Experiment - 2


To determine the direct axis reactance (Xd) and quadrature axis reactance (Xq) of synchronous machine.


  1. Ammeter: 0 - 15A; 0 - 2.5A.
  2. Voltmeter: 0 – 40V; 0 – 250V.
  3. Wattmeter: 0 -1400 W.


Fig 2.1: Circuit diagram for Slip Test

Fig 2.2: Circuit diagram for Static Test

(a). Slip Test

  1. Check the phase sequence and voltage of the synchronous machine and the bus bar.
  2. Remove the field excitation and connect RYB terminals of machine to RYB of bus bar through a three phase variac.
  3. Start DC motor and adjust its speed near to synchronous speed, switch on the AC supply and apply a small voltage. The voltmeter connected across field winding should fluctuate and remains within the rated value, if the slip is small and direction of rotation is correct. The armature current should also fluctuate.
  4. Increase the AC voltage applied to the armature from the variac, such that maximum current is nearly equal to the rated full load current of the armature.
  5. Note down the maximum and minimum value of armature applied voltage and current.
  6. Calculate the applied voltage per phase (Vp=VL/√3).
  7. Record the readings for different applied voltage.
(b). Maximum lagging current test for determination of Xq

  1. First run the synchronous machine as an alternator with the help of prime mover (dc machine) and synchronize it with ∞ bus bar.
  2. Switch off the DC supply to the motor so that the synchronous machine will run as a synchronous motor.
  3. Gradually reduce the excitation to zero.
  4. Reverse the field connection with the help of DPDT switch.
  5. Increase the excitation slowly in the negative direction till the machine shows sign of falling out of step. Note this field current, also note the line current.
Static test for determination of Sub-transient reactance

  1. With the rotor of the synchronous machine standstill apply single phase voltage from a variac across two phase winding connected in series. The field is short circuited through an ammeter.
  2. Rotate the rotor manually such that two positions of flux linking are apparent from the induced field current.
  3. Let the armature voltage and current corresponding to the minimum induced field current be Vq and Iq'' and armature voltage and current corresponding to the maximum induced field current be Vd and Id''.
  4. Record the readings for different applied voltages.
For slip test

  1. Zd – maximum phase voltage / minimum phase current
    Xd =√ (Zd2 – Re2)
  2. Zq – minimum phase voltage / maximum phase current
    Xq =√ (zq2 – Re2) Re2 – Equivalent resistance /Phase of the alternator
  3. Static Test for Xd'' and Xq''
    Zd'' = Vd / (2Id'') Xd'' = √(Zd'2 – Re2)
    Zq'' = Vq / (2Iq'') Xq'' = √(Zq'2 – Re2)


  1. Connect as shown in Figure (Click to see fig.). Set the output to zero.
  2. Switch on the d.c. supply and run the d.c. motor at a speed close to the synchronous speed of alternator but less than synchronous speed.
  3. Switch on a.c. supply and increase the variac output to a suitable value. Observe the variations in voltmeter and ammeter readings.
  4. Adjust the speed of the d.c. motor finely to get maximum swings in ammeter and voltmeter pointers.
  5. Note maximum and minimum readings of voltage and current.
  6. Take additional sets of readings by adjusting different variac outputs.
  7. Now adjust the d.c. motor speed to a value little higher than synchronous speed and take similar readings as above.