14-1 copyright © 2003 by john wiley & sons, inc. chapter 14 induction motor drives chapter 14...

Copyright © 2003 by John Wiley & Sons, Inc.

Chapter 14 InductionMotor Drives

14-1

Chapter 14

Induction Motor Drives

• Extremely large potential as adjustable speed drives



14-2

Pump Application: Adjustable Flow rate

• Fixed versus adjustable speed drive



14-3

Per-Phase Representation

• Assuming sinusoidal steady state



14-4

Important Relationships in an Induction Machine

• Not necessary for our purposes to know the exact expressions for constants used here



14-5

Torque-Speed Characteristics

• The linear part of the characteristic is utilized in adjustable speed drives



14-6

Plot of Normalized Rotor Current

• It increases with slip and slip frequency



14-7

Acceleration Torque at Startup

• Intersection represents the equilibrium point



14-8

Torque Speed Characteristics at various Frequencies of Applied Voltage

• The air gap flux is kept constant



14-9

Adjusting Speed of a Centrifugal Load

• The load torque is proportional to speed squared



14-10

Frequency at Startup

• The torque is limited to limit current draw



14-11

Increasing Speed at Startup

• The ramp rate of frequency depends on load inertia



14-12

Phasor Diagram at Small Value of Slip Frequency

• The rotor branch is assumed to be purely resistive



14-13

Voltage Boost to Keep Air Gap Flux at its Rated Value

• Depends on the torque loading of the machine



14-14

Induction Motor Drive Capability Curves

• Mainly two regions



14-15

Generator Mode of Operation

• Rotor speeds exceed the synchronous speed



14-16

Regenerative Braking Mode to Slow Down

• Machine is made to go into the generator mode



14-17

Per-Phase Equivalent Circuit at Harmonic Frequencies

• The magnetizing branch is ignored



14-18

Torque Pulsations due to Harmonics

• Rotations of fields due to the fifth and the seventh harmonics are in opposite directions



14-19

Classification of Converter Systems

• PWM-VSI is now most commonly use



14-20

PWM-VSI System

• Diode rectifier for unidirectional power flow



14-21

PWM-VSI System

• Options for recovered energy during regenerative braking



14-22

General-Purpose Speed Controller

• High dynamic performance is not the objective here



14-23

Change in Switching Frequency based on the required Fundamental Frequency

• Can be significant in large power ratings



14-24

Field-Oriented Control

• A concise coverage is presented in “Advanced Electric Drives: Analysis, Control and Modeling using Simulink” by N. Mohan (www.MNPERE.com)



14-25

Square-Wave VSI Waveforms

• Large peak-peak ripple in currents



14-26

CSI Drives

• Mostly PWM-VSI drives are used



14-27

Comparison of Three Types of Inverter Systems

• PWM-VSI is by far the most commonly selected system now



14-28

Speed Control by Adjusting the Stator Voltage

• Highly inefficient in most cases



14-29

Controlling the Stator Voltage Magnitude

• Results in distorted current and torque pulsations



14-30

Torque-Speed Curves for Wound-Rotor Machines

• Highly energy-inefficient unless using energy recovery schemes



14-31

Static Slip Recovery

• Applications in very large power ratings where the speed is to be adjusted over a very limited range

14-1 copyright © 2003 by john wiley & sons, inc. chapter 14 induction motor drives chapter 14...

Documents