A generator of electric current can be defined as a machine which converts mechanical energy into electrical energy, the direct-current generator, or dynamo, being one which supplies a unidirectional, as distinct from an alternating current. It is, therefore, necessary that the dynamo shall be driven by some kind of prime mover which has to provide an amount of energy equal to that supplied by the dynamo, plus the various losses that occur in the dynamo. When the dynamo is on no-load, that is when it is not supplying any energy to the circuit to which it is connected, the prime mover has to supply energy sufficient for the losses that take place in the dynamo at no-load.

Essentials for Production of a Current. So that an electric current may flow through a circuit it is necessary that there shall be an electro-motive force (e. m. f.) acting in the circuit, and in the dynamo this e. m. f. is produced by electromagnetic induction. For an e. m. f. to be produced in this manner it is necessary that conductors of electricity shall be moved in such a way as to cut the lines of force of a magnetic field; in other words, there must be three things, namely, a magnetic field, a system of conductors, and motion of those conductors in the magnetic field, the motion being such that the conductors move across, and not along, the magnetic lines of force. That part of the dynamo which produces the magnetic field is called the field-magnet, while the part which carries the system of moving conductors is called the armature. We shall see that the dynamo gives an unidirectional, and not an alternating, voltage at its terminals, but to transfer this voltage to fixed terminals a third part called the commutator is necessary.

To understand the modern dynamo it is advisable to consider the simplest possible form, since this is the easiest way of obtaining a grasp of the principles which are the same for the more complicated types necessitated by commercial requirements. We will therefore consider, first of all, a single-turn rectangular coil rotating in a bi-polar field. 16. The diagram shows the coil ABCD rotating between the poles N. and S. in a clockwise direction when looked at from the end AC. In the position shown, the coil side AB is moving under the N. pole in a direction from left to right (in the end view) so that it is cutting the magnetic lines of force, which are represented by the dotted lines.

Fleming's Right-Hand Rule. — The direction of the e. m. f. induced in AB is given by Fleming's Right-Hand Rule, which is applied as follows: Arrange the thumb and the first and second fingers of the right hand mutually at right angles. Point the first finger in the direction of the magnetic lines of force, and the thumb in the direction of motion of the conductor: then the direction in which the second finger is pointing gives the direction of the e. m. f. induced in the conductor. Applying this rule to the coil side AB in Fig. 16, and making use of the end view, we point the first finger downwards, i.e. from TV". to S., the thumb to the right, and the second finger then points into the paper. The e. rn. f. induced in AB is, therefore, into the paper in the end view, or from A to B in the side view, as indicated by the arrow head. Similarly, we find that the e. m. f. induced in the coil side CD is from D to C. Following these directions round the coil we see that the e. m. f. s induced in the two coil sides act in the same direction round the coil, a condition which, as we shall see, applies to the coils in commercial armature windings. Also, with the coil in the position shown in the figure, if the two ends E and F are connected to an external circuit, the current set up by the induced e. m. f. will leave the coil at E and re-enter the coil at F. In other words, E will be the positive and F the negative terminal of the coil.

Next consider what is happening" at the instant the coil is perpendicular to the lines of force, the coil having made one-quarter of a complete revolution from the first position. Its position will then change, from which we see that each coil side is, at that instant, moving along the direction of the lines of force. The magnitude of the induced e.m.f. will thus be zero. Now, whatever the position of the coil may be, the magnitude of the induced e.m.f. is proportional to the rate  at which the coil sides cut the lines of force, and if the angular velocity of the coil is uniform, this  rate  will be the greatest when the coil sides are moving across the lines in a direction perpendicular to them. In the first diagram a conductor moves a distance d from P to Q perpendicular to the lines of force, and in its passage it cuts ten of these lines. In the second diagram the conductor moves the same distance d, but this time the direction of motion is oblique to the lines of force with the result that fewer of the lines are cut; five in the figure. Hence in the case of our rectangular coil rotating in a two-pole field, we see that the induced e.m.f. will have its maximum value when the coil sides are directly opposite the pole centres, and that the e.m.f. will gradually diminish as the coil rotates, becoming zero by the time the coil has rotated one-quarter of a revolution from the position of maximum e.m.f.

When the coil has made still another quarter-revolution, that is, half a revolution altogether from the position of Fig. 16, the coil side AB will be directly opposite the south pole, while CD will be directly opposite the north pole. The direction of the induced e.m.f. in the side AB will now be from В to A, while that in side CD will be from С to D. Therefore, while the coil has made half a revolution the e.m.f. induced in it has reversed in direction, and if the coil has been connected all the time to an external circuit, the current in this circuit will also have reversed in direction, the end F now being the positive terminal, and E the negative terminal. Also in this position the induced e.m.f. will have its maximum numerical value because the coil's sides will, at the instant, be moving in a direction perpeadicular to the lines of force. After a third quarter-revolution the coil will be again horizontal, the coil sides will at the instant be moving along, instead of across, the lines of force, and therefore the magnitude of the induced e.m.f. will be zero. Finally, when the coil has made a complete revolution, it will be back in the position of Fig. 16, and the e.m.f. in it will have its maximum value in the original direction. The changes that will have taken place are best represented by a graph, as in Fig. 20 which illustrates the following facts.

(a)     The e.m.f. induced in the coil is an alternating one, since it is alternately positive and negative in direction.

(b)    The e.m.f. undergoes one complete "cycle" of changes in the time taken by the coil to make one complete revolution. This is shown by the fact that the graph begins to repeat itself after one revolution, as indicated by the dotted continuation. There will be, in fact, as many of these repeats as there are complete revolutions of the coil.

(c) When the e.m.f. has zero magnitude, e.g. when it has made one-quarter or three-quarters of a revolution, it is either on the point of changing from positive to negative direction, or vice versa.

It can be said generally of all heteropolar electrical machines, that is, machines having alternate north and south polarity, that the e.m.f. induced in the armature is an alternating one. This applies both to direct- and to alternating-current machinery.

Magnitude of the Induced E.M.F.— The magnitude of the induced e.m.f. depends upon the linear velocity of the conductor measured in a direction perpendicular to the lines of force, the length of the conductor, and the strength of the magnetic field. It will be remembered that the strength of a magnetic field is measured by the number of lines which cross each square inch, or square centimetre, of normal cross section.

Let B=field strength in lines per sq. cm.

l= length of conductor in cms.

v — velocity of conductor in cms. per sec. in a direction perpendicular to the lines of force.

E = e.m.f. in volts induced in one conductor i.e. one-half turn of a single-turn coil.

Then E = Blv X 10^-8 volts.

If the length is expressed in inches, then, since 1 in, is equal to 2.54 cms., a length of 1 in. will be equal to 2.54 X 1 cms. Also, if the field strength is in lines of force per square inch, then since there are (2*54)² = 6*45 sq. cms. to 1 sq. in., a field strength of B lines per sq. in. corresponds to B/6*45 lines per sq. cm. Again, a velocity of v ft. per sec. is equal to a velocity of 12 X 2*54 X v = = 30*5 v cms. per. sec. Hence, if B is expressed in lines per sq. in., l in inches, and v in ft. per sec, the expression for E is

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