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Electrical Machines 2 Lab Experiments

To plot the ‘v’ and ‘inverted v’ curves of Synchronous motor


To plot the ‘v’ and ‘inverted v’ curves of Synchronous motor

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

Name plate details:

DC Motor(prime mover)

Rated power:5HP


Line voltage: 415V



Frequency 50Hz

Rated Current:6.5A

Field current(If)1.5A

Field Voltage (Vf):220V

3- ɸ Auto transformer details:

Input voltage: ___415_________________(Volt)

Output Voltage : ____(0-470)_______________(Volt)

Frequency. :________50________________(Hz)

Current: _________________________(Amp)

Circuit Diagram:



  1. Connections are made as per the circuit diagram.
  2. Opening the SPST switch connected across the field DC supply is given to the field and field current is adjusted to 0.3A ( 20% of rated field current)
  3. The DC supply to the field is removed and SPST switch is connected across the field by closing the switch
  4. As 3- ɸ , 440V, 50Hz AC supply is applied to 3- ɸ dimmer stator keeping it in minimum output position, keeping it prior to that motor is kept in no load state.
  5. Gradually supply voltage to synchronous motor is increased and then motor starts running as squirrel cage induction motor. The direction of rotation is observed. if it is not proper then supply phase sequence is altered.
  6. Observing Ia, the voltage is gradually increased. It will reach a high value and suddenly falls to a low value.
  7. At that instant, open SPST switch connected across the field. The DC supply is then given to the field. Then the motor is pulled into synchronism and motor now works as a synchronous motor.
  8. Gradually the supply voltage to stator is increased by observing the armature current. If Ia, increases above the rated value then increase If such that Ia will be within limits and thus full rated supply voltage is gradually given to the motor. Now motor will work as synchronous motor with full rated voltage.
  9. By varying If in steps, armature currents are recorded at no-load.
  10. By applying half of full load on motor, If and Ia are recorded again. The same experiment is repeated at 3/4th load, full load and corresponding readings are recorded.
  11. Completely removing the load on motor, the 3- ɸ supply to stator and then the DC supply to the field are switched OFF.

Observations Table: Field current If(Amp) Armature current Ia(Amp) Cos ɸ
1 0.05 1
2 1 0.5
3 0.15 0.6
4 02 0.4
5 0.25 0.2
6 0.35 0
7 0.45 0 0.5lag
8 0.5 0.2 0.8lag
9 0.55 0.4 0.98lag
10 0.6 0.5 0.8lead
11 0.65 0.6 0.75
12 0.7 0.7 0.7 lead
13 0.8 0.8 0.6 lead
14 0.85 1 0.55lead
15 0.9 1.5 0.5lead


Power factor = Cos [Tan-1[√3(W11-W2)/(W1-W2)]]

Φ = [Tan-1[√3(W11-W2)/(W1-W2)]]



V and inverted V curves of synchronous motor are drawn.

Viva Questions:

  1. What are the difficulties in starting a synchronous motor?
  2. What are the commonly employed methods of starting a synchronous motor?
  3. What are the applications of synchronous motor?
  4. What is synchronous condenser?
  5. What do you understand by hunting?

Electrical Machines 2 Lab Experiment list


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