ………….. A ( From SCC )
Resultant field current, 𝐼𝑓 = …………………… (……….. p.f lag )
Resultant field current, 𝐼𝑓 = …………………… (……….. p.f lead)
( 𝐼𝑓 = Vector sum of 𝐼𝑓1 & 𝐼𝑓2 , where 𝐼𝑓2 is plotted 90 + ϕ for lagging p.f and 90 − ϕ for leading p.f
or 𝐼𝑓 = √𝐼𝑓1 2 + 𝐼𝑓2 2 + 2𝐼𝑓1 𝐼𝑓2 Cos(180 − (90 ± ϕ) )
Open circuit voltage 𝐸0 corresponding to resultant field current 𝐼𝑓 = …………V (……….. p.f lag )
Open circuit voltage 𝐸0 corresponding to resultant field current 𝐼𝑓 = …………V (……….. p.f lead )
𝐸0 − 𝑉
% Voltage regulation at half load ………. Power factor lag = X 100 =
𝑉
𝐸0 − 𝑉
% Voltage regulation at half load ………. Power factor lead = 𝑉
X 100 =
SAMPLE GRAPG
A. MMF METHOD
B. POWERFACTOR Vs % VOLTAGE REGULATION
TABULAR COLUMN (% VOLTAGE REGULATION Vs POWERFACTOR)
MMF METHOD
% VOLTAGE REGULATION
Sl PF FULL LOAD HALF LOAD
No I E0 V % I E0 V %
V.REG V.REG
1 0 lag
2 0.2 lag
3 0.4 lag
4 0.6 lag
5 0.8 lag
6 1
7 0.8 lead
8 0.6 lead
9 0.4 lead
10 0.2 lead
11 0 lead
RESULT
a. pre-determined the percentage regulation of three phase alternator by MMF method at various
power factors
b. Plotted Power factor Vs % Voltage regulation curves for both full load & ½ full load
