THE elementary principle upon which the action of all classes of dynamo-electric machinery depends is that whenever a conducting material is moved so as to cut the lines of force of a magnetic field, an E.M.F. is set up in the conductor. The induction of currents in conductors when moved across a magnetic field has already been fully explained on page 230. We may also here at once state the converse, and say that when a conductor carrying an electric current is placed in a magnetic field, it will tend to place itself so that its own field coincides with that in which it is placed, and if movement is needed to effect this, it will move so as to cut the lines of force.

The action of any type of dynamo or generator of electric current is dependent upon the former conditions being fulfilled. Similarly, the working of an electric motor requires the fulfilment of the second condition. We are here stating the matter broadly, without reference to the character of the current which the generator delivers, whether continuous or alternating, and in the case of motors the same remark applies. Consider a single loop of conducting wire arranged to revolve upon its axis as shown in Fig. 365. Imagine pole pieces placed on the right and left of the loop, as in Fig. 368, and if the loop of wire is now revolved, we shall find that an

E.M.F. is set up in each side of the loop, for if we carry wires from each of the sliding contacts shown in the sketch,

FIG. 365.-SINGLE LOOP

GENERATING AN ALTER-
NATING CURRENT.

and connect these wires to a galvanometer, we shall find that as long as the loop continues to turn a current will persist in the circuit, as shown by the galvanometer deflection. If we consider one side of the rectangular loop, it is obvious that in every revolution the magnetic field is cut twice, the conductor cutting the flux first as it leaves a N pole, and then again as it enters a S pole. We know from the laws of the induction of currents, that the direction of the induced current depends upon the direction of the magnetic flux, and the

so as to form two insulated segments, which are each connected to one end of the loop, as in Fig. 366. Two strips of conducting material make contact upon this split ring at opposite diameters, in such a position that the circuit through the "brushes," as they are called, changes from one segment to the other at the moment when the loop is not cutting the flux between the poles.

A simple commutator of this description is shown in Fig. 367. The action is simple; by means of this mechanical arrangement we automatically change the connections to the loop, and the current in the external circuit continues to flow in the same direction while the loop revolves. This is the elementary

type of the continuous FIG. 367.—" SPLIT current dynamo or

TUBE" OR TWO

PART COMMU

TATOR.

motor, and in Fig. 368 we see such a simple machine represented with the field magnets in situ. We can conveniently represent the alternating current produced by our simple alternator in the

The

no other disturbing factors, we should. obtain no such gradual curve. E.M.F. would rise quickly to its maximum value, remain at this maximum during the whole time that one side of the loop was under a pole, and then fall abruptly to zero. Actually, as we shall see later, owing to the spread of the magnetic field at the pole face, and the effect of self-induction, no such The loop, sudden changes occur. before it comes actually under the tips of the poles, experiences the effect of a weak field, which induces a small voltage, and the density of the field in which the conductor moves gradually increasing, there results a wave of E.M.F. of approximately the shape shown, namely a "sine" wave.

Fig. 370 enables us to understand the effect of the commutator in reversing the second wave, giving a unidirectional current, but it is hardly correct to call this a continuous current, because it will be seen that the E.M.F. pulsates in this case when we have only two segments or so, and at one instant approaches to zero. A practical dynamo provides a current in

which the amount of this pulsation is practically nothing, for instead of employing two commutator segments, at least a dozen such segments are used, even in the smallest machines, and generally a very much larger number.

Fig. 371 shows the result of using eight commutator segments and four loops. The current never tends to die down to zero, owing to the fact that some of the loops are always active. In the diagram, the lower curves represent the induced voltages in the several coils, and the upper thick curve, the resultant of these.

It will be seen that there is very little pulsation in the voltage, and, as a matter of fact, there are other effects which tend to wipe out even such small pulsations as these, so that for all intents and purposes the E.M.F. is continuous, especially with

in 1831, was a machine which gave continuous currents, but without the use of a commutator. Fig. 372 shows the construction of this machine: the edge of a copper

N

FIG. 372.-FARADAY'S DISC.

disc was inserted between the poles of a powerful electromagnet, and Faraday found on revolving the disc that a unidirectional current could be produced if rubbing contacts were placed, one at the the periphery of the disc, and the other on the shaft, which was in metallic contact with the body of the copper disc.

The currents in this case flow radially from the centre of the disc to the periphery, and as there is no reversal of the direction in which the conductor cuts the magnetic field, no commutator is necessary, the machine furnishing a perfectly steady, continuous current, the voltage depending upon the strength of the magnetism and the speed of the disc. At first sight it would seem that machines constructed on this principle would possess advantages over those employing commutators, but practically this is not so, as it is impossible to get any considerable voltage from a machine of this type except by the employment of very high speeds and very heavy field magnets, and though many attempts have been made to construct practical dynamos on this principle, they have only come into a very limited practical use. There is, however, some hope that in future such machines may have an increased applica

tion, and in Chapter III. we make some further reference to this type, called “unipolar" or "homopolar" dynamos.

THE MAGNETIC CIRCUIT.

The laws of magnetism and the calculations of the general magnetic circuit have been fully set forth in Chapter III., Section IV., but the demand of practice. requires a few additional remarks.

As British units of length are adopted throughout this section, the magnetisation or permeability curves in Fig. 373 are plotted in lines per square inch and ampere turns per inch instead of the usual "B" (lines per square cm.) and "H" (the unit of magnetomotive force), and are representative of the materials as obtained in actual practice.

Variations in the quality of the material may in some samples cause some little variation in the permeability, but samples of commercial cast steel and sheet iron vary little from one another in their permeability.

Samples of cast iron vary considerably owing to the fact that manufacturers usually employ the ordinary foundry iron, which is not expressly intended for electrical use, as is the case with cast steel and sheet iron. The curves for sheet iron are plotted to both ampere-turn scales, A and B, enabling the permeability of the iron at high flux densities to be readily scaled off; the straight line showing the ampere turns required for inch of air is plotted to the scale A, the cast steel and cast iron curves corresponding to the same scale.

The calculation of the magnetic circuit of an actual machine is a comparatively simple matter, providing the quality of the steel or iron is well known and the leakage of the circuit be not considerably under or over estimated.

The accurate estimation of the excitation necessary for the air gap demands careful

150,000

140,000

130,000

120,000

Flux density

Lines per

square

inch.

200

consideration, as in the magnetic circuit of continuous current machines and alternators the reluctance of the air gap is usually equal to or greater than the sum of the reluctances of the rest of the circuit.

Having been given the length of the air gap, it is necessary to estimate the average magnetic density of the flux in the gap, and as in all modern continuous current machines, and in the majority of alternators, the armature surface is slotted, the effect is to increase the mean density in the air gap compared with what the density would be

if the armature presented an unbroken iron surface to the flux emanating from the poles.

As no simple rules can be given for this estimation, recourse must be had either to the rather complicated methods of Arnold and Carter, or, as is the case with many practical designers, to experience and

common sense.

The calculation of the excitation necessary for the armature teeth again requires some judgment if they are highly saturated as is usually the case, especially with continuous current machinery, this being