ELECTRIC MOTORS - THEIR ACTION, CONTROL AND APPLICATION
BY FRANCIS B. CROCKER
NEW YORK, D. VAN NOSTRAND COMPANY, 1910
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The design and construction of electrical apparatus are covered fairly well by existing literature, the books on such subjects being very numerous, and many of them are comprehensive and authoritative. On the other hand, the operation of electrical machinery has received comparatively little attention. This latter fact appears to be anomalous when we consider that there are undoubtedly several hundred users for every designer or constructor of such apparatus, because each builder supplies a large number of customers. Hence the authors have endeavored to supply information that may be useful to those who operate or are interested in the operation of electric motors. Included among these are electrical engineers who install or run electric power plants, managers of manufacturing or other establishments in which electric devices are employed, as well as students and others who desire to acquaint themselves with the working of various kinds of electric motors and their application to useful purposes.
The general method herein adopted is an outgrowth of the course of lectures on electric motors and their applications given in Columbia University. It is based upon the consideration of counter e.m.f. and its relation to impressed e.m.f. as the important criterion of motor action. This point of view is, of course, not original, but it is claimed that the conception is more explicitly and widely applied than heretofore. Furthermore, this idea brings together the motor and generator so that they may be regarded as identical except for slight differences easily seen, and our knowledge concerning one is applicable to the other. The plan of treatment also links voltage with speed, and current with torque, since in general they are respectively proportional. Thus we consider one pair of quantities at a time instead of four. The synchronous ac motor differs so radically from the dc type that the treatment must be modified, but even in this case a similar standpoint is adopted as closely as possible.
TABLE OF CONTENTS
- Types of Motors and Advantages of Electric Drive
- Action of Shunt Motors
- Shunt-Motor Starting Boxes
- Shunt-Motor Speed Control by Variation of Armature Voltage
- Speed Control of Shunt Motors by Variation of Field Current
- Speed Control of Motors by Variation of Field Reluctance
- Multiple- Voltage Systems of Motor Speed Control
- Direct-Current Series Motors
- Control of Direct- Current Series Motors
- Compound-Wound Motors
- Alternating-Current Motors - Introduction
- Synchronous Alternating-Current Motors
- Polyphase Induction Motors
- Starting of Polyphase Induction Motors
- Speed Control of Polyphase Induction Motors
- Single-Phase Induction Motors
- Commutating Alternating - Current Motors
- Service Conditions and Applications of Electric Motors
- Standardization Rules, Electric Motors
CHAPTER I. INTRODUCTION
An electric motor is a machine which converts electrical power into mechanical power. In function, therefore, it is the exact converse of the dynamo-electric generator. On the other hand, identically the same machine may be and often is employed to perform either function, which fact is known as the reversibility of the dynamo-electric machine. In the earlier periods of their development, however, the two machines were usually regarded as quite different in character and were constructed on wholly different lines.
Strange to say, the motor historically precedes either the magneto or dynamo-electric generator. Barlow’s wheel of 1823, the first electric motor, was similar in construction to Faraday's disc of 1831, which was the original magneto-electric generator. The Jacobi electric motor of 1838 was large enough to propel a boat carrying fourteen passengers at three miles per hour, and Page in 1851 constructed a car driven by a 16-horsepower electric motor at nineteen miles per hour. These as well as other electric motors of those times were far more powerful and were regarded as more practical or more promising than the contemporaneous magneto- electric generators. The Pacinotti ring of 1861, the prototype of modem armatures, was primarily intended to be used in a motor, although the inventor suggested that it could also be employed to generate electric currents.
All of these early electric motors depended upon primary batteries for their supply of electrical energy, and it was found that the cost of operation was excessive for any considerable power, especially with the low-efficiency motors and crude forms of battery then available. The result was that the motor had to wait while the generator was being developed to commercial success, which cannot be said to have really begun before 1880. Even then the electrical energy produced was used entirely for arc and incandescent lighting. In fact, it was not until about 1887 that central stations with their systems of distribution had become sufficiently large and well regulated so that the use of electric motors was encouraged or even permitted except in a few isolated cases.
The electric light having been practically introduced and more or less generally established, inventors, manufacturers, also those who produced electrical energy, turned some attention to electric power, which, from about 1888, has been a prominent part of electrical engineering, including railway as well as stationary motors. The former type, also the induction and synchronous alternating-current motors, began to be commercially introduced about that time or soon after. Since this comparatively recent epoch the progress of electric power in all its branches has been at an extraordinarily rapid rate and with most far-reaching results, unequaled by any other art or industry in anything like the same period of time.
Relation between Generator and Motor. - Either a dynamo-electric generator or a motor may be regarded as made up of a certain number of centimeters of wire located in a magnetic field of given density in lines per square centimeter. The former machine will generate e.m.f. when the wire moves; the latter machine exerts torque if current flows in the wire, that being the essential distinction between the two. We may have, for example, a generator (separately excited) producing full emf with no current in its armature and we may have a motor exerting full torque with its armature prevented from turning, but we cannot have a generator without motion or a motor without armature current. In practical operation, however, the generator has current flowing in the wire so that torque, opposed to driving force, is also exerted; and in the motor e.m.f., counter to energizing current, is set up by the motion of the wire. Hence either machine while working develops both e.m.f, and torque, the only difference between them under these conditions being the fact that this emf is positive with respect to current in the generator and negative in the motor, while torque is negative with respect to motion in the generator and positive in the motor. It follows therefore that electrical power is positive in the generator and mechanical power is negative, whereas electrical power is negative in the motor and mechanical power positive. In fact the exact function of these machines is expressed in the above statement, which means that they convert mechanical power into electrical power and vice versa. These distinctions in function or action do not, however, involve any necessary difference in the construction of generators and motors. As already stated, identically the same machine is equally operative for either purpose because the dynamo-electric machine is perfectly reversible. In practice, motors and generators are made somewhat differently, but merely with respect to details of form or connections, so that they will be more convenient for the special uses to which they are applied. As a matter of fact motors differ among themselves, railway and stationary types, for example, fully as much as they differ from generators.
While these differences in construction are for the most part mere matters of adaptability, the operation of generators is radically unlike the operation of motors. The former are almost universally driven at constant speed by steam engines, gas engines, turbines or other sources of mechanical power. Of course in practice the speed varies somewhat, but this is very undesirable and avoided as much as possible by the most careful design as well as adjustment of governors. The few cases in which the speed variation is large, as, for example, the driving of a generator from the axle of a railway car, involve serious mechanical as well as electrical difficulties, special and often complicated auxiliary apparatus being employed.
On the other hand, the speed of electric motors is very commonly variable or adjustable, the range in many cases being from zero to a maximum in either direction, as in railway or elevator service, and ratios of three or four to one or higher are common in factories, machine shops, etc. The means and methods used to accomplish such speed variation constitute an important branch of engineering, and it is the particular purpose of this book to discuss this subject of motor control. In those applications for which constant speed is desired, the motors may depart somewhat from this condition owing to their own action, which matter will also be given special attention, because it is often of practical importance to reduce or allow for these undesired changes of speed.
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