Electric Drives E-Book

Marcel Jufer

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 124

Veröffentlichungsjahr: 2013

Beschreibung

Electric Drives: Design Methodology

An electric drive that is designed or adapted to a specific application must take into account all the elements of the chain of constituent elements in its use and deployment. In addition to the motor, the transmission, power electronics, control, sensors, and electrical protection systems must be taken into account.

The motor and the transmission can be optimized and designed to obtain the best energy efficiency assessment, in particular for dynamic nodes. An inventory and a characterization of these various components is proposed as part of this book's examination and explanation of the different technology elements, as well as a dynamic model of the system, with the whole system constituting a methodology for integrated electric drive design.

Leseprobe

Table of Contents

Chapter 1. Introduction - Electric Drive Components

1.1. Definition

1.2. Electric drive components

Chapter 2. Driven Bodies

2.1. Function of the driven body

2.2. Reference or rated running

2.3. Transient behavior

2.4. Specifications

Chapter 3. Transmission

3.1. Transmission types and characterization

3.2. Resolution

3.3. Speed adaptation

3.4. Dynamic behavior

3.5. Oscillatory torque

3.6. Position transfer

Chapter 4. Motors

4.1. Characterization

4.2. Rotating and linear motors

4.3. Induction motors

4.4. DC motors

4.5. Synchronous motors

4.6. Variable reluctance motors

4.7. Linear motors

4.8. Piezoelectric motors and actuators

4.9. Appendix - BLDC motor characteristics

Chapter 5. Motors: Characterization

5.1. Characteristics

5.2. Scaling laws

5.3. Parametric expression

Chapter 6. Global Design of an Electric Drive

6.1. Introduction

6.2. Dynamic equations

6.3. Example

6.4. Conclusions

Chapter 7. Heating and Thermal Limits

7.1. Heating importance

7.2. Thermal equations

7.3. Energy dissipated at start-up

7.4. Cooling modes

Chapter 8. Electrical Peripherals

8.1. Adaptation

8.2. Sources

8.3. Voltage adjustment

8.4. Current adjustment devices

Chapter 9. Electronic Peripherals

9.1. Power electronic

9.2. Simple switch

9.3. H bridge

9.4. Element bridge

Chapter 10. Sensors

10.1. Functions and types

10.2. Optical position sensors

10.3. Hall sensors

10.4. Inductive position sensors

10.5. Resolver-type rotating, inductive, contactless sensors

10.6. Other position sensors

10.7. The motor as a position sensor

10.8. Sensor position

10.9. Current sensors

10.10. Protection sensors

Chapter 11. Direct Drives

11.1. Performance limits

11.2. Motor with external rotor

11.3. Example

Chapter 12. Integrated Drives

12.1. Principle

12.2. Realization

Symbols

Indices

Bibliography

Index

First published 2010 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

ISTE Ltd27-37 St George’s RoadLondon SW19 4EUUKJohn Wiley & Sons, Inc.111 River StreetHoboken, NJ 07030USAwww.iste.co.ukwww.wiley.com

© ISTE Ltd 2010

The rights of Marcel Jufer to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Cataloging-in-Publication Data

Jufer, Marcel, 1941-

Electric drives / Marcel Jufer.

p. cm.

Includes bibliographical references and index.

ISBN 978-1-84821-217-6

1. Electric driving. 2. Electric motors. I. Title.

TK4058.J84 2010

621.46--dc22

2010022200

British Library Cataloguing-in-Publication Data

A CIP record for this book is available from the British Library

ISBN 978-1-84821-217-6

Chapter 1

Introduction – Electric Drive Components

1.1. Definition

An electric drive is a system providing an electromechanical conversion using an electric motor and including all the peripherals necessary for transmission, supply and control.

The concept of a system, in opposition to the motor only, is characteristic of the electric drive. The quality of this system is evaluated based on the weakest component.

The electric drive must be adapted to the application considered, using some of its components. The driven body (pump, machine tool, tracer, computer peripheral, robot, etc.), if it is not an integral part of the drive, determines its characteristics via the specifications which follow.

1.2. Electric drive components

The main components of an electric drive are as follows (Figure 1.1):

– the transmission, which enables us to adapt the movement (rotating-linear conversion, for example), the speed, the resolution or the dynamics of the system;

– the electric motor, defined by its external characteristics and by regulation;

– the supply, which enables us to adapt the primary electric source to the motor;

– the command and the regulation which assure the control of the system dynamic behavior (positioning, speed, current, torque, etc.);

– one or several sensors which supply the information necessary for control;

– the safety and protection elements such as thermal protection, over-speed protections, overload, etc.

Figure 1.1.Electric drive components

In a traditional way, each of these elements is designed individually, to enable the widest possible range of applications. The current trend goes towards the integration of some of these components into the motor (transmission, supply, sensor), while increasing the flexibility of use by means of a command with a programmable microprocessor.

The approach of a system analysis of electric drive involves, first of all, a study of components:

– the driven body which enables us to define the specifications and the constraints compulsory for the electric drive;

– the transmission which enables the adaptation of the electric drive to the load;

– the motor, its supply, its command and its peripherals which are strictly connected and conditioned by the desired function.

There are two steps when choosing synthesis:

– the weighting of the diverse choice criteria. Certain factors are a priority in an evident way, while others are more difficult to control. In particular, the economic aspects are strongly connected to specific conditions such as quantities, manufacturing etc., thus with more a priori known difficulty;

– the process of iteration enables a comparison of several variants, so as to make a considered choice that is put into perspective.

The following chapters will cover the analytical aspects, while the final chapters put the focus on synthesis.

Chapter 2

Driven Bodies

2.1. Function of the driven body

Any driven body is characterized by a function: pumping, position transfer, machining, oscillating movement, speed control, etc. The specifications objective is to translate this function into the terms of electric drive. Furthermore, a certain number of constraints, bound to the environment associated with the driven body, can intervene: primary electric source, atmosphere and ambient temperature, dimensional constraints (diameter, length, mass), etc.

2.2. Reference or rated running

Most electric drives can be characterized by the reference or rated running of the load. This corresponds to the torque that the system can permanently supply, without over­heating any of its components. It is related to a reference speed ΩN. This reference speed is not the maximum possible speed. By rule and without particular precision, any drive has to withstand an over-speed of 20% of the rated speed.

[2.1]

Any rotating system is also characterized by a torque or a rated output with:

[2.2]

As per usual, motors are defined by their torque rates and their speed for the small powers; and by their rated output and their speed for the higher powers (1 kW approximately).

For many driven bodies, the power (or the torque) and the rated speed are clearly defined values: a pump is generally working at constant or weakly variable power and speed. On the other hand, for other driven systems, this concept has no sense a priori; only the transient behavior is characterized and the concepts of rated power and speed are only defined in an equivalent way, a posteriori.

2.3. Transient behavior

Transient behavior is defined by an evolution of one or several parameters in a duration lower than or comparable to the biggest time constant of the system. Very frequently, this last one corresponds to the thermal time constant of the motor. Two types of transient behavior are to be considered:

– an exceptional behavior, occurring in a periodicity clearly greater than the thermal time constant. Typically, start up, braking with recovery or a short-term overload are cases of an exceptional transient regime. Generally, it does not influence the design of the motor, instead influencing the design of some peripherals: electric supply, specific command, protection, etc;

– a reference behavior consisting of a succession of periodic transitory regimes such as acceleration, transfer with constant speed, deceleration, dead time. In a given configuration – transmission, supply, control defined – equivalent rated design can be deduced. The rated speed corresponds to the maximum speed. The rated equivalent torque is defined as the one which leads to the same losses, in permanent regime, as the average losses in transient regime.

A value characterizes an intermittent use of an electric drive: it is the rate of use that establishes the ratio between the switch on time and the reference duration, as a rule 300 s. This concept is frequently used for actuators such as switches or pushers, less frequently for motors. Indeed, in this last case, the conditions and the relative duration of start up or braking play an important role, not only defined by the rate of use.

2.4. Specifications

2.4.1. Basic data

The specifications include as a rule the following basic information, enabling us to characterize the load in terms of electric drive:

– the rated or reference torque MN;

– the rated speed ΩN;

– the starting up torque Md;

– the supply voltage and the type of primary supply source UN (AC mains, DC mains, battery, etc.).

These elements constitute the basic data. A lot of additional information allows us to better characterize the system. We can classify them in characteristics of regulation, start up, transitory and relative to the peripheral elements.

2.4.2. Characteristics of regulation

These characteristics are relative to the motor behavior under load. They allow us to define ranges of speed, torque and resolution as well as the required precision.

In general, a domain of speeds is defined by two limit values, by a relative precision and by an acceptable maximum rate of oscillation:

– range of speed Ωmin-Ωmax;

– relative precision pr;

– oscillation rate Ωosc/ΩN.

Regarding the torque, a characteristic of maximum torque to be supplied according to the speed is generally imposed. The oscillatory torque due to reluctant effects or to the switching of the electronic supply is frequently limited:

– characteristic torque-speed under load M(Ω);

– maximum oscillatory torque Mosc;

– or Mosc/MN.

The resolution requirements are generally fixed to the stopping position, and are more rare in dynamic behavior. The resolution is generally fixed by increments (steps per revolution) or by an angle:

– Resolution αp (angle) Np (steps/rev).

– Precision (position difference relative to the step) Δα/αp.

Some specifications also impose constraints in terms of regulation time constants.

2.4.3. Start-up and braking characteristics

The objective of the operation of start-up is to bring the motor into the range of functioning, in terms of speed and torque. This can implicitly be part of the usual domain of regulation, but can distinguish itself by constraints of specific resisting torque, acceleration torque, position transfer time or precision.

In terms of driven body, start-up is essentially characterized by the following parameters:

– the characteristic of resisting torque in the start-up mode Md (Ω);

– the load inertia (or the mass for a linear movement) I me;

– the frequency of the start-up (or the duration separating two start-ups), possibly the number of consecutive start-ups fd;

– the maximum acceptable time of start-up Td max.

Braking is sometimes the object of particular specifications, if it is not performed by slowing down in free mode. It is then characterized in a symmetric way with the start-up mode:

– characteristic of resisting torque in deceleration mode Mf(Ω);

– braking time (or characteristic speed-time) Tf;

– possibilities of energy saving Ωf(t);

– stopping conditions: precision, resolution, oscillation, no overshooting.

2.4.4. Transient characteristics

Some systems are working in a mode corresponding to a succession of transient regimes: axes control of machine tools, robots, plotters, etc. They are generally characterized by a typical cycle. This will generally be defined as follows:

– characteristic of speed according to time Ω (t), v (t);

– corresponding characteristic of load torque Mr (t);

– cycle and timeout time or rate of use, Tc, Tm, τut.

2.4.5. Characteristics of the peripheral devices

The specifications frequently impose certain constraints or characteristics relative to the peripheral devices.

Regarding the electrical supply, the conditions mainly concern the primary source of energy:

– nature of the primary source, type of source (AC or DC, network, battery, etc);

– voltage level UN;

– limit current Imax;

– rate of harmonic maximum rejection τh %;

– possibility of energy recovery or not.

The transmission can be a priori imposed only in principle, in execution (transmission ratio and inertia) or on the contrary left free. In case the transmission is imposed, the following elements have to characterize it:

– transmission ratio Ωm, Ωe Ωm, xe;

– equivalent transmission inertia or mass Jt mt;

– transmission efficiency ηt(M, Ω).

The sensors are sometimes directly integrated into the driven body. In that case, which is relatively rare, all characteristics of the sensor must be specified. In a more general case, the characteristics of the chosen sensor ensue mainly from information relative to the characteristics of regulation and from the requested resolution.

2.4.6. Thermal aspect

Any electric drive generates losses which must be evacuated by the environment. The characteristics and the possibilities of evacuation must be specified. The data necessary for this aspect are as follows:

– evacuation type of the losses:

- natural convection,

- forced convection,

- heat exchanger air-air,

- heat exchanger air-water,

– maximum ambient temperature Tamb (without precision, this value is 40°C);

– insulation class or maximum bearable temperature of the insulation (without precision, class B):

Y 90 [°C]

A 105

E 120

B 130

F155

H180

C220

– particular corrosive, explosive atmospheres;

– degree of protection, corresponding in particular to the resistance to humidity and water.

2.4.7. Constraints

Unlike the characteristics, the constraints have a restrictive aspect with regard to the realization of specific components. They will generally be of a geometrical nature:

– dimensions constraints: diameter, length, volume, mass;

– constraints relative to a characteristic: inertia lower or greater than a limit value;

– supply constraints: limit current or power;

– thermal constraints: limited losses, extreme ambient temperatures;

– noise limits;

– electromagnetic compatibility limits;

– cost limits, etc.

2.4.8. Specification list

Table 2.1 below summarizes the characteristics of the driven body and its peripheral components. It offers a typical list which it is enough to fill and which allows us to fix the specifications and to define the criteria of choice and design.

Table 2.1. Summary of the characteristics of the driven body and its peripheral components

Chapter 3

Transmission

3.1. Transmission types and characterization

3.1.1. Rotating-rotating transmissions

The conversion of a rotating movement in another rotating movement has several possible objectives:

– reduction or increase of speed;