GATE Electrical Engineering - Complete Master Library

 

📘 GATE Previous Year Questions – Induction Motor

These problems follow real GATE pattern. Concept clarity + calculation accuracy required.

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🔹 PYQ 1 – Slip & Rotor Copper Loss

A 4-pole, 50 Hz induction motor runs at 1440 rpm. Rotor input power = 10 kW. Find rotor copper loss. Step 1: Synchronous speed Ns = 120f / P = 120 × 50 / 4 = 1500 rpm Step 2: Slip s = (1500 − 1440)/1500 = 0.04 Step 3: Rotor copper loss = s × Rotor input = 0.04 × 10000

Rotor copper loss = 400 W

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🔹 PYQ 2 – Slip at Maximum Torque

If rotor resistance R2 = 0.3 Ω Rotor reactance X2 = 1.5 Ω Find slip at maximum torque. Formula: s_max = R2 / X2 = 0.3 / 1.5

s_max = 0.2 (20%)

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🔹 PYQ 3 – Speed Change Due to Rotor Resistance

A motor develops maximum torque at 5% slip. If rotor resistance is doubled, find new slip for maximum torque. Since: s_max ∝ R2 New slip = 2 × 0.05

New slip = 10%

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🔹 PYQ 4 – Mechanical Power Developed

Rotor input power = 8 kW Slip = 0.05 Mechanical power: = (1 − s) × Rotor input = 0.95 × 8000

Mechanical power = 7600 W

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🔹 PYQ 5 – Torque Calculation

Motor output power = 15 kW Speed = 1450 rpm Torque: T = (9550 × P) / N = (9550 × 15)/1450

T ≈ 98.8 Nm

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🔹 PYQ 6 – Starting Torque Concept

Statement: Maximum torque is independent of rotor resistance. True or False?

Answer: TRUE

But slip at maximum torque depends on R2. Very common conceptual question.

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🎯 PYQ Focus Areas

• Slip-based power distribution
• Rotor copper loss
• Maximum torque condition
• Effect of rotor resistance
• Torque calculation

Induction Motor = High Probability GATE Questions