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 run 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. In order to realize the essential connection between the two modes of operation consider the diagrams in Fig. 23.

The first diagram shows one armature conductor of a dynamo rotating in a clockwise direction under a N. pole. Fleming' 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 to A, Fig. 23, 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 is impossible for two sets of lines of force to intersect one another as shown in A, the actual arrangement resulting from the combination of the two fields being as shown in B. 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 l 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

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 shown in A and B 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. This is illustrated by Fig. 24.

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.

 М.А. Беляева и др. «Сборник технических текстов на англ. языке»