In a synchronous motor, at no-load condition, and with normal excitation the armature current drawn by a synchronous motor is
Right Answer is:
Lags the applied voltage V by a small angle
In a synchronous motor, at no-load condition, and with normal excitation, the armature current drawn by a synchronous motor lags the applied voltage V by a small angle.
Synchronous Motor Phasor Diagrams
Consider a normally-excited synchronous motor (i.e. Eb = V) supplied with fixed excitation. Since field excitation and speed of the motor = synchronous speed are fixed, the value of Eb remains the same.
V = supply voltage/phase
Eb = back e.m.f./phase
Er = Net voltage
Zs = synchronous impedance/phase
(i) Motor on no-load with no losses:-
- When the synchronous motor is on no-load and has no losses, the center lines of rotor poles and stator poles coincide and the torque angle δ is practically 0°.
- Under this condition, the back e.m.f. Eb, is equal and opposite to the supply voltage V as shown in the phasor diagram in Fig.
- Since net voltage Er = V − Eb is zero, the armature current Ia = Er/Zs is zero. This is expected because there is neither load nor motor losses. Now, the motor just floats.
Motor on no-load with losses
- If the motor is on no-load but it has losses, the torque angle increases by a small angle δ.
- Consequently, the vector Eb (its magnitude is constant as excitation is fixed) falls back (vectors are rotating in an anticlockwise direction) by angle δ as shown in Fig.
- The resultant or net voltage Er in the stator causes the stator or armature current Ia = Er/Zs to have some finite value to supply the no-load losses.
- Note that armature current Ia lags behind Er by a small angle θ = tan−1 Xs/Ra. Since Xs >> Ra, Ia, lags Er by nearly 90° (∵ θ = tan−1 Xs/0 = 90°). The phase angle between V and Ia is φo so the no-load power factor is cosφo.
No-load input power/phase = VI, cosφo,
Thus at no-load, the motor takes a small power VI, cosφo/phase from the supply to meet the no-load losses while it continues to run at synchronous speed.
Hence Under no-load conditions with normal excitation, armature current Ia drawn by a synchronous motor lags the applied voltage V by a small angle.
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