Construction. A direct-current motor consists of the same essential parts as a direct-current generator, namely, field magnet, armature with its commutator, and brush gear. The armature and commutator are constructed on exactly the same principles as the armature and commutator of a dynamo, and any difference in external appearances of dynamos and motors is due to a modification in the mechanical arrangement of the field magnets and frame, designed to give the motor the maximum amount of protection. Dynamos are employed mostly in a central power station where they are not exposed to any mechanical danger, such as the risk of heavy bodies falling on them, and as a result they can be of open construction. This is a great advantage since they are accessible for repairs, and also they are easily ventilated.
Motors, on the other hand, often work in very exposed situations thus necessitating partial or complete enclosure of the working parts. The type of duty to be performed also has an influence on the construction of the motor. The motor must, of course, be totally enclosed, but at the same time must be capable of rapid dismantling for inspection.
General Principles. It is often thought that the principle of operation of a dynamo is quite unconnected with that of a motor; actually the two cannot be separated, since dynamo and motor actions go on field-magnets, the direction of rotation of a machine when running as a motor is opposite to its direction when running as a dynamo. On the other hand, if the directions of rotation are the same and the polarities are the same, then the directions of the armature currents will be different for the two modes of operation.
Now when a machine is running as a motor, the conductors on the armature cut the lines of force of the magnetic field just as they do when the machine is acting as a dynamo. As a result they have e.m.f.'s induced in them. The direction of one such induced e.m.f. in an individual conductor is, of course, given by the right-hand rule, and applying this rule to the conductor, we see that the induced e.m.f. acts outwards, that is, in opposition to the current. This induced e.m.f. in the case of a motor is, therefore, called the "back e.m.f."
Application of the Principle of Work. It is interesting to look at the above problems from another point of view. We know that an electric motor does mechanical work, and we also know that in order that any machine may do work, an equal amount of work (plus the losses in the machine) has to be put into it. Again
when the work is done some force has to be overcome. Now, it is the supply e. m. f. which puts work into the motor by driving the current through the armature, and since work is only done when some force is overcome, we see that in order that the motor may perform mechanical work, the supply e.m.f. must have some opposition. This opposition must obviously come from a force of the same nature, namely an e.m.f., from which it follows that the armature must set up a back e.m.f. A similar process of reasoning shows that a magnetic drag must be set up on the armature of a dynamo delivering current.
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It will thus be seen that the motor action and the dynamo action, which for the sake of convenience are studied separately, cannot, as a matter of fact, have separate existences. They are inextricably bound up together, and one cannot come into operation without the other. As soon as a dynamo delivers current, the motor action comes into play and sets up the resistance to motion called the magnetic drag; and when a motor is made to perform work the dynamo action immediately comes into play and sets up the back e.m.f.
Notes
commutator -
gear - ;
modification -
to dismantle - ;
outward - ,
obviously - ,
thus - ;
to reason - ; ;
inextricable - ,
Comprehension questions:
1. What is the construction of the direct-current motors?
2. What can you say about the general principles of a dynamo operation?
3. Compare with your partner the motor action and the dynamo action. Give the reasons.
TEXT22. GENERATORS
Principles of Operation
Generation of E.M.F. The fundamental principle on which all a.c. generators depend is that an e.m.f. is induced when a conductor is made to cut across a magnetic flux. In order to obtain an e.m.f. of suitable magnitude, many conductors in series and a strong magnetic flux are employed, while the rate of cutting is made as high as practicable.
During the first 90 of rotation the rate of cutting flux is gradually increasing, while during the second 90 it is gradually decreasing, owing to the alteration in the angle at which the conductors cut the flux. During the second half of the revolution the conductors cut the flux in the reverse direction, and during this period the reverse half of the e.m.f. wave is induced. One complete cycle of e.m.f. is induced in one complete revolution.
If a second turn were rigidly attached to the first, but at right angles to it, an e.m.f. of the same r.m.s. value would be induced in this second turn, but it would be 90 out of phase with the e.m.f. induced in the first turn. The arrangement would constitute a two-phase generator.
In practice three-phase generators are the most common, and a three-phase supply would be obtained by mounting three turns on the game shaft, these three turns being rigidly fixed at 120to one Another.
Rotating Field. In d.c. generators the armature winding is mounted on the rotating member which rotates between the poles of n fixed inatfiiut system, but with a.c. generators, or alternators, the standard practice is to place the armature winding upon the stationary clement, now called the stator, while the field system is mounted on the rotating* element, now called the rotor. This method of construction has many advantages from the designer's point of view, and is always adopted except in the case of a few special purpose machines.
The d. exciting current has to be led into the rotor, this being done by means of two insulated slip rings on which press a number of carbon brushes. This d.c. exciting current is usually provided by a special d.c. generator called an exciter, this being directly driven from the shaft of the main a.c. generator. In certain cases where a d.c. supply is available, this exciter is omitted, but in power station work each a.c. generator is provided with its own d.c. exciter.
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Notes
flux - ;
horizontal -
inactive -
merely - , ;
angle -
rigid -
r.m.s. = root means square
to omit - ; ;
Comprehension questions:
1. What is the fundamental principle on which all a.c. generators depend?
2. What did you learn from the text about the behavior of e.m.f. during the generation?
3. What can you say about d.c. exciting current? Is it important in rotating field or not?
