Text 21. Direct-current motors

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.

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 - комутація

gear - шестерня;прилад

modification - видозмінювання

to dismantle - розбирати; знімати

outward - зовні, назовні

obviously - очевидно, явно

thus - так; таким чином

to reason - міркувати; робити висновок;

переконувати

inextricable - складний, заплутаний

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.