=Ques. What is the difference between a compound and an over compounded dynamo?=

Ans. In the first instance, there are just enough turns in the series winding to maintain the voltage constant at the brushes for variable load.

If a greater number of turns be used in the series winding than is required for constant voltage at the brushes for all loads, the voltage will rise as the load is increased, and thus make up for the loss or drop in the transmission lines, so that a constant voltage will be maintained at some distant point from the generator. The machine is then said to be _over compounded_.

=Ques. For what service is over compounding desirable?=

Ans. For incandescent lighting where there is considerable length of transmission lines.

=Ques. What is the usual degree of over compounding?=

Ans. Generally for a rise of voltage of from five to ten per cent.

In construction, the field coils are wound with a greater number of turns than actually required, the machine being accurately adjusted by a running load test after completion.

=Ques. How is the degree of over compounding varied?=

Ans. A rheostat is placed in shunt with the series winding so that the current pa.s.sing through the winding may be regulated to control the voltage of the machine.

[Ill.u.s.tration: FIG. 195.--Compound wound dynamo, used when better automatic regulation of voltage on constant pressure circuits is desired than is possible with the shunt machine. The compound dynamo is a combination of the series and shunt types, that is, the field magnet is excited by both series and shunt windings. With a proper selection of the number of turns in the series coils, the voltage may be kept automatically constant for wide fluctuations in the load. When the machine is _over compounded_ its characteristic is to slightly increase the voltage with increase of load, a desirable feature for long transmission lines in order to compensate for the line drop.]

=Ques. How are the ends of the shunt winding of a compound dynamo connected?=

Ans. There are two methods of connection, being known as the short shunt and the long shunt.

=Ques. Describe the short shunt.=

Ans. In the short shunt, the ends of the shunt winding are connected directly to the brushes as in fig. 196.

=Ques. Describe the long shunt.=

Ans. In the long shunt, one end of the shunt winding is connected to one of the brushes and the other end to the terminal connecting the series winding with the external circuit as in fig. 197.

=Ques. Which is the more desirable?=

Ans. Theoretically, the long shunt is preferable as being the more efficient; however, in practice, the gain is not very appreciable and the short shunt is generally used.

[Ill.u.s.tration: FIGS. 196 and 197.--Short and long shunt types of compound wound dynamos. The distinction between the two is that the ends of the short shunt connect direct with the brush terminals, while in the long shunt type, fig. 197, one end of the shunt connects with one brush terminal and the other with the terminal connecting the series winding with the external circuit. R is the shunt field rheostat for regulating the current through the shunt.]

=Ques. What may be said regarding the voltage in short, and long shunt machines?=

Ans. In a short shunt machine, the shunt winding is subjected to a higher voltage than with a long shunt. The pressure applied through a shunt winding with a long shunt, for any particular load, is equal to the voltage at the brushes plus the drop in the series winding.

=Ques. For what other service besides incandescent lighting are compound dynamos adapted?=

Ans. They are employed in electric railway power stations where the load is very fluctuating.

=Ques. What is the effect of a short circuit on a compound dynamo?=

Ans. It overloads the machine, since the excessive current flowing through the series field tends to keep the voltage at its normal value.

Unless the line be automatically opened under such a condition either by a fuse or circuit breaker, the machine and its driving engine may be damaged. To avoid this danger fuses or automatic circuit breakers are employed.

=Ques. Mention another service for which the compound dynamo is used.=

Ans. In some isolated plants, as small country residences where it is frequently necessary to have a dynamo capable of charging a storage battery during the day, and of furnishing current for lighting during a certain portion of the evening.

Under such conditions the compound machine with slight modification is used, the ordinary shunt dynamo not being capable of maintaining the necessary consistency of voltage, without attention to the shunt regulator in driving the lamps direct, the ordinary compound dynamo on the other hand, being unsatisfactory for charging storage batteries.

=Ques. How is the compound dynamo modified to adapt it to the dual service of lighting and battery charging?=

Ans. It is furnished with _alternative compound winding_, in which the series winding is provided with a switch, which may be fixed either upon the machine itself or upon the switchboard. This switch permits the series coils to be either short circuited in part or cut out of the circuit entirely while the machine is charging the storage battery, being again cut into circuit when the machine is required to furnish current for the lamps.

[Ill.u.s.tration: FIG. 198.--Separately excited dynamo. Current for field excitation is supplied by a second and smaller generator.]

=Separately Excited Dynamos=.--In this cla.s.s of machine the current required to excite the field magnets is obtained from some independent external source. Though used by Faraday, the separately excited dynamo did not come into favor until, in 1866, Wilde employed a small auxiliary magneto machine to furnish currents to excite the field magnets of a larger dynamo.

A separately excited dynamo is shown in fig. 198. This method of field excitation is seldom used except for alternators; it is, however, to be found occasionally in street railway power houses, the shunt fields of all the dynamos being separately excited by one dynamo.

In common with the magneto, the separately excited machine possesses the property that, with the exception of armature reactions, the magnetism in its field and therefore the total voltage of the machine is independent of variations in the load.

[Ill.u.s.tration: FIG. 199.--Diagram showing principle of Dobrowolski three wire dynamo. This type of machine is shown in more detail in fig. 795 on page 708.]

=Dobrowolski Three Wire Dynamo.=--This type of dynamo was designed to operate a three wire system of distribution without a balancer. The armature is provided with insulated slip rings connected to suitable points in the armature winding and (by means of brushes) with choking coils meeting at a common point, to which the neutral wire of the system is connected, the main terminals being connected with the outside wires.

The machine is capable of feeding unbalanced loads without serious disturbance of the pressure on either side of the system.

The principle of the Dobrowolski three wire dynamo is ill.u.s.trated in fig. 199. The armature A is tapped at two points, B and B', and connected to slip rings C C'. A compensator or reactance coil D, between the two halves of which there is minimum magnetic leakage, is connected to C and C' by brushes, and has its middle point tapped and connected to the neutral wire E.

[Ill.u.s.tration: FIG. 200.--Armature of Westinghouse three wire dynamo.

Collector rings are mounted at one end of the armature as shown, and the leads to them with the armature winding are similar to those employed on the alternating current side of a rotary converter armature. The connections from the armature to collector rings may be either single phase, two phase, or three phase. The two phase connection with four collector rings and two balance coils is used in the Westinghouse three wire dynamo.]

It is clear, from the symmetry of the arrangement, that the center point of the coil must always be approximately midway in pressure between that of the brushes, and hence any unbalanced current will return into the armature, dividing equally between the two halves of the coil.

The arrangement forms a cheap and effective subst.i.tute for a balancer set, but lacks the adjustable properties of the latter.

There are various modifications of the arrangement. Thus more than two slip rings may be used. The compensator windings, however, should always be arranged so that the magnetizing effect of the neutral current is self-neutralized in the windings, as otherwise saturation occurs causing a very heavy alternating magnetizing component.

CHAPTER XVI

FIELD MAGNETS

The object of the field magnet is to produce an intense magnetic field within which the armature revolves. It is constructed in various forms, due in a large measure to considerations of economy, and also to the special conditions under which the machine is required to work.

Electromagnets are generally used in place of permanent magnets on account of: 1, the greater magnetic effect obtained, and 2, the ability to regulate the strength of the magnetic field by suitably adjusting the strength of the magnetizing current flowing through the magnet coils.