Question

In all rotating electrical machine, electrical torque is developed when relative speed between stator field and rotor field is ____________

(a) zero

(b) equal to rotor speed

(c) equal and opposite to rotor speed

(d) dependent upon the type of electrical machine

The question was asked by my college professor while I was bunking the class.

This intriguing question originated from Production of Torque in Non-Salient Pole Machines topic in section Synchronous Machines of Electrical Machines

Answer 1

Right choice is (a) zero

The explanation is: If the relative speed between stator field and rotor field is not zero, then the load angle δr varies with time.

From equation Te = -(Π/8)P^2ΦFrsinδr Nm, we can say that average torque over a complete cycle is zero, implies Te=0.