Synchronous Motor Excitation - Over, Under & Normal Excitation


     In the last article, we have seen the behavior of synchronous on loading i.e, by varying load on the motor. For a constant field excitation, when the load on the synchronous motor increases the current drawn by the motor increases. But by keeping the load constant we can operate a synchronous motor at different power factors by varying the field excitation. Let us see such a desirable operation of a synchronous motor.


Effect of Excitation on Synchronous Motor at Constant Load :


     When a synchronous motor is loaded and load is kept constant, the input power drawn by the motor will remain constant i.e., √3 VI cos φ is constant. Since the input voltage V and input power are constant, so I cos φ i.e., the active component of current is constant, for a constant load.

     When excitation is changed, the magnitude of induced emf changes. The load angle, α is also constant for a constant load.
  • Er = Resultant voltage between V̅ and E̅b = Ia Za
  • θ = Internal angle = Angle between E̅r and I̅a = Tan-1 ( Xs / Ra )
  • Ia = Armature current
  • φ = Power factor angle = Angle between V̅ and I̅a


Case - 1 :

     When the excitation is reduced in such a way that induced emf is equal to applied voltage Eb = V as shown in the below figure (a). Such excitation is called 'Normal Excitation'. At this condition motor works at lagging power factor i.e., Ia lags V by an angle φ.



Case - 2 :

     When the excitation is reduced in such a way that Eb < V, the motor is said to 'Under Excited'. The resultant emf Er advanced in a clockwise direction and as an angle θ is constant, Ia also moves in the clockwise direction.

     As seen from figure (b) angle φ is increased and the power factor is decreased. To maintain Ia cos φ constant, Ia is increased. Therefore for low values of excitation, Ia increases and the power factor is lagging in nature.

Synchronous Motor Excitation - Over, Under & Normal Excitation

Case - 3 :

     When excitation is increased in such a way that Eb > V, the motor is said to be 'Over-Excited'. The resultant emf Er is moved in the anti-clockwise direction and so the Ia also moves in an anti-clockwise direction ( as an angle between Er and Ia is θ ). Current Ia leads voltage by an angle φ as shown in figure (c).

     As excitation goes on increasing, Er and Ia go on moving in the anti-clockwise direction, angle φ increases. power factor decreases and Ia increases as Ia cos φ = constant. Thus for high values of excitation, current Ia increases and power factor is leading in nature.



Case - 4 :

     For the unity power factor, Eb is slightly greater than V ( Eb ≅ V ). This is shown in figure (d). The excitation for which the motor is operated at unity p.f. is called 'Critical Excitation'. Then Ia is in phase with V. Now Ia cos φ = constant, cos φ = 1 is at its maximum hence Ia is minimum at this condition.


The Above Conditions Clearly Indicates the Following :

Under Excitation

Lagging p.f.

Eb < V

Over Excitation

Leading p.f.

Eb > V

Critical Excitation

Unity p.f.

Eb ≅ V

Normal Excitation

Lagging p.f.

Eb = V

  • A synchronous motor draws a lagging current when it is under-excited.
  • At critical excitation, it draws a minimum armature current and the power factor is unity.
  • The motor draws a leading current when it is over-excited.


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