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`PETITIONERS' EXHIBIT 1124
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`PETITIONERS' EXHIBIT 1124
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`Electric Motors and Drives
`Fundamentals, types and applications
`Second edition
`
`Austin ~ughes
`Senior Lecturer, Department of Electronic and Electrical
`Engineering, University of Leeds
`
`fi)NEWNES
`
`PAGE50F12
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`

`

`Newnes
`An imprint of Butterworth-Heinemann Ltd
`Linacre House, Jordan Hill, Oxford OX2 SDP
`-@.. A member of the Reed Elsevier group
`
`OXFORD LONDON BOSTON
`MUNICH NEW DELHI SINGAPORE SYDNEY
`TOKYO TORONTO WELLINGTON
`First published J9?~ ~~~NG R t:o
`Reprinted ~~J®l', 1~
`<:J'J'
`Second e iil'l!n J.mN 7 \994
`ghes J\&OPtW
`
`All rights reserved. No par o this publication
`may be reproduced in any material form (including
`photocopying or storing in any medium by electronic
`means and whether or not transiently or incidentally
`to some other use of this publication) without the
`wriucn permission or the copyright bolder e~ccpl in
`accordance with the provisions or the Copyright,
`Designs and Patents Ae1 1988 or under the tenns of a
`lioence issued by the Copyright Licensing Agency Ltd,
`90 Touenhum Court Rond, London, England WlP 9HE.
`Applications for the copyright holder's written permission
`to reproduce any part of this publication should be addressed
`to the publishers
`
`British Library Cataloguing in Publication Data
`Hughes, Austin
`Electric motors and drives - 2nd ed.
`l. Electric motors
`I. Title
`621.46'2
`
`ISBN 0 7506 1741 1
`
`Typeset by Vision Typesetting, Manchester
`Printed and bound in Great Britain by
`Biddies Ltd, Guildford and King's Lynn
`
`PAGE60F12
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`CONTENTS
`
`Preface
`
`1 ELECTRIC MOTORS
`
`Introduction
`
`Producing rotation
`Electromagnetic force
`Magnetic flux and flux
`Force on a conductor
`
`Magnetic Circuits
`Magnetomotive force C
`Electric circuit analogy
`The air-gap
`Air-gap flux densities
`Saturation
`Magnetic circuits in m<
`
`Torque Production
`Magnitude of torque
`Slotting
`
`Specific Loadings and Sp~
`Specific loadings
`Torque and motor vol
`Specific output power ·
`
`Motional EMF
`Power relationships -
`Power relationships - ·
`constant speed
`
`

`

`222 Electric Motors and Drives
`
`Split-phase motors
`The main winding is of thick wire, with a low resistance and high
`reactance, while the auxiliary winding is made of fewer turns of
`thinner wire with a higher resistance and lower reactance (Figure
`6.19(a)). The inherent difference in impedance is sufficient to give
`the required phase-shift between the two currents without
`needing any external elements in series. Starting torque is good
`at typically 1.5 times full-load torque, as shown in Figure
`6.19(b ). As with the capacitor type, reversal is accomplished by
`changing the connections to one of the windings.
`
`Torque
`
`Auxi liary
`Winding
`
`•
`Figure 6.19 Si11gle-phase split-phase induction motor
`
`Speed
`
`b
`
`Shaded pole motors
`There are several variants of this extremely simple, robust and
`reliable cage motor, which predominates for low-power appli(cid:173)
`cations such as hair-dryers, oven fans, tape decks, office
`equipment, display drives etc. A common 2-pole version from
`the cheap end of the market is shown in Figure 6.20.
`The rotor, typically between 1 and 4 em diameter, has a
`die-cast aluminium cage, while the stator winding is a simple
`concentrated coil wound round the laminated core. The stator
`pole is slotted to receive the 'shading ring' which is a single
`short-circuited turn of thick copper or aluminium.
`Most of the pulsating flux produced by the stator winding
`by-passes the shading ring and crosses the air-gap to the rotor.
`
`Figure 6
`
`But so
`becaus;
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`PAGE70F12
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`Operating Characteristics of Induction Motors 223
`
`Laminated
`Stator
`
`Figure 6.20 Shaded-pole induction motor
`
`But some of the flux passes through the shading ring, and
`because it is alternating it induces an e.m.f. and current in the
`ring. The opposing MMF of the ring current diminishes and
`retards the phase of the flux through the ring, so that the flux
`through the ring reaches a peak after the main flux, thereby
`giving what amounts to a rotation of the flux across the face of
`the pole. This far from perfect travelling wave of flux produces
`the motor torque by interaction with the rotor cage. Efficien(cid:173)
`cies are low because of the rather poor magnetic circuit and the
`losses caused by the induced currents in the shading ring, but
`this is generally acceptable when the aim is to minimise first
`cost. Series resistance can be used to obtain a crude speed
`control, but this is only suitable for fan-type loads. The
`direction of rotation depends on whether the shading ring is
`located on the right or left side of the pole, so shaded pole
`motors are only suitable for uni-directional loads.
`
`QUESTIONS ARISING
`• Some cage rotors appear to have bars which are not parallel to
`the axis, but show a slight spiral from one end to the other: why
`is this?
`
`eandrugh
`er turns of
`ce(Figure
`enttogive
`s without
`ueisgood
`in Figure
`plished by
`
`Speed
`
`obust and
`wer appli-
`:ks, office
`·sion from
`10.
`ter, has a
`sa simple
`fhe stator
`ts a single
`
`r winding
`the rotor.
`
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`PAGE80F12
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`224 Electric Motors and Drives
`
`Plate 6.3 Single-phase shaded-pole motor. These motors are produced
`in very large numbers for use in small domestic fans. office equipment
`etc. (Photograph by courtesy of Brook Crompton)
`
`Rotor bars are often 'skewed' (a) to minimise the tendency of the
`rotor and stator teeth to act like a reluctance motor, and produce
`pulsating torques as the rotor turns, and (b) to minimise the
`harmonic effects (see below). A modest measure of skewing has
`little effect on the fundamental motor performance.
`
`• What is 'crawling'?
`The stator MMF wave in an induction motor is not a perfect
`sinewave (see Figure 5.6), but consists of the fundamental wave
`and a series of (smaller) 'space harmonic' fields. So in addition
`to the main travelling field there are harmonic fields whose
`synchronous speeds are inversely proportional to their order.
`For example a 4-pole, 50 Hz motor will have a main field
`rotating at 1500 rev/min, but in addition there may be a 5th
`harmonic (20-pole) field rotating in the reverse direction at 300
`rev/min, a 7th harmonic (28-pole) field rotating forwards at 214
`
`PAGE9 OF 12
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`

`

`Operating Characteristics of Induction Motors 225
`
`rev/min, etc. These 'space harmonics' are minimised by stator
`winding design, but can seldom be eliminated. If the rotor
`reacts to these fields, there will be unwanted dips in the
`torque-speed curve at the synchronous speed of the harmonic
`field. In extreme cases the motor might for example stabilise on
`the 7th harmonic, and 'crawl' at about 200 rev /min, rather than
`running up to 4-pole speed. Skewing of the rotor bars (see
`above) helps to limit the rotor's response to space harmonics.
`
`• I want to use a 25 kW (33.5 h.p.), 550 V, 60 Hz, 3-phase,
`4-pole, 1750 revjmin induction motor on a 50 Hz supply. What
`voltage is needed, and what speed and power will be appropri(cid:173)
`ate?
`To get the best out of the motor we must ensure that the air-gap
`flux is at the designed value: if it is too high the magnetic circuit
`will saturate, while if it is too low the performance will obviously
`suffer. Looking at equation 5.5 we see that if we reduce the
`frequency, we will need to reduce the voltage in proportion, so
`the correct voltage at 50 Hz is 50/60 x 550 = 458 V.
`To define the new full load we should aim for conditions in
`the rotor to be the same as they were originally. By scaling the
`voltage as above, we ensure that the rotating field has the same
`amplitude, so all we need to do in order for the rotor to behave
`the same is to specify the same absolute value of slip speed. The
`synchronous speed at 60Hz is 1800 rev /min, so the original slip
`speed was 1800- 1750 = 50 revjmin. The new synchronous
`speed is 1500 rev /min, so to keep the same rated slip speed the
`new rated speed will be 1450 revjrnin.
`The torque at 1450 rev/min will be the same as it was
`originally at 17 50 rev /min, because the flux density and slip are
`unchanged. Hence the new power output is given by
`
`1450
`pout= 1750 X 25 = 20.7 kW or 27.8 h.p.
`
`The rotor losses will be the same as before because conditions
`on the rotor are identical. The full-load stator current will also
`be the same because both the magnetising and torque compo-
`
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`226 Electric Motors and Drives
`
`nents are unchanged. Hence the stator copper loss is the same;
`but the stator iron loss will reduce somewhat because of the
`lower frequency, so the total losses will reduce a little. If the
`motor is totally enclosed, the lower speed will mean that there
`will be less air flow to cool the stator, so a modest de-rating may
`be advisable; the best thing would be to try it.
`
`• The diagram (Fig 6.16) indicates that the starting torque of a
`wound-rotor induction motor can be increased by adding
`resistance in the rotor circuit. This seems very surprising, as I
`would expect less torque to be produced if the rotor current is
`reduced. Does adding rotor resistance really increase the
`torque?
`To make sense of what admittedly seems paradoxical, we need
`to recall two points. Firstly, the flux density in the air-gap is
`determined primarily by the voltage and frequency applied to
`the stator, so the flux density does not change appreciably
`when we alter the rotor resistance. And secondly, the torque
`depends not only on the amplitude of the induced rotor current
`wave, but also on the space phase of the current wave with
`respect to the flux density wave. Increasing the rotor resistance
`at standstilJ reduces the magnitude of the rotor current wave,
`but this is more than compensated by the fact that it
`simultaneously moves the rotor current wave more into phase
`with the flux density wave, thereby producing more torque.
`• How can I tell if an induction motor driving a pump is working
`at full load without using costly instruments?
`If the supply voltage and frequency are correct, and the motor
`is running at its rated speed (as given on the rating plate) it will
`be working at full load. Checking the speed is sometimes
`difficult when there is no shaft extension, because a hand-held
`tachometer cannot be used. A convenient method is to use
`either a calibrated stroboscope shining directly onto some of
`the rotating parts, or one of the digital speed indicators which
`reflect light from a piece of shiny tape stuck onto the shaft.
`
`• How can I be sure in which direction an induction motor will
`rotate? I need to know in advance of commissioning the motor
`
`PAGE 11 OF 12
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`Operating Characteristics of Induction Motors 227
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`because it will not be easy to swap over the heavy~current
`cables once the motor is hooked up.
`If the manufacturer's data does not indicate the phase sequence
`needed for a particular direction of rotation, there is no easy
`alternative but to carry out a trial. This is not difficult as long as
`the motor is not coupled to the load, because a low~power
`supply at reduced voltage should provide enough torque to
`cause the rotor to begin turning.
`
`• In many cage rotors it looks as if the bars are not insulated from
`the laminations. Does this mean that induced currents flow in
`the laminations?
`There is often no .insulation, but very little axial current can
`flow in the laminations since the surface of each is insulated.
`Some current can flow circumferentially, especially in the
`laminations near to the ends of the rotor, but the resistance of
`the bars and end-rings is so low in comparison with the paths in
`the iron that the currents in the latter are very small.
`
`