Of the four classes of Induction Motor torque speed curves s

Of the four classes of Induction Motor torque speed curves shown which is likely to be the most efficient? Class A Class B Class C Class D

Solution

We know high starting torque is possible only when rotor resistance is high. Which implies that rotor copper loss will be high under full load condition.

Efficiency of induction motor is Eff = Pout/Pin ;

if high rotor losses then Pout will be less for same input power.

Observing four classes of induction motor torque speed curves starting torque of Class A, B are equal and less compared to Class C and D.

Efficiency will be more for Class A and B compare to Class C and D.

Comparing to Class A and B , Class A has higher efficiency because rated output power obtained at low slip compared to Class B. If slip is low then rotor loss will be low and Pout will be high for same input power.

Thus efficiency of Class A induction motor is high compared to B,C and D.

option A.