Hybrid motor

A cross-sectional view of a typical 1.8 hybrid motor is shown in Figure 9.6. The stator has eight main poles, each with five teeth, and each main pole carries a simple coil. The rotor has two steel end-caps, each with 50 Figure 9.6 Hybrid (200 steps per revolution) stepping motor. The detail shows the rotor and stator tooth alignments, and indicates the step angle of 1.8 Figure 9.6 Hybrid (200 steps per revolution) stepping motor. The detail shows the rotor and stator tooth alignments, and...

Inverter waveforms

When we looked at the converter-fed d.c. motor we saw that the behaviour was governed primarily by the mean d.c. voltage, and that for most purposes we could safely ignore the ripple components. A similar ap proximation is useful when looking at how the inverter-fed induction motor performs. We make use of the fact that although the actual voltage waveform supplied by the inverter will not be sinusoidal, the motor behaviour depends principally on the fundamental (sinusoidal) component of the...

Rotor induced emf current and torque

The rate at which the rotor conductors are cut by the flux - and hence their induced e.m.f. - is directly proportional to the slip, with no induced e.m.f. at synchronous speed (s 0) and maximum induced e.m.f. when the rotor is stationary (s 1). Figure 5.12 Variation of rotor induced e.m.f and frequency with speed and slip The frequency of rotor e.m.f. is also directly proportional to slip, since the rotor effectively slides with respect to the flux wave, and the higher the relative speed, the...

Chopperfed Dc Motor Drives

If the source of supply is d.c. (for example in a battery vehicle or a rapid transit system) a chopper-type converter is usually employed. The basic operation of a single-switch chopper was discussed in Chapter 2, where it was shown that the average output voltage could be varied by periodically switching the battery voltage on and off for varying intervals. The principal difference between the thyristor-controlled rectifier and the chopper is that in the former the motor current always flows...

Rotor construction

Two types of rotor are used in induction motors. In both the rotor 'iron' consists of a stack of steel laminations with evenly spaced slots punched around the circumference. As with the stator laminations, the surface is coated with an oxide layer, which acts as an insulator, preventing unwanted axial eddy currents from flowing in the iron. The cage rotor is by far the most common each rotor slot contains a solid conductor bar and all the conductors are physically and electrically joined...

Scaling down the excitation problem

We can get to the essence of the matter by imagining that we take a successful design and scale all the linear dimensions by half. We know that to fully utilise the iron of the magnetic circuit we would want the air-gap flux density to be the same as in the original design, so because the air-gap length has been halved the stator MMF needs to be half of what it was. The number of coils and the turns in each coil remains as before, so if the original magnetising current was Im, the magnetising...

Review Questions

The first nine questions test general understanding questions 10 to 18 are numerical problems based mainly on the equivalent circuit questions 19 to 26 are discursive questions related to d.c. machines and the remaining questions are more challenging, with an applications bias. 1) What is the primary (external) parameter that determines the speed of an unloaded d.c. motor 2) What is the primary external factor that determines the steady-state running current of a d.c. motor, for any given...

Steadystate operation Importance of achieving full flux

Three simple relationships need to be borne in mind to simplify understanding of how the inverter-fed induction motor behaves. Firstly, we established in Chapter 5 that for a given induction motor, the torque developed depends on the strength of the rotating flux density wave, and on the slip speed of the rotor, i.e. on the relative velocity of the rotor with respect to the flux wave. Secondly, the strength or amplitude of the flux wave depends directly on the supply voltage to the stator...

Selfsynchronous closedloop operation

In the open-loop scheme outlined above, the frequency of the supply to the motor is under the independent control of the oscillator driving the switching devices in the inverter. The inverter has no way of knowing whether the rotor is correctly locked-on to the rotating field produced by the stator, and if the pull-out torque is exceeded, the motor will simply stall. In the self-synchronous mode, however, the inverter output frequency is determined by the speed of the rotor. More precisely, the...

Significance of equivalent circuit parameters

If the study of transformers was our aim, we would now turn to some numerical examples to show how the equivalent circuit was used to predict performance for example, we might want to know how much the secondary voltage dropped when we applied the load, or what the efficiency was at various loads. But our aim is to develop an equivalent circuit to illuminate induction motor behaviour, not to become experts at transformer analysis, so we will avoid quantitative study at this point. On the other...

Power Electronic Converters For Motor Drives

In this chapter we look at examples of the power converter circuits which are used with motor drives, providing either d.c. or a.c. outputs, and working from either a d.c. (battery) supply, or from the conventional a.c. mains. The treatment is not intended to be exhaustive, but should serve to highlight the most important aspects which are common to all types of drive converters. Although there are many different types of converters, all except very low-power ones are based on some form of...

Torque per unit volume

For motors with similar cooling systems, the rated torque is approximately proportional to the rotor volume, which in turn is roughly proportional to the overall motor volume. This stems from the fact that for a given cooling arrangement, the specific and magnetic loadings of machines of different types will be more or less the same. The torque per unit length therefore depends first and foremost on the square of the diameter, so motors of roughly the same diameter and length can be expected to...

Performance of chopperfed dc motor drives

We saw earlier that the d.c. motor performed almost as well when fed from a phase-controlled rectifier as it does when supplied with pure d.c. The chopper-fed motor is, if anything, rather better than the phase-controlled, because the armature current ripple can be less if a high chopping frequency is used. Typical waveforms of armature voltage and current are shown in Figure 4.13(c) these are drawn with the assumption that the switch is ideal. A chopping frequency of around 100 Hz, as shown in...

Thyristor Dc Drives General

Drive Schematic Diagram

For motors up to a few kilowatts the armature converter can be supplied from either single-phase or three-phase mains, but for larger motors three-phase is always used. A separate thyristor or diode rectifier is used to supply the field of the motor the power is much less than the armature power, so the supply is often single-phase, as shown in Figure 4.1. The arrangement shown in Figure 4.1 is typical of the majority of d.c. drives and provides for closed-loop speed control. The function of...

Harmonic effects skewing

A further cautionary note in connection with the torque-speed curves shown in this and most other books relate to the effects of harmonic airgap fields. In Chapter 5, it was explained that despite the limitations imposed by slotting, the stator winding magnetic flux MMF is remarkably close to the ideal of a pure sinusoid. Unfortunately, because it is not a perfect sinusoid, Fourier analysis reveals that in addition to the predominant fundamental component, there are always additional unwanted...

Base speed and field weakening

Field Weakening Constant Power

Returning to our consideration of motor operating characteristics, when the field flux is at its full value the speed corresponding to full armature voltage and full current i.e. the rated full-load condition is known as base speed see Figure 3.10 . The motor can operate at any speed up to base speed, and at any torque current up to the rated value by appropriate choice of armature voltage. This full flux region of operation is indicated by the shaded area Oabc in Figure 3.10, and is often...

Double cage rotors

Double cage rotors have an outer cage made up of relatively high resistivity material such as bronze, and an inner cage of low resistivity, usually copper, as shown in Figure 6.10. The inner cage is sunk deep into the rotor, so that it is almost completely surrounded by iron. This causes the inner bars to have a much higher leakage inductance than if they were near the rotor surface, so that under starting conditions when the induced rotor frequency is high their inductive reactance is very...

A8 Disturbance Rejection Example Using Dc Machine

Motor Block Diagram Torque Disturbance

We will conclude our brief look at the benefits of feedback by considering an example that illustrates how a closed-loop system combats the influence of inputs or disturbances that threaten to force the output of the system from its target value. We already referred to the matter qualitatively in Section A.2.2, when we looked at how we would drive a car at a constant speed despite variations in wind or gradients. Throughout this book the self-regulating properties of electric motors have been...

Armature reaction

In addition to deliberate field-weakening, as discussed above, the flux in a d.c. machine can be weakened by an effect known as 'armature reaction'. As its name implies, armature reaction relates to the influence that the armature MMF has on the flux in the machine in small machines it is negligible, but in large machines the unwelcome field weakening caused by armature reaction is sufficient to warrant extra design features to combat it. A full discussion would be well beyond the needs of most...

Cycloconverter Drives

Cyclo Convertor Output Voltage Waveform

We conclude this chapter with a discussion of the cycloconverter variable-frequency drive, which has never become very widespread but is sometimes used in very large low-speed induction motor or synchronous motor drives. Cycloconverters are only capable of producing acceptable output waveforms at frequencies well below the mains frequency, but this, coupled with the fact that it is feasible to make large induction or synchronous motors with high-pole numbers e.g. 20 means that a very low-speed...

Ideal transformer noload condition flux and magnetising current

Ideal Transformer

We will begin by asking how the ideal transformer behaves when its primary winding is connected to the voltage source as shown in Figure 7.2, but the secondary is open circuited. This is known as the no-load Figure 7.2 Single-phase transformer supplying secondary load Z Figure 7.2 Single-phase transformer supplying secondary load Z Figure 7.3 No-load condition i.e. secondary open-circuited , with secondary winding omitted for the sake of clarity Figure 7.3 No-load condition i.e. secondary...

Torque Production

Having designed the magnetic circuit to give a high flux density under the poles, we must obtain maximum benefit from it. We therefore need to arrange a set of conductors, fixed to the rotor, as shown in Figure 1.11, and to ensure that conductors under a N-pole at the top of Figure 1.11 carry positive current into the paper , while those under the S-pole carry negative current. The tangential electromagnetic 'BIl' force see equation 1.2 on all the positive conductors will be to the left, while...

Field produced by each phase winding

Crompton Phase Motor Under Binding

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...

Graphical interpretation via phasor diagram

Locus Diagram Circuit

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...

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

Closed Loop Speedometer Block Diagram

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

Deep Bar Rotor

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

Upper Brush Motor

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...

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

Transformer 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

Quadrant Motor

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

Synchronous Motor Phasor Diagram

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

Magnetic Circuit With Air Gap

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...