Exercises

I. Give all the synonyms you know for the following words:

to convert, to take place, common, exactly, in the same way, to receive, in fact, apparent, to connect, readily, to perform, speed, to require, to slow down, motion, main, to revolve, several.

II. Form nouns from the following words and translate them:

to convert, to receive, to reason, to link, to demand, to increase, to move, to supply, to depend, to transform, to perform, inductive, various, free.

III. Form adverbs from the following adjectives and translate them:

ready, large, high, late, short, recent, direct.

IV. Form adjectives from the following words and translate them:

power, electricity, to relate, to add, station, reason, to depend, to compare, care, value, to adjust, to move, to create.

V. Give the different meanings of the following words and illustrate their use in sentences:

as, to make, even, that.

VI. Describe the induction-motor principle.

VII. Describe the induction-motor action.

VIII. Translate the following sentences:

В электрическом моторе происходит превращение электрической энергии в механическую. Индукционный мотор можно рассматривать как вращающийся трансформатор. Ток в роторе является ре­зультатом электромагнитной индукции. Ротор состоит из пластин­чатого сердечника, содержащего обмотку. Главное его поле имеет четное число полюсов. В статоре полюса образуются взаимодей­ствием полей двух или трех фаз.

IX. Supplementary reading.

Text 12 direct-current motors

Back E.M.F. – Suppose the machine, instead of generating its own electrical power, is uncoupled from the prime mover and is connected to an external supply, and current sent through the armature and field windings in the same directions as before. Then it is obvious that the magnetic fields produced by the field magnet and by the armature will be the same as before and therefore the shape of the lines of force will still be as shown in B Fig. 23. Therefore, the magnetic drag will be set up in the same direction as before, and since there is now no engine to force the armature round against this drag, the armature will rotate in the same direction as the drag, as shown in C. The ma­chine will now be running as a motor. We therefore see that for the same direction of the armature currents and the same polarity of the 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 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 shown in C, 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.