SMJE 1103�Electrical Power System

Induction Motor

References & Circuit Simulator

Can be downloaded from;

​

1. http://elearning.utm.my/22232/
2. https://people.utm.my/rasli1/

​

Circuit simulator: Matlab

Observation

• [9]p171 – p203
• [1]p432 – p445
• [10]p279 – p281
• [4]p723 – p751
• [12]p379 – p395

​

Induction Motor

Scope of discussion

• Structure
• Basic concept
• Equivalent Circuit
• Power and Torque
• Speed Control
• Testing

Structure

Squirrel Cage – no winding and slip ring

Wound Rotor – the winding connected to slip rings

Basic Concept

Induced Torque:

• Speed of magnetic field
• Induced Voltage
• Induced Torque

Rotor Slip:

Rotor Frequency

Problems/Examples

Look at the given problems/examples;

-[9]p171, problem 7-3

-[9]p186, problem 7-15

-[9]p187, problem 7-16

Problems/Examples

Look at the given problems/examples;

-[9]p171, problem 7-2

-[9]p172, problem 7-4

​

Problem

A 208V, 10hp, 4 poles, 50 Hz, Y-connected induction motor has a full-load slip of 5%.

​

1. What is the synchronous speed of this motor?
2. What is the rotor speed of this motor at the rated load?
3. What is the rotor frequency of this motor at the rated load?
4. What is the shaft torque of this motor at the rated load?

[8]p6

Some Important Points on�Basic Structure of Induction Motor

• Synchronous speed
• Rotor speed
• Rotor Frequency
• Speed of magnetic field
• Slip
• Pullout torque
• Motor efficiency

• Rotor current in standstill
• Rotor current in running
• Power factor
• Speed at maximum torque

​

Equivalent Circuit

Rotor voltage – Locked-rotor voltage

Rotor frequency – Frequency of the induced voltage

Problems/Examples

Look at the given problems/examples;

-[9]p194, problem 7-19

-[9]p203, problem 7-25

​

Problems/Examples

Look at the given problems/examples;

-[9]p175, problem 7-8

-[9]p184, problem 7-14

​

Some Important Points on�Equivalent Circuit

• Stator current
• Conversion power
• Output power
• Induced torque
• Load torque

• Air gap power
• Stator core loss
• Rotor core loss
• Motor speed
• Efficiency

​

Quiz 19

A 460V, 25hp, 60Hz, 4 pole, Y-connected induction motor has the following impedances in ohms per phase referred to stator circuit:

​

​

The total rotational losses are 1100W and are assumed to be constant. The core loss is lumped in with the rotational losses. For a rotor slip of 2.2% at rated voltage and rated frequency, find the motor’s

1. speed d) P_conv and P_out
2. stator current e) τ_ind and τ_load
3. power factor f ) efficiency

Power and Torque

Problems/Examples

Look at the given problems/examples;

-[10]p279- example 13-5

-[10]p280, example 13-6

-[9]p177, problem 7-9

Some Important Points on�Torque and Losses

• Air gap loss
• Core rotor loss
• Conversion power
• Output power
• Mechanical power

• Efficiency
• Core stator loss
• Copper loss
• Line current
• Efficiency

​

Quiz 20

A 480V, 60Hz, 50hp, 3 phase induction motor is drawing 60A at 0.85 Pf lagging. The stator copper losses are 2 kW, and the rotor losses are 700W. The friction and windage losses re 600W, the core losses are 180W, and the stray losses are negligible. Find:

1. The air gap power P_AG
2. The power converted _Pconv
3. The output power P_out
4. The efficiency of the motor

Speed Control

• Two methods
• Varying stator and rotor magnetic field speed

- electrical frequency or changing the number of poles

2. Varying slip

- varying rotor resistance or terminal voltage

Some Important Points on�Motor Speed

• Air gap loss
• Core rotor loss
• Conversion power
• Output power
• Mechanical power

• Efficiency
• Core stator loss
• Copper loss
• Line current
• Efficiency

​

Problems

[9]p172, Problem 7-5

[9]p173, Problem 7-6

​

Quiz 21

The power input to the rotor of a 440 V, 50 Hz, 3-phase, 6-pole induction motor is 60 kW. It is observed that the rotor emf makes 90 complete cycle per minute. Calculate;

1. the slip
2. rotor speed
3. rotor Cu loss per phase
4. mechanical power developed
5. the rotor resistance per phase if rotor current is 60 A

Testing

There are basically 3 types of test in modeling induction motor parameters:

1. No-load test
2. DC test
3. Locked rotor test or blocked rotor test

​

Equivalent circuit element – R₁, R₂, X₁, X₂ and X_M

Testing (no load)

Measuring rotational losses and providing info about its magnetizing current (X₁ + X_M)

Testing (DC test)

To find R1 (stator resistance).

1. Dc voltage is applied and adjusted to rated condition.
2. Voltage and current flow are recorded.

Testing (Locked rotor)

Steps:

1- locked

2- AC voltage is applied and currents flow is adjusted to full load condition

3- Measure voltage, current and power flow

Problem

[9]p171, Problem 7-1

[9]p173, Problem 7-7

[9]p180, Problem 7-11

Quiz 22

Find the equivalent of this motor.