CHAPTER XV
CLASSES 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 classified 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 class.
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 classed 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 class 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.
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.
=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 passed 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 classes 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 passes through the coils of the field magnets to the external circuit. The current in passing 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?=
Ans. It may be used for series arc lighting, series incandescent lighting, and as a _booster_ for increasing the pressure on a feeder carrying current furnished by some other generator.
=Ques. What is the effect of the series winding in the operation of the machine?=
Ans. Its characteristic is to furnish current at an increased voltage as the load increases. If sufficient current be drawn to overload the machine, the voltage will drop.
Since the armature coils, field magnets and external circuits are in series, any increase in the resistance of the external circuit lessens the power of the machine to supply current, because it diminishes the current in the coils of the field magnets and therefore diminishes the effective magnetism. Again, a decrease in the resistance of the external circuit will increase the voltage because more current will flow through the field magnets. Accordingly, when the external circuit has lamps in series (as is common in an arc light circuit) the switching on of an additional lamp both adds to the resistance of the circuit and diminishes the power of the machine to supply current. When the lamps are in parallel, the switching on of additional lamps not only diminishes the resistance of the circuit, but causes the field magnets to be further excited by the increased current, so that the greater the number of lamps put on, the greater becomes the risk of inducing too much current.
The series dynamo has also the disadvantage of not starting action until a certain speed has been attained, or unless the resistance of the external circuit be below a certain limit.
=Regulation of Series Dynamos.=--The series dynamo is ordinarily used for operating arc lamps connected in series. The current generally consumed is about 10 amperes, and it is necessary that it should remain at this strength to keep the lights burning steadily. If it increase, the lights will be too bright, and if it decrease, they will be too dim or flicker.
With all the lamps connected in series it is evident that the resistance of the circuit will vary widely as they are turned on or off, the resistance increasing as the lamps are turned on, and decreasing as they are turned off. It is necessary, therefore, that some means of regulation be provided to enable the dynamo to increase or decrease the voltage in proportion to the load. There are several methods of regulation, as by:
1. Variation of armature speed; 2. Variation of position of brushes; 3. Variation of field strength.
Whatever method be used the necessary regulation should be accomplished by automatic devices, as it would not be practical to station a man in constant attendance to regulate the voltage every time one or more lamps were thrown on or off.
=Ques. When is the first method of regulation used?=
Ans. It is only used in special cases, as for constant load; if the voltage be not just right to give the required current, it may be adjusted by changing the speed of the engine.
=Ques. What may be said of the second method?=
Ans. In both the “ring” and “drum” types of armature, rotating in a bipolar field, there are two points situated at opposite extremities of a diameter of the commutator, at one of which the potential is a maximum and at the other a minimum, and it is at these points that the brushes must be placed in order to obtain the greatest difference of pressure, the difference being less at other points. Hence, by rocking the brushes around the commutator the pressure at the terminals of the machine may be varied and regulated as required.
=Ques. What difficulty is experienced in rocking the brushes to regulate the voltage?=
Ans. Sparking takes place at the brushes when they are moved any considerable distance from the neutral position.
Special dynamos have been designed to overcome this objectionable feature, still this method of regulation is not extensively used.
=Ques. What may be said of the third method of regulation?=
Ans. The third method, that of variation of field strength, is the one in general use.
=Ques. How is the field strength varied?=
Ans. This may be done by the _two path method_, or by the _variable field coil method_.
=Ques. Describe the two path method of field regulation.=
Ans. An adjustable resistance or _rheostat_ is connected in parallel with the field winding as shown in fig. 191. This shunts more or less of the current from the field winding according to the amount of resistance made active by the lever, _L_.
Thus, if the current in the armature and main circuit be 10 amperes and the resistance of the field winding 10 ohms, a resistance of 40 ohms in parallel with the winding would cause the current to split in the ratio of 40 to 10, or 4 to 1; 2 amperes would pass through the resistance and 8 amperes through the field.
=Ques. Describe the variable field coil method of field regulation.=
Ans. This consists in dividing the field winding into a number of sections and throwing the sections in and out of circuit as shown in fig. 192.
Since the strength of any magnet depends on the number of ampere turns in its field winding, reducing or increasing the number of turns will respectively reduce or increase the field strength, the current being kept constant.
=Ques. What is the objection to this method?=
Ans. This arrangement is undesirable for magnets of large size, because of the tendency to flashing at the contacts of the regulating switch.
=The Shunt Dynamo.=--The shunt wound dynamo differs from the series wound machine, in that an independent circuit is used for exciting its field magnet. This circuit is composed of a large number of turns of fine insulated copper wire, which is wound round the field magnets and connected to the brushes, so as to form a shunt or “by pass” to the brushes and external circuit, as shown in fig. 193. Two paths are thus presented to the current as it leaves the armature, between which it divides in the inverse ratio of the resistance. One part of the current flows through the magnetizing coils, and the other portion through the external circuit.
In all well designed shunt dynamos, the resistance of the shunt circuit is always very great, as compared with the resistance of the armature and external circuit, the strength of the current flowing in the shunt coils being very small even in the largest machines.
=Ques. For what service is the shunt dynamo adapted?=
Ans. It is used for constant voltage circuits, as in incandescent lighting.
=Ques. In the operation of a shunt dynamo what is its characteristic feature?=
Ans. The voltage at the dynamo remains practically unchanged, and the current varies according to the load.
=Ques. Does the voltage remain constant for all loads?=
Ans. There is a certain maximum load current that the shunt dynamo is capable of supplying at constant voltage; beyond this, the voltage will decrease, the machine finally demagnetizing itself, and ceasing to generate current.
=Ques. Why does the voltage not remain constant for all loads?=
Ans. Because there is a drop in the voltage in forcing the current through the armature windings which increases with the load.
=Ques. What is the usual method of regulation for shunt dynamos?=
Ans. The method of varying the current through the field coils by means of a rheostat inserted in series with the field winding as shown in fig. 194.
Moving the lever L of the rheostat to the right increases the resistance in series with the field winding, and this reduces the amount of current in that winding, thus reducing the strength of the magnet and consequently the voltage at the brushes. The contrary movement of the lever, by cutting out the resistance, produces the opposite effect.
=The Compound Dynamo.=--This class of generator is designed to automatically give a better regulation of voltage on constant pressure circuits than is possible with a shunt machine. It possesses the characteristics of both the series and shunt machines, of which it is in fact a combination.
The field magnets of the compound dynamo, as shown in fig. 195, are wound with two sets of coils, one set being connected in series, and the other set in parallel, with the armature and external circuit. The purpose of the series winding is to strengthen the magnets by the current supplied from the armature to the circuit, and thus automatically sustain the pressure. If the series winding were not present, the pressure at the terminals would fall as the load increased. This fall of pressure is counteracted by the excitation of the series winding, which increases with the load and causes the pressure to rise. The number of turns and relative current strengths of the series and shunt windings are so adjusted that the pressure at the terminals is maintained practically constant under varying loads.
With respect to the ratio between the number of turns of the two field windings, the dynamo is spoken of as:
1. Compound; 2. Over compounded.
=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 passing through the winding may be regulated to control the voltage of the machine.
=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.
=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.
=Separately Excited Dynamos=.--In this class 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.
=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 illustrated 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.
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 substitute 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.