Chapter 14 Induction Motor Drives Extremely large potential

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 Copyright © 2003 by John Wiley & Sons, Inc. Chapter 14 Induction Motor Drives 14 -2

Per-Phase Representation • Assuming sinusoidal steady state Copyright © 2003 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 Copyright © 2003 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 Copyright © 2003 by John Wiley & Sons, Inc. Chapter 14 Induction Motor Drives 14 -5

Plot of Normalized Rotor Current • It increases with slip and slip frequency Copyright © 2003 by John Wiley & Sons, Inc. Chapter 14 Induction Motor Drives 14 -6

Acceleration Torque at Startup • Intersection represents the equilibrium point Copyright © 2003 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 Copyright © 2003 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 Copyright © 2003 by John Wiley & Sons, Inc. Chapter 14 Induction Motor Drives 14 -9

Frequency at Startup • The torque is limited to limit current draw Copyright © 2003 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 Copyright © 2003 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 Copyright © 2003 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 Copyright © 2003 by John Wiley & Sons, Inc. Chapter 14 Induction Motor Drives 14 -13

Induction Motor Drive Capability Curves • Mainly two regions Copyright © 2003 by John Wiley & Sons, Inc. Chapter 14 Induction Motor Drives 14 -14

Generator Mode of Operation • Rotor speeds exceed the synchronous speed Copyright © 2003 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 Copyright © 2003 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 Copyright © 2003 by John Wiley & Sons, Inc. Chapter 14 Induction Motor Drives 14 -19

PWM-VSI System • Diode rectifier for unidirectional power flow Copyright © 2003 by John Wiley & Sons, Inc. Chapter 14 Induction Motor Drives 14 -20

PWM-VSI System • Options for recovered energy during regenerative braking Copyright © 2003 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 Copyright © 2003 by John Wiley & Sons, Inc. Chapter 14 Induction Motor Drives 14 -25

CSI Drives • Mostly PWM-VSI drives are used Copyright © 2003 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 Copyright © 2003 by John Wiley & Sons, Inc. Chapter 14 Induction Motor Drives 14 -27

Speed Control by Adjusting the Stator Voltage • Highly inefficient in most cases Copyright © 2003 by John Wiley & Sons, Inc. Chapter 14 Induction Motor Drives 14 -28

Controlling the Stator Voltage Magnitude • Results in distorted current and torque pulsations Copyright © 2003 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 Copyright © 2003 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