EE-362 ELECTROMECHANICAL ENERGYCONVERSION-II

Starting Methods and Operating Modes ofInduction Machines

Ozan Keysan

keysan.me

Office: C-113 • Tel: 210 7586

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Induction Motors

20 MW Induction Motor for HMS Queen Elizabeth3 / 26

Induction Motors

Single Line Diagram of HMS Queen Elizabeth Power System4 / 26

Typical Torque Curve of an Induction Motor

Torque Graphs

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What is wrong with directly connecting motor to aconstant voltage supply (i.e. grid)?

High Start-up Current

Low(or high) Torque at Start-up

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Starting Methods (Source Side):

1- Use Auto-transformer (Variac)

Apply a smaller voltage during start-up, and increase it gradually.

Remember: Torque ∝ V2

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Starting Methods (Source Side):

2- Use ( ) switchY − Δ

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Starting Methods (Source Side):

3- Soft Starters

Soft Starters, Soft starter vs Motor Drive

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Starting Methods (Source Side):

4- Induction Motor Drives

Variable Voltage-Frequency Source (or Variable Frequency Drives)

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Starting Methods (Source Side):

4- Induction Motor Drives

Variable Voltage-Frequency Source (or Variable Frequency Drives)

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Starting Methods Comparison:

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Starting Methods (Machine Side):

How to increase the starting torque, but reduce thestarting current at the same time?

=smaxT

r′

2

+ ( +R2

1X1 X ′

2)2‾ ‾‾‾‾‾‾‾‾‾‾‾‾‾‾√

= 3Tmax0.5V 2

ωs

1

( +R1 + ( +R2

1X1 X ′

2)2‾ ‾‾‾‾‾‾‾‾‾‾‾‾‾‾√

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Starting Methods (Machine Side):

Increase rotor resistance ( )

Torque Graphs

r′

2

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Starting Methods (Machine Side):

Increase rotor resistance ( )r′

2

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How to modify ( )?

1 - Add External Resistor: Easy for wound rotorinduction motors by using external resistance

r′

2

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How to modify ( ) for squirrel cage

motors?

2 - Use Deep Rotor Bars: Utilize rotor resistancechange with skin e�ect

r′

2

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How to modify ( ) for squirrel cage

motors?

2 - Use Deep Rotor Bars: Utilize rotor resistancechange with skin e�ect

r′

2

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Rotor Bar Shapes

For curious students: Rotor design, Motor design classes

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Complete Torque Characteristics

Can slip be larger than 1, or can it be less than 0?

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Operation Modes of Induction Motors

1- Motoring

2- Generating

3- Braking

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Motoring

Power Flow: Electrical to Mechanical

Slip: 0 < s < 1

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Generating

Power Flow: Mechanical to Electrical

Slip: s < 0

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Braking (Plugging)

Power Flow: Mechanical+Electrical to Heat

Slip: s > 1

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Machine Dynamics

Torque Balance Equation

− − = JTelec Tload Tf riction

dω

dt

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Four-Quadrant Operation

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