Question

The flux/pole in a synchronous motor with stator not connected to supply is Φ₁ and when connected to supply it is \(\frac{3}{4}{{\rm{\Phi }}_1}\). The no-load current drawn from the supply under this condition would be
1. lagging the supply voltage
2. leading the supply voltage
3. in phase with the supply voltage
4. zero

Answer 1

Correct Answer - Option 2 : leading the supply voltage

Concept:
In a synchronous motor,

• For a lagging power factor, the motor takes the lagging reactive power from the supply and hence the effect of armature reaction is Magnetizing.
• For a leading power factor, the motor takes the leading reactive power from the supply and hence the effect of armature reaction is demagnetizing (taking leading VAR means supplying lagging VARs i.e. the magnetic energy) and there becomes deficiency of magnetic energy in the motor.

Application:

The flux/pole in a synchronous motor with stator not connected to supply is Φ₁

The flux per pole when connected to supply it is \(\frac{3}{4}{{\rm{\Phi }}_1}\)

That means after connecting with supply, the flux per pole is decreased. This indicates that the armature current is demagnetizing in nature.

Therefore, the motor operates under leading power factor i.e. the no-load current drawn from the supply under this condition would be leading the supply voltage.