Field produced by each phase winding
The aim of the winding designer is to arrange the layout of the coils so that each phase winding, acting alone, produces an MMF wave and hence an air-gap flux wave of the desired pole number, and with a sinusoidal variation of amplitude with angle. Getting the desired pole number is not difficult we simply have to choose the right number and pitch of coils, as shown by the diagrams of an elementary 4-pole winding in Figure 5.3. In Figure 5.3 a we see that by positioning two coils each of which...
Main airgap flux and leakage flux
Broadly speaking the motor designer shapes the stator and rotor teeth to encourage as much as possible of the flux produced by the stator to pass Figure 5.7 Resultant air-gap flux density wave produced by a complete 3-phase, 4-pole winding at three successive instants in time Figure 5.7 Resultant air-gap flux density wave produced by a complete 3-phase, 4-pole winding at three successive instants in time right down the rotor teeth, so that before completing its path back to the stator it is...
Graphical interpretation via phasor diagram
We will look at the current phasor diagram as the slip is varied, for two motors, both having the same leakage reactance, XT. One motor will be representative of the 'low-resistance' end of the scale R2 0.1 XT while the other will represent the 'high-resistance' end R2 XT . As before, the voltage and frequency are constant throughout. The reason for choosing total leakage reactance as the common factor linking the two motors is simply that the current loci see below are then very similar, and...
Torquespeed Characteristics Constant Vf Operation
When the voltage at each frequency is adjusted so that the ratio V f is kept constant up to base speed, and full voltage is applied thereafter, a family of torque-speed curves as shown in Figure 8.3 is obtained. These curves are typical for a standard induction motor of several kW output. As expected, the no-load speeds are directly proportional to the frequency, and if the frequency is held constant, e.g. at 25 Hz in Figure 8.3, the speed drops only modestly from no-load point a to full-load...
Vector Fieldoriented Control
Where very rapid changes in speed are called for, however, the standard inverter-fed drive compares unfavourably with d.c. drive. The superiority of the d.c. drive stems firstly from the relatively good transient response of the d.c. motor, and secondly from the fact that the torque can be directly controlled even under transient conditions by controlling the armature current. In contrast, the induction motor has inherently poor transient performance. For example, when we start an unloaded...
Synchronous Motors
In Chapter 5 we saw that the 3-phase stator windings of an induction motor produce a sinusoidal rotating magnetic field in the air-gap. The speed of rotation of the field the synchronous speed was shown to be directly proportional to the supply frequency, and inversely proportional to the pole number of the winding. We also saw that in the induction motor the rotor is dragged along by the field, but that the higher the load on the shaft, the more the rotor has to slip with respect to the field...
Injection braking
This is the most widely used method of electrical braking. When the 'stop' button is pressed, the 3-phase supply is interrupted, and a d.c. current is fed into the stator via two of its terminals. The d.c. supply is usually obtained from a rectifier fed via a low-voltage high-current transformer. We saw earlier that the speed of rotation of the air-gap field is directly proportional to the supply frequency, so it should be clear that since d.c. is effectively zero frequency, the air-gap field...
Series motor steadystate operating characteristics
The series connection of armature and field windings Figure 3.13 a means that the field flux is directly proportional to the armature current, and the torque is therefore proportional to the square of the current. Reversing the direction of the applied voltage and hence current therefore leaves the direction of torque unchanged. This unusual property is put to good use in the universal motor, but is a handicap when negative braking torque is required, since either the field or armature...
A22 Closedloop systems
To illustrate the origin and meaning of the term 'closed-loop' we will consider another familiar activity, that of driving a car, and in particular we will imagine that we are required to drive at a speed of exactly 50 km h, the speed to be verified by an auditor from the bureau of standards. The first essential is an accurate speedometer, because we must measure the output of the 'process' if we are to control it accurately. Our eyes convey the 'actual speed' signal from the speedometer dial...
Deep bar rotors
The deep bar rotor has a single cage, usually of copper, formed in slots which are deeper and narrower than in a conventional single-cage design. Construction is simpler and therefore cheaper than in a double cage rotor, as shown in Figure 6.11. The deep bar approach ingeniously exploits the fact that the effective resistance of a conductor is higher under a.c. conditions than under d.c. conditions. With a typical copper bar of the size used in an induction motor rotor, the difference in...
Conventional Dc Motors
Until the 1980s the conventional brushed d.c. machine was the automatic choice where speed or torque control is called for, and large numbers remain in service despite a declining market share that reflects the move to inverter-fed induction motors. Applications range from steel rolling mills, railway traction, to a very wide range of industrial drives, robotics, printers, and precision servos. The range of power outputs is correspondingly wide, from several megawatts at the top end down to...
Autotransformer starter
A 3-phase autotransformer is usually used where star delta starting provides insufficient starting torque. Each phase of an autotransformer consists of a single winding on a laminated core. The mains supply is connected across the ends of the coils, and one or more tapping points or a sliding contact provide a reduced voltage output, as shown in Figure 6.3. The motor is first connected to the reduced voltage output, and when the current has fallen to the running value, the motor leads are...
Real transformer approximate equivalent circuit
The justification for the approximate equivalent circuit see Figure 7.11 a rests on the fact that for all transformers except very small ones i.e. for all transformers that would cause serious harm if they fell on ones foot the series elements in Figure 7.10 b are of low impedance and the parallel elements are of high impedance. Actually, to talk of low or high impedances without qualification is nonsense. What the rather loose language in the paragraph above really means is that under normal...
Real transformer on load exact equivalent circuit
The equivalent circuit showing the transformer supplying a secondary load impedance Z2 is shown in Figure 7.10 a . This diagram has been annotated to show how the ideal transformer at the centre imposes the relationships between primary and secondary currents. Provided that we know the values of the transformer parameters we can use this circuit to calculate all the voltages, currents and powers when either the primary or secondary voltage are specified. However, we seldom use the circuit in...
Fourquadrant Operation And Regenerative Braking
As we saw in Section 3.4, the beauty of the separately excited d.c. motor is the ease with which it can be controlled. Firstly, the steady-state speed is determined by the applied voltage, so we can make the motor run at any desired speed in either direction simply by applying the appropriate magnitude and polarity of the armature voltage. Secondly, the torque is directly proportional to the armature current, which in turn depends on the difference between the applied voltage V and the back...
Phasor diagram and Powerfactor control
To see how the magnitude of the e.m.f. influences behaviour we can examine the phasor diagrams of a synchronous machine operating as a motor, as shown in Figure 10.4. The first point to clarify is that our sign convention is that motoring corresponds to positive input power to the machine. The power is given by VI cos f, so when the machine is motoring positive power the angle f lies in the range 90 . If the current lags or leads the voltage by more than 90 the machine will be generating....
Step position error and holding torque
In the previous discussion the load torque was assumed to be zero, and the rotor was therefore able to come to rest with its poles exactly in line with the excited stator poles. When load torque is present, however, the rotor will not be able to pull fully into alignment, and a 'step position error' will be unavoidable. The origin and extent of the step position error can be appreciated with the aid of the typical torque-displacement curve shown in Figure 9.8. The true step position is at the...
Preface
Like its predecessors, the third edition of this book is intended primarily for non-specialist users and students of electric motors and drives. My original aim was to bridge the gap between specialist textbooks which are pitched at a level too academic for the average user and the more prosaic 'handbooks', which are full of useful detail but provide little opportunity for the development of any real insight or understanding. The fact that the second edition was reprinted ten times indicated...
Electric circuit analogy
We have seen that the magnetic flux which is set up is proportional to the MMF driving it. This points to a parallel with the electric circuit, where the current amps that flows is proportional to the EMF volts driving it. In the electric circuit, current and EMF are related by Ohm's Law, which is Current - i.e. I 1.3 For a given source EMF volts , the current depends on the resistance of the circuit, so to obtain more current, the resistance of the circuit has to be reduced. We can make use of...
Austin Hughes
Senior Fellow, School of Electronic and Electrical Engineering, University of Leeds AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO Newnes is an imprint of Elsevier Linacre House, Jordan Hill, Oxford OX2 8DP 30 Corporate Drive, Suite 400, Burlington, MA 01803 First edition 1990 Second edition 1993 Third edition 2006 Copyright 1990, 1993, 2006, Austin Hughes. Published by Elsevier Ltd. All rights reserved The right of Austin Hughes to be...