Synchronous Motor - Effect of Excitation on Armature Current & Power Factor


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


     When excitation is changed, the magnitude of induced emf changes. The load angle, is also constant for a constant load.

Let,
 
   Er  =  Resultant voltage between  and  =  IZa

   θ  =  Internal angle  =  Angle between r and  =  Tan -1 ( Xs  / Ra )

   I =  Armature current 

   φ  =  Power factor angle  =  Angle between V̅ and I̅a

                                           

(i) When the excitation is reduced in such a way that Eb  =  V. Shown in below figure. Ia lags ' V ' by an angle φ.

Synchronous Motor - Effect of Excitation on Armature Current & Power Factor

(ii) When the excitation is reduced in such a way that Eb  <  V, the motor is said to under excited. Resultant  ' Er ' advanced in clockwise direction and an angle ' θ ' is constant, Ia also moves in clockwise direction.

Synchronous Motor - Effect of Excitation on Armature Current & Power Factor

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


(iii) When excitation is increased in such a way that Eb  >  V, the motor is said to be over excited. Resultant  ' Er ' is moved in anti-clockwise direction and so the ' Ia ' also moves in anti-clockwise direction ( as angle between Er and  Ia is  θ ). Current Ia leads voltage by an angle ' φ '.

Synchronous Motor - Effect of Excitation on Armature Current & Power Factor

     As excitation goes on increasing. Er and Ia go on moving in 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.


(iv) For unity power factor. Eb is slightly greater than V ( Eb  ≈  V ). This is shown in below figure. The excitation for which the power factor of the motor is unity is called as normal excitation. Then Ia is in phase with V. 

Synchronous Motor - Effect of Excitation on Armature Current & Power Factor

     Now Ia Cos φ  =  Constant, Cos φ  = 1 is at its maximum hence Ia, is minimum for normal excitation.



The Above Conditions Clearly Indicates the Following 


(i) A synchronous motor draws a lagging current, when it is under excited.

(ii) At normal excitation, it draws minimum armature current and power factor is unity. 

(iii) The motor draws a leading current, when it is over excited.




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