Chapter 14
Induction Motor Drives
• Extremely large potential as adjustable speed drives
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-1
Pump Application: Adjustable Flow rate
• Fixed versus adjustable speed drive
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-2
Per-Phase Representation
• Assuming sinusoidal steady state
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-3
Important Relationships in an Induction
Machine
• Not necessary for our purposes to know the
exact expressions for constants used here
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-4
Torque-Speed Characteristics
• The linear part of the characteristic is utilized in
adjustable speed drives
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-5
Plot of Normalized Rotor Current
• It increases with slip and slip frequency
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-6
Acceleration Torque at Startup
• Intersection represents the equilibrium point
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-7
Torque Speed Characteristics at various
Frequencies of Applied Voltage
• The air gap flux is kept constant
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-8
Adjusting Speed of a Centrifugal Load
• The load torque is proportional to speed squared
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-9
Frequency at Startup
• The torque is limited to limit current draw
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-10
Increasing Speed at Startup
• The ramp rate of frequency depends on load inertia
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-11
Phasor Diagram at Small Value of Slip
Frequency
• The rotor branch is assumed to be purely
resistive
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-12
Voltage Boost to Keep Air Gap Flux at its
Rated Value
• Depends on the torque loading of the machine
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-13
Induction Motor Drive Capability Curves
• Mainly two regions
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-14
Generator Mode of Operation
• Rotor speeds exceed the synchronous speed
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-15
Regenerative Braking Mode to Slow Down
• Machine is made to go into the generator mode
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-16
Per-Phase Equivalent Circuit at Harmonic
Frequencies
• The magnetizing branch is ignored
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-17
Torque Pulsations due to Harmonics
• Rotations of fields due to the fifth and the
seventh harmonics are in opposite directions
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-18
Classification of Converter Systems
• PWM-VSI is now most commonly use
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-19
PWM-VSI System
• Diode rectifier for unidirectional power flow
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-20
PWM-VSI System
• Options for recovered energy during
regenerative braking
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-21
General-Purpose Speed Controller
• High dynamic performance is not the objective
here
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-22
Change in Switching Frequency based on the
required Fundamental Frequency
• Can be significant in large power ratings
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-23
Field-Oriented Control
• A concise coverage is presented in “Advanced Electric
Drives: Analysis, Control and Modeling using Simulink” by
N. Mohan (www.MNPERE.com)
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-24
Square-Wave VSI Waveforms
• Large peak-peak ripple in currents
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-25
CSI Drives
• Mostly PWM-VSI drives are used
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-26
Comparison of Three Types of Inverter
Systems
• PWM-VSI is by far the most commonly selected
system now
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-27
Speed Control by Adjusting the Stator Voltage
• Highly inefficient in
most cases
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-28
Controlling the Stator Voltage Magnitude
• Results in distorted current and torque pulsations
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-29
Torque-Speed Curves for Wound-Rotor
Machines
• Highly energy-inefficient unless using energy
recovery schemes
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by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-30
Static Slip Recovery
• Applications in very large power ratings where the
speed is to be adjusted over a very limited range
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 14 Induction
Motor Drives
14-31