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Electrical Machines 2 Lab Experiments

To find the regulation of a 3 - ɸ alternator by using synchronous impedance method.


To find the regulation of a 3 - ɸ alternator by using synchronous impedance method.

Apparatus Required: Equipment Type Range Quantity
1 Voltmeter MI (0-300/600)V 1nos
2 Ammeter MI (0-5/10)A 1nos
3 Ammeter MI (0-2.5/5)A 1nos
4 rheostat Wire Wound 400Ω/1.7A 145Ω/2A 1no/2nos
5 Techometer Digital (0-10000) RPM 1no
6 Connecting Wires -- (0-20)A Required

Name plate details:-

DC Motor(prime mover)

Rated power:5HP

Armature Voltage:220V




3-ɸ Alternator

Power Rating:3KVA

Voltage:415 V

Rated Current: 3.8A


Excitation:DC Generator

Circuit Diagram:



Open Circuit Test:

  1. Make the connections as per the circuit diagram
  2. Before starting the experiment, the potential divider network in field circuit and field regulator rheostat of motor circuit is set minimum resistance position.
  3. Switch ON the supply and close the DPST switch. The DC motor is started by moving starter handle
  4. Adjust the field rheostat of DC motor to attain rated speed (equal to synchronous speed of alternator)
  5. By decreasing the field resistance of Alternator, the excitation current of alternator is increased gradually in steps
  6. Note the readings of field current, and its corresponding armature voltage tabular column
  7. The voltage readings are taken upto and 10% beyond the rated voltage of the machine.

Short Circuit Test:

  1. For Short circuit test, before starting the experiment the potential divider is brought back to zero output position, i.e., resistance should be zero in value.
  2. Now close the TPST switch.
  3. The excitation of alternator is gradually increased in steps until rated current flows in the machine and note down the readings of excitation current and load current (short circuit current).
  4. Switch OFF the supply.


OC test Field current in Amp.(If) OC voltage per phase (Vo)
1 0.25 140
2 0.3 160
3 0.35 175
4 0.4 205
5 0.45 228
6 0.5 250
7 0.55 270
8 0.6 265
9 0.7 320
10 0.75 335

S.C. test Field current If(Amp.) SC current ISCAmp.
1 0.24 0.5
2 0.3 1
3 0.35 1.2
4 0.4 1.6
5 0.45 1.9
6 0.5 2.15
7 0.55 2.4
8 0.6 2.8
9 0.65 3.1
10 0.7 3.4

Procedure to find Armature resistance of Alternator:

  1. Connections are made as per the circuit diagram.
  2. Switch ON the supply. By varying the rheostat, take different readings of ammeter and voltmeter in a tabular column.
  3. From the above readings, average resistance Ra of a armature is found out

Connection diagram to find Ra:


Observations: Armature current I(amp) Armature voltage Va (volts) R DC= V/I

Procedure to find synchronous impedance from OC and SC tests:

  1. Plot open circuit voltage, short circuit current verses field current on a graph sheet.
  2. From the graph, the synchronous impedance for the rated value of excitation is calculated.
  3. The excitation emf is calculated at full load current which is equal to the terminal voltage at No load.
  4. The voltage regulation is calculated at rated terminal voltage

Model calculations:

Zs= VOC/IsCfor the same If and speed:Xs=√Zs2-Ra2(Ra = Rdc)

Generated e.m.f. of alternator on no load is

E0= √(vCosɸ + IaRa)2+ (vsinɸ ± Ia XS)2

+ for lagging p.f.

- for leading p.f.

The percentage regulation of alternator for a given p.f. is

1. Re g = E 0-V/V x100


E 0– generated emf of alternator (or excitation voltage per phase)

V – full load, rated terminal voltage per phase


Model graphs:

Draw the graph between IfVS E0 per phase and If VS ISC



  1. Connections must be made tight
  2. Before making or breaking the circuit, supply must be switched off

Result:The O.C. and S.C. tests were conducted on the given 3-Φ Alternator and the regulation of Alternator was predetermined by e.m.f. and m.m.f. method.

Electrical Machines 2 Lab Experiment list


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