Exercises

I. Find in the text synonyms for the following words:

to link, dimension, to be called, to determine, to do, demand, the use, for instance, stable, irrespective of, almost, velocity, usually, to make use of, may be, some.

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

to vary, to establish, to apply, to satisfy, to choose, to improve, to offer, to add, to characterize, to differ, to use, to desire.

III. Form adjectives from:

number, to compare, to define, to consider, to desire, to involve, value.

IV. Find in the text English equivalents for:

имеется в наличии, независимо от, общепринято.

V. Memorize the following terms:

rating, starting torque, reluctance motor, shaded-pole motor, split-phase motor, repulsion-start motor, trouble-free, load.

VI. Translate the following sentences:

1. In the figure are shown the conclusions they arrived at after having studied the problem of current and voltage in phase. 2. The thing one must have in view while dealing with resistance is that it is impossible to construct a circuit "with resistance only". 3. The curve we referred to represents the angle of a lag or lead of the current under consideration. 4. It is very important to know the time the current passes the midway between the given points, because of its being of great importance for the test in question. 5. Not only will the oscillo­gram show the number of times a second the current alternates, but it will also show how closely the current approaches a sine wave in form.

VII. Be ready to answer the following questions:

1. What kinds of motors are technically referred to as "small motors"? 2. Under what conditions are they designed to operate? 3. Where can single-phase motors be made use of? 4. What types of such machines have been developed by the motor-manufacturing industry? 5. What type of motor works equally well both on direct and alternating current up to 60 cycles? 6. What characteristic features of such motors do you know? 7. What type of motors connected to a single-phase source of supply is the most widely used one?

VIII. Choose one of the topics below and prepare to talk on it:

1. The split-phase type. 2. The repulsion-induction and repuls­ion-start motors. 3. Synchronous motors.

IX. Write a summary of the text.

Х. Supplementary reading.

Text 11 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 at the same time in both dynamos and motors. Any direct-current dy­namo will fun as a motor, that is, convert electrical power to mechan­ical power, if its field and armature are connected to a suitable elec­tric supply. Also any direct-current motor will function as a dynamo provided that the conditions for self-excitation are fulfilled. .

The first diagram shows one armature conductor of a dynamo rotating in a clockwise direction under a N. pole. Fleming's right-hand rule shows that the e. m. f. induced in the conductor acts inwards, and this also will be the direction of the current in the conductor, since, in the case of a dynamo, the current flows under the influence of the e. m. f. induced in the armature. Now whenever a current flows through a straight conductor a magnetic field is set up, the lines of force of which are concentric circles having their centre in the conductor. The direction, or sense, of these lines of force is given by the corkscrew rule, which states that if the current through a straight conductor is in the same direction as the bodily motion of a corkscrew, then the direction in which the handle of the corkscrew has to be ro­tated gives the direction of the circular lines of force. Applying this rule we should have to rotate a corkscrew in a clock­wise direction to drive it into the paper, i. e. in the direction of the current in the conductor, and this clockwise direction, therefore, gives the direction of the lines of force set up by the current. For simplicity only one of these lines of force is shown, and it is repre­sented by the dotted circle. Now the lines of force of the main field from the N. pole cross the air-gap from the pole to the armature, i. e downwards in the figure and, therefore, on comparing the directions of the lines of force of the two fields, we see that the armature field acts in the same direction as the main field on the right-hand side of the conductor, and in opposition to the main field on the left-hand side. As a result there is a strong field on the right-hand side and a weakened field on the left-hand side. It will be seen that some of the lines of force are bent-round the conductor. Now magnetic lines of force are always in a state of tension and therefore, the bent lines of force will set up a mechanical force on the conductor much in the same way that the bent elastic of a catapult produces a mechanical force on the stone. In the case of the conductor in the figure this force obviously acts from right to left, i.e. opposite to the motion of the conductor. This applies to all the conductors on the armature of a generator delivering current, and it follows that the steam engine or other prime mover has to drag the armature round against this opposing force. For this reason the force is called the "magnetic drag."

For a current of J amperes flowing through a conductor of length / cms., placed at right angles to the lines of force of a magnetic field of strength B lines per sq. cm., the magnitude of the drag is given by the expression

f = BJl

10 dynes

The denominator 10 being introduced because the practical unit of current, the ampere, is one-tenth of the C.G.S. unit of current. The di­rection of the force is opposite to the direction of motion in the case of the generator and, therefore, since the right-hand rule gives the relationship between the directions of field, current and motion, a similar left-hand rule will give the relationship between field current and force. The rule is therefore as follows: hold the thumb and first finger of the left hand at right angles, and bend the second finger so as to point at right angles-to the plane of these two. Then if the first finger is pointed in the direction of the field, and the second finger in the direction of the current the thumb will point in the direction of the force.