Experiment- 3


To determine positive sequence, negative sequence and zero sequence reactances of an alternator.


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

DETERMINATION OF X1 (Positive Sequence reactance): –

It is a reactance of a synchronous machine under steady state condition in the direct axis. The positive sequence impedance can also be defined as the impedance offered by the machine to the flow of positive sequence currents in the armature windings create a magnetic field that rotates in the normal direction in the air gap.


1. Open Circuit Test

  1. Run the machine at rated speed.
  2. Connect a voltmeter and ammeter according to the circuit diagram.
  3. Note the reading at different exciting current.

2. Short Circuit Test

  1. Run the machine at rated speed.
  2. Apply low voltage to the field circuit so that exciting current is small. Alternately connect a high resistance rheostat in the field circuit with full field voltage applied connect an armature in the field circuit.
  3. Apply three-phase short circuit at the synchronous machine terminal with an ammeter connected in any phase.
  4. Measure the short circuit current corresponding to the field current given by the ammeter reading.

DETERMINATION OF X2 (Negative Sequence Reactance): –

The negative sequence reactance X2 can be determined by driving the machine at rated speed with low excitation. Short circuit ant two phases and open circuit one phase. Measure the short circuited current Isc and voltage of open circuited phase Vos. The current coil of voltmeter is excited by Isc and voltage coil by Vos. The negative sequence impedance can be calculated.

Z2=Vos / 3.Isc

X2=Z2sinQ , where Q = cos-1(P / Vsc.Vsc)

  1. Run the machine at rated speed
  2. Short circuit two phases of the alternator through an ammeter and the current coil of the wattmeter.
  3. Connect the voltage coil of the wattmeter and the voltmeter between the open phase and any short circuited phase.
  4. Gradually increase the excitation such that the short circuit current does not exceed its full load value.
  5. Take reading of voltmeter, Ammeter and Wattmeter.

Determination of X0 (Zero Sequence Reactance):-

The machine is driven at rated speed. The windings of all phases are connected in parallel. Connect the voltmeter and ammeter according to the circuit diagram shown in Fig 3.1.

  1. Connect the armature winding in parallel according to the circuit diagram.
  2. Run the machine at rated speed.
  3. Apply low voltage from a variac and measure both voltage and current taken by the armature windings
  4. X0= 3. V0 / I0