Again considering the action of the four ring coil shown in fig. 185, and starting at the beginning of the revolution, the variation of electromotive force induced in coils AA' is indicated by the dotted sine curve 1, and of BB' by dotted curve 2. It will be seen that 1 begins at the axis or line of no pressure, and 2 at maximum pressure.

[Ill.u.s.tration: FIG. 187.--The resultant curves of figs. 183, 185 and 186 are here shown for comparison to ill.u.s.trate the approach to uniform pressure as the number of coils are increased. It should be noted that the number of pulsations per cycle depends on the number of coils, and that as the pulsations increase in number, the variation in pressure decreases.]

The two curves overlap each other, and in order to determine the effect of this it is necessary to trace the resultant curve, 3.

This is easily done, as the resultant electromotive force induced at any point in the revolution of the armature is equal to the sum of the pressures induced in AA' and BB'. Thus, at the beginning of the revolution the pressure induced in AA' is at zero point, and in BB' at its maximum J, hence, the resultant curve begins at the point J. Again, for any point in the revolution, as N, the height of the resultant curve is equal to NP + NT = NV. For 45 or 1/8 revolution, the resultant curve reaches its amplitude, which is equal to 2 RZ = RW, and at 90 it again reaches its minimum, XY.

=Ques. State the conditions upon which the steadiness of the current depends.=

Ans. _It depends on the number of coils and the manner in which they are connected._

Comparing curves 1 and 3, in fig. 185, it will be noted that with four coils the variation of pressure or amplitude of the pulsations is less than half that obtained with two; moreover, with four coils the number of pulsations per cycle is doubled.

In order to further observe the approach to continuous current obtained by increasing the number of coils, the effect of a six coil armature is shown in fig. 186, the resultant curve being obtained in the same manner as just explained. For comparison, the curves for the three cases of two, four, and six coils are reproduced under each other in fig. 187.

As the number of coils is further increased, the amplitude of the pulsations decreases so that the resultant curve approaches nearer the form of a straight line.

In the actual dynamo there are a great many coils, hence the amplitude of the pulsations is exceedingly small; accordingly, it is customary to speak of the current as "continuous,"

although as previously mentioned such is not the case.

CHAPTER XV

CLa.s.sES OF DYNAMO

In order to adapt the dynamo to the varied conditions of service, its design is modified in numerous ways, giving rise to the different "types."

These may be cla.s.sified with respect to:

1. Field magnets; 2. Field excitation; 3. Field winding.

The first division relates to the number of magnetic poles, as unipolar, bipolar, and multi-polar dynamos; also inter-polar dynamos. Under the second division are included the following:

1. _Self-exciting machines_ of which the magneto is the simplest. Its magnetic field is obtained from permanent magnets, hence the electromotive force generated is comparatively small. The more important type of self-exciting machine is provided with electromagnets in which the field of force is "built up" from the residual magnetism of the soft iron or steel cores of the field magnets of the dynamo itself. Nearly all commercial types of dynamo are of this cla.s.s.

2. _Separately excited machines_ in which the field magnets are magnetized when the machine is in operation by current supplied from a separate source such as a battery or magneto generator.

With respect to the third division, based on the field winding, dynamos are cla.s.sed as:

1. Series wound; 2. Shunt wound; 3. Compound wound.

In addition to the foregoing there are further distinctions with respect to the mechanical features. Most dynamos have a revolving armature and stationary field magnets; however, in some cases, both the armature and field magnets are stationary, a revolving iron inductor being provided to intercept the magnetic lines intermittently which produces the same effect as is obtained in cutting the magnetic lines by a revolving armature.

=Ques. What may be said of bipolar and multi-polar dynamos?=

Ans. Dynamos with bipolar field magnets were universally used prior to 1890, but since that time machines of this type are only made in very small sizes; the multi-polar dynamo is the type now in general use.

=Ques. State some of the features of the multi-polar dynamo.=

Ans. In this cla.s.s of machine, the armature and field magnets are surrounded by a circular frame, or _ring yoke_ to which the field magnets are attached. This ring arrangement has the advantages of strength, simplicity, symmetrical appearance, and minimum magnetic leakage, since the pole pieces have the least possible surface and the path of the magnetic flux is shorter.

=Ques. What important advantage is gained by the use of multi-pole field magnets?=

Ans. Commercial voltages are obtained at moderate armature speed.

The difficulty experienced with bipolar machines is that, with a dynamo of large output, the speed at which its armature would have to rotate to generate commercial voltages would be excessive.

[Ill.u.s.tration: FIGS. 188 and 189.--Circuit diagrams to ill.u.s.trate the difference between a dynamo and a magneto. The former has its field magnets F F magnetized by means of a small current flowing around a shunt circuit. In a magneto the field magnets are permanently magnetized. The strength of the magnet field of a magneto is constant while that of a dynamo varies with the output.]

It is evident that with two or more magnetic fields, secured by increasing the number of poles, the armature inductors revolving between them cut more magnetic lines in one revolution than with a single field, hence, a given voltage is obtained with less speed of the armature than in the bipolar machine.

For instance, if a bipolar dynamo be required to run at say 900 revolutions per minute to generate 125 volts, a four pole machine of equal output will require only 450 revolutions, and one of eight poles only 225 revolutions per minute.

=Ques. What is a self-exciting dynamo?=

Ans. A machine in which the initial excitation of the field is due to the residual magnetism retained by the cores.

=Ques. What may be said of the field due to this residual magnetism?=

Ans. It presents a very weak field, and the voltage that could be generated by the armature revolving in such a field would be only about two to ten volts.

[Ill.u.s.tration: FIG. 190.--Series wound dynamo, used for series arc lighting, and as a booster for increasing the pressure on a feeder carrying current furnished by some other generator. The coils of the field magnet are in series with those of the armature and external circuit, and consists of a few turns of heavy wire. The characteristic of the series dynamo is to furnish current with increasing voltage as the load increases. If overloaded, the voltage will drop.]

=Ques. How then can commercial voltages such as 100 or more volts be obtained with a self-exciting dynamo?=

Ans. Part or all of the current induced in the armature is pa.s.sed through the windings of the field magnets, thus strengthening the field. The voltage, therefore, will "build up," increasing until the maximum has been reached.

The maximum voltage will depend upon the capacity of the field magnets as determined by the construction, and upon the strength of current used to excite them.

=Ques. How long does the process of "building up" require?=

Ans. The time required to fully excite the field magnets is from ten to twenty seconds, the rise in field strength being indicated on the voltmeter or by the gradual increase in the brilliancy of the _pilot lamp_.

=Ques. Name three important cla.s.ses of dynamo.=

Ans. Series wound, shunt wound, and compound wound.

=Ques. Describe the winding of a series dynamo.=

Ans. In this machine, the field magnets are wound with a few turns of thick wire joined in series with the armature brushes as shown in fig.

190.

=Ques. What is the effect of this arrangement?=

Ans. All of the current generated by the machine pa.s.ses through the coils of the field magnets to the external circuit. The current in pa.s.sing through the field magnets, energizes them and strengthens the weak field due to the residual magnetism of the magnet cores, resulting in the gradual building up of the field.

=Ques. For what service is the series dynamo adapted?=