Question

A 3 phase induction motor is driving full load torque which is independent of speed if the line voltage drops to 90% of the rated value,% increase in motor copper losses

Answer 1

phase induction motor is driving full load torque which is independent of speed if the line voltage drops to 90% of the rated value

Answer 2

When T remains constant,  

s₁V₁ ² = s₂ V₂ ²

so, s₂/s₁=(V₁/V₂)²=(1/0.9)²=1.23

we know that , I ₂ ∝ sV , therefore

I’₂/I₂ =s₂V₂/s₁V₁ = 1.2 × 0.9 = 1.107  

Now, Cu losses are nearly proportional to I₂²

∴ Cu loss in the 2nd case/ Cu loss in the 1st case = (I ′₂)²/(I₂)² = 1.1072 = 1.23

Reduction of 10% in line voltage causes about 23% increase in Cu losses.