Test: Electrical Machines- 1 - Electrical Engineering (EE) MCQ

In practice, the interpoles of appropriate polarity are strengthened so that in interpolar zone, the armature cross flux is neutralized and in addition some flux is produced there. This additional flux in the interpolar zone induces rotational emf in the commutated coil in such a direction as to oppose the reactance voltage. So, the resultant emf in the commutated coil would be zero and therefore zero current in that coil would amount to sparkless commutation. This is the reason why interpoles are designed to provide more mmf than the armature mmf in the commutating zone. In practice, the interpole mmf may be 1.2 to 1.3 times the armature mmf per pole.

E_a = V – I_a R_a
E_a = 200 = 220 – I_a × 0.5 ⇒ Ia = 40 A 

If shunt field resistance is more than the critical field resistance, there will be no voltage build up beyond residual voltage but voltage would not be zero.

Refer, harmonic asynchronous (induction) torques. Running stably at low speed (around 1/7^th normal motor speed) is known as Crawling.

If rotor slots are skewed by 2 harmonic polepitches, then effect of slot harmonics (Parasitic torques) can be eliminated.
Since the skewed rotor slot is not parallel to stator slot in an induction motor, there is more leakage reactance. As a result, induction motor has lower starting and maximum torques.
With skewed rotor bars, the revolving flux, in effect, encounters an air gap of uniform reluctance and this results in a uniform torque and quieter operation.