Induction Motor MCQs Part 16

Contents

- 1 The stator of 3-phase induction motor produces magnetic field that is
- 2 A 430 V, 50 Hz, 50 HP, 3-phase induction motor is drawing 60 A at 0.85 lagging PF
- 3 A415 V, 50 Hz, star-connected, 3-phase squirrel cage induction motor is operated from 415V, 60 Hz supply
- 4 What happens to the power factor of rotor circuit of 3 phase induction motor when its slip increases
- 5 The approximate efficiency of a 60 Hz, 6 pole, 3 phase induction motor running at 1080 is

## The stator of 3-phase induction motor produces magnetic field that is

The stator of 3-phase induction motor produces magnetic field that is:

- Steady in nature
- Rotating
- Alternating
- None of above

Correct answer: 2. Rotating

## A 430 V, 50 Hz, 50 HP, 3-phase induction motor is drawing 60 A at 0.85 lagging PF

A 430 V, 50 Hz, 50 HP, 3-phase induction motor is drawing 60 A at 0.85 lagging PF. The stator copper losses are 2 kW and rotor copper losses are 800 watts. The friction and windage losses are 700 W and stator iron losses are 2000 W. The motor output efficiency is:

- 83%
- 92.5%
- 98%
- None of these

Correct answer: 1. 83%

Solution:

The input to 3-phase induction motor is electric power.

The air-gap power is just input power to induction motor minus the stator copper losses and stator iron losses. This is power that is transferred to rotor across air-gap.

Input power (Pin) = √3 VL * IL cos φ = √3 * 430 * 50 * 0.85 = 31653.2285083212 = 31.7 kW

——————-

Air gap power Pag = Pin – Stator losses = 31.7 – 2 – 2 = 27.7 kW

——————-

Pconv = Pag – Rotor Cu losses = 27.7 – 0.8 = 26.9 kW

——————-

Pout = Pconv -Friction losses – Windage losses

Pout = 26.9 – 0.7 = 26.2 kW

——————-

Motor efficiency = Pout/Pin * 100 = 26.2/31.7 * 100 = 83%

## A415 V, 50 Hz, star-connected, 3-phase squirrel cage induction motor is operated from 415V, 60 Hz supply

A415 V, 50 Hz, star-connected, 3-phase squirrel cage induction motor is operated from 415V, 60 Hz supply. The torque that motor can now provide while drawing rated current from supply:

- Increases
- Decreases
- Remains the same
- Depends on rotor resistance

Correct answer: 2. Decreases

In the present scenario, the frequency increases from 50 Hz to 60 Hz, while supply voltage remains the same. As the supply frequency increases, the synchronous speed and motor speed increases. (Ns and N). Motor torque T ∝ 1/N. When motor speed increases, the motor torque decreases.

## What happens to the power factor of rotor circuit of 3 phase induction motor when its slip increases

What happens to the power factor of rotor circuit of 3 phase induction motor when its slip increases:

- P.F increases
- P.F decreases
- P.F remains constant
- P.F is independent of slip

Correct answer: 2. P.F decreases

Explanation:

Rotor P.F = Rotor Resistance/Rotor reactance

With an increase in slip the rotor reactance increases (due to increase in frequency) while the rotor resistance is constant. Thus with increase of slip, the P.F decreases

## The approximate efficiency of a 60 Hz, 6 pole, 3 phase induction motor running at 1080 is

The approximate efficiency of a 60 Hz, 6 pole, 3 phase induction motor running at 1080 is:

- 80%
- 85%
- 90%
- 95%

Correct answer: 3. 90%

Synchronous speed = 120*f/P = 1200 RPM

Fractional slip, s = (Ns – N)/Ns = (1200 – 1080)/ 1200 = 0.1

Approximate efficiency = 1 – s = 1 – 0.1 = 0.9 = 90%