THE DYNAMO: CURRENT COMMUTATION
=How the Dynamo Produces Direct Current: The Commutator.=--The essential
difference between an alternator and a dynamo is that the alternator
delivers alternating current to the external circuit while the dynamo
delivers direct current. In both machines, as before stated, alternating
currents are induced in the armature, but the kind of current delivered to
the external circuit depends on the manner in which the armature currents
are collected.
In the case of an alternator, the method is quite simple. As
previously explained, each end of the loop is connected with an
insulated collector ring carried by the shaft, the current being
collected by means of brushes which bear against the rings. This
principle, rather than the actual construction, is shown in the
preceding illustrations. Its important point, as distinguished
from other methods of collecting the current, is that _each end
of the loop is always in connection with the same brush_.
=Ques. How is direct current obtained in a dynamo?=
Ans. A form of switch called the _commutator_ is placed between the
armature and the external circuit and so arranged that it will reverse the
connections with the external circuit at the instant of each reversal of
current in the armature.
=Ques. How is a commutator constructed?=
Ans. It consists of a series of copper bars or segments arranged side by
side forming a cylinder, and insulated from each other by sheets of mica
or other insulating material.
[Illustration: FIGS. 174 to 178.--Commutation of the current. These
figures show how a dynamo transforms alternating into the so-called direct
current. During the first half of the revolution the current flows in the
direction A B, out through segment F of the commutator and brush M,
returning through brush S and segment G, figs. 174 and 175. At the
beginning of the second half of the revolution, fig. 176, the current in
the armature reverses and flows around the loop in the direction B A. At
this instant the brushes M and S pass the gaps between the commutator
segments, thus reversing contact with the segments, and causing the
current in the external circuit to remain in the same direction.]
=Ques. Where is the commutator placed?=
Ans. It is attached to the shaft at the front end of the armature.
=Ques. What are inductors?=
Ans. The insulated wires wound on the armature core, and in which the
electric current is induced.
=Ques. How are the inductors connected to the commutator?=
Ans. The ends of each conducting loop or coil must be connected with the
commutator segments in a certain order to correspond with the type of
winding.
=Ques. Explain in detail how direct current is obtained in a dynamo.=
[Illustration: FIGS. 179 to 181.--Elementary dynamo armatures. Fig. 1,
single turn loop; fig. 2, coil of two turns _in series_; fig. 3, coil of
two turns _in parallel_. In operation the amplitude or maximum pressure
induced with the two turn coil, fig. 180, is double that of a single turn
loop, fig. 179. In fig. 180, the pressure is double that induced in fig.
181, while the amount of current generated with series turns, fig. 180, is
only half that generated with turns in parallel fig. 181.]
Ans. It will be easily seen by the aid of a series of illustrations just
how the alternating armature currents are transformed into direct current.
Figs. 174 to 178 show, in several positions, a single loop of wire with
its ends joined to a commutator; the latter has only two segments, one for
each end of the loop. In fig. 174 the loop is shown in the vertical
position, and it should be noted that the division between the two
segments forming the commutator is in the same plane as the loop. When the
loop is in the vertical position, as shown in fig. 174, brush M is in
contact with segment F, and S with G. As the armature rotates, the current
flows for one half revolution in the direction A B, through segment F and
out to the external circuit through brush M as shown in figs. 174 and 175,
returning through brush S and segment G. At the beginning of the second
half of the revolution, fig. 176, the current in the loop reverses and
flows in the opposite direction B A as indicated by the arrows. At this
instant, however, the brushes M and S pass out of contact with segments F
and G, and come into contact with G and F respectively; that is, M leaves
F and contacts with G, while S leaves G and contacts with F. The effect of
this is _to reverse the connections with the external circuit at the
instant the alternation or reversal of current in the armature takes
place_, thus keeping the current in the external circuit in the same
direction.
[Illustration: FIG. 182.--Gramme ring armature with one coil, and
characteristic sine curve below. With one coil as shown, there are two
pulsations of the current per revolution of the armature.]
=Ques. How is this indicated by the sine curve?=
Ans. The sine curve, instead of falling below the axis, as in figs. 169 to
173, again rises as in the first half of the period, that is G′H′I′ is
identical with E′F′G′.
=Ques. Is the direct current indicated by the sine curve in figs. 174 to
178 continuous?=
Ans. _No_; it is properly described as a _pulsating current_, or one,
constant in direction, but periodically varying in intensity so as to
progress in a series of throbbings or pulsations instead of with uniform
strength.
=Ques. What is generally understood by the word “continuous” as applied to
the current obtained from a dynamo?=
Ans. It is usually accepted as meaning a steady or non-pulsating direct
current; one that has a uniform pressure and constant direction of flow as
opposed to an alternating current.
=Ques. Is a continuous current ever obtained with a dynamo?=
Ans. _No._
[Illustration: FIG. 183.--Gramme ring armature with two coils placed 180°
apart. This arrangement gives double the pressure of the one coil
armature, fig. 182.]
It should be clearly understood at the outset that it is
impossible to obtain a continuous current with a dynamo. The
so-called continuous current which it is said to produce is in
reality a pulsating current, but with pulsations so minute and
following each other with such rapidity that the current is
practically continuous, and as such is generally called
continuous.
=Ques. How is the so-called continuous current produced by a dynamo?=
Ans. In order to obtain a large number of small pulsations per revolution
of the armature instead of two large pulsations, as with the single loop
armature, the latter must be replaced by one having a great number of
loops properly connected to commutator segments and so arranged that the
successive loops begin the cycle progressively.
* * * * *
The difficulties encountered in connecting up numerous loops were overcome
by Gramme, who, in 1871 invented a “ring” armature. His method consists in
winding a ring with a continuous coil of wire, connections being made at
suitable intervals with the commutator.
[Illustration: FIG. 184.--Four separate coils wound on ring to illustrate
the action of a Gramme ring armature. If the ring be rotated the
electromotive forces induced in adjacent coils will be equal and tend to
produce currents in opposite directions; hence, if the inner ends be
joined, the junctions would be at a higher potential (+ or -) than the
loose ends. With proper connections current may be collected at the
junctions.]
In order to understand the action of such an arrangement, it will be well
to first consider four separate coils wound on a ring as shown in fig.
184. These coils are all similar, but at the moment occupy different
magnetic positions on the ring. The rotation being clockwise, 1 is about
to enter the field adjacent to the north pole, while 2 is emerging from
the field in the region of the south pole. Again, 3 is approaching the
south pole and 4 receding from the north pole.
=Ques. Describe in detail the action of the four coils wound around the
ring as in fig. 184.=
Ans. According to the laws of electromagnetic induction, pressures are set
up at the ends of the coils such as tend to produce currents in the
directions indicated by the arrows. Now, assuming the electromotive forces
in coils 1 and 2 to be equal, if the adjacent ends be joined, no flow of
current will take place, but the junction will be at a higher pressure
than the loose ends of the coils and if a wire be attached to this
junction, and the necessary circuits completed, a current will flow along
the wire outward from the junction. Similarly, if the adjacent ends of
coils 3 and 4 be joined, there will be no flow of current, but the
junction will be at a lower pressure than the loose ends, and if a wire be
attached to the junction and the necessary circuits completed, current
will flow from the junction around the coils.
[Illustration: FIG. 185.--Gramme ring armature with four coils. The
electromotive force induced in coils A, A′ reaches the zero point at the
instant that of coils B, B′ is at a maximum; hence, sine curve No. 1,
beginning at zero, and No. 2, at the maximum, show the pressure changes
for A, A′ and B, B′, respectively. The summation of these curves gives
_the resultant curve_ No. 3, showing changes in pressure of current
delivered to the external circuit.]
=Ques. What may be said with respect to the four coil Gramme ring armature
shown in fig. 185?=
Ans. According to the laws of electromagnetic induction, with the north
pole of the field at the left and clockwise rotation, the induced currents
flow _upward_ on both sides of the ring, hence, _the electromotive forces
oppose each other at only two of the junctions, namely: at the one
connected to brush M where the pressures on either side are both directed
toward the junction and the other at the junction connected to brush S, at
which the pressures are both directed from the junction._
[Illustration: FIG. 186.--Gramme ring armature with six coils. The sine
curves 1, 2 and 3, represent the conditions due to coils AA′, BB′ and CC′,
respectively, and 4, the resultant pulsations.]
It is evident, then, that the pressure at M is higher than at S;
that is, M is positive and S negative; consequently, the current
flows from M to the external circuit and returns through S.
=Ques. In what other way may the four coils of the armature in fig. 185 be
regarded?=
Ans. They may be considered as two pairs A A′ and B B′, the action of
either pair being identical with the two coil armature shown in fig. 183;
this, in turn, produces the same effect as the one coil armature of fig.
182, with the exception that the amplitude of the current generated with
two coils is twice as great as that with one coil of the same number of
turns.
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.
[Illustration: FIG. 187.--The resultant curves of figs. 183, 185 and 186
are here shown for comparison to illustrate 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.