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SarkariResultOutAdditional Assistant Engineer (Electrical) · 2026

Previous Year Question

What is the condition for maximum efficiency in a DC motor?
  1. Copper losses are zero
  2. Total losses are equal to input power
  3. Mechanical losses are zero
  4. Armature copper losses are equal to constant losses

Correct answer: D

Explanation

For a DC machine operated with essentially constant flux, maximum efficiency occurs when the variable copper loss equals the constant losses. In a motor this is commonly expressed as armature copper loss equal to constant losses.

Formula

I_a²R_a = constant losses

Step-by-step solution

Maximum Efficiency Condition explanatory diagram

Correct Answer

D — Armature copper losses are equal to constant losses

Explanation

For a DC machine operated with essentially constant flux, maximum efficiency occurs when the variable copper loss equals the constant losses. In a motor this is commonly expressed as armature copper loss equal to constant losses.

Key Concept

Maximum Efficiency Condition

Formula

I_a²R_a = constant losses

Exam Point

Maximum efficiency: variable loss = constant loss.

Exam tip

Maximum efficiency: variable loss = constant loss.

Quick method

Maximum efficiency: variable loss = constant loss.