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.
[Illustration: FIGS. 188 and 189.--Circuit diagrams to illustrate 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.
[Illustration: 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 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.
[Illustration: FIG. 191.--The two path method of regulating a series
dynamo. The ends of the series winding are connected by a shunt containing
a rheostat. The current induced in the armature, divides and flows through
the two paths thus offered, the amount flowing through the shunt being
regulated by the rheostat. In this way the field strength is easily
regulated.]
[Illustration: FIG. 192.--Regulation of series dynamo by variable field. A
multipoint switch is provided with connections to the field winding at
various sections, thus permitting more or less of the field winding to be
cut out to regulate its strength.]
=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.
[Illustration: FIG. 193.--Shunt wound dynamo for parallel circuit
incandescent lighting, and for mill and factory power. The coils of the
field magnet form a shunt to the main circuit; they consist of many turns
of fine wire and consequently absorb only a small fraction of the current
induced in the armature. The characteristic of the shunt dynamo is that it
gives practically constant voltage for all loads within its range. If
overloaded the pressure will drop and the machine cease to generate
current.]
=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.
[Illustration: FIG. 194.--Regulation of shunt dynamo by method of varying
the field strength. A rheostat is placed in series with the field coils,
and by varying the resistance, more or less current will flow through the
coils, thus regulating the field strength.]
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.
[Illustration: 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.
[Illustration: 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.
[Illustration: FIG. 198.--Separately excited dynamo. Current for field
excitation is supplied by a second and smaller generator.]
=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.
[Illustration: 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
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.
[Illustration: 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 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.