COMMUTATION AND THE COMMUTATOR
The act of commutation needs special study. If it be incorrectly
performed, the imperfection at once manifests itself by sparks which
appear at the brushes. In the study of this chapter on commutation it
would be advisable for the student to first review the basic principles
of commutation as given in chapter XIV, which contains a brief and
simple explanation of how the alternating current in the armature is
converted into direct current by the action of the commutator.
=Ques. What is the period of commutation?=
Ans. The time required for commutation, or the angle through which the
armature must turn to commute the current in one coil.
=Ques. Upon what does the period of commutation depend?=
Ans. Upon the width of the brushes as shown in fig. 300.
This fixes the angle through which the armature must revolve
to commute the current in one coil. This angle is formed, as
shown in the figure, by two intersecting planes, M and S, which
pass through the axis of the armature and the two edges of the
brush. Commutation then, begins at M and ends at S.
=Ques. What is the position of the commutating plane with respect to
M and S, in fig. 300?=
Ans. It bisects the angle formed by the planes M and S.
[Illustration: FIG. 300.--Armature with one brush in position
to illustrate the _period of commutation_ and _commutating plane_. The
latter is called "commutating line" by some writers. The period of
commutation depends on the thickness of the brush end in contact with
the commutator. Careful distinction should be made between _commutating
plane_, _neutral plane_, and _normal neutral plane_ as defined
elsewhere.]
=Ques. What is the the commutating plane?=
Ans. An imaginary plane passing through the axis of the armature and
the center of contact of the brush.
=Ques. What two planes are referred to in stating the position of the
brushes?=
Ans. The normal neutral plane and the commutating plane.
The angle intercepted by these two planes represents the
_lead_, thus in stating that the brushes have a lead of 6°,
means that the angle intercepted by the normal neutral plane
and the commutating plane is 6°.
[Illustration: FIG. 301.--The proper position of the brushes,
if there were no field distortion and self-induction in the armature
coils, would be in the normal neutral plane. In the actual dynamo
these two disturbing effects are present which makes it necessary to
advance the brushes as shown in figs. 302 and 303 to secure sparkless
commutation.]
=Ques. What is the difference between the normal neutral plane and
the neutral plane?=
Ans. This is illustrated in figs. 301 and 302. _The normal neutral
plane is the position of zero induction assuming no distortion of
the field_ as in fig. 301. _The neutral plane is the position of zero
induction with distorted field_ as in fig. 302 and as is found in
the actual machine; the distortion is exaggerated in the figure for
clearness.
[Illustration: FIG. 302.--Brush adjustment for field
distortion. The effect of the latter is to twist the lines of force
around in the direction of rotation, thus maximum induction takes place
in an inclined plane. The brushes then must be advanced to the _neutral
plane_ which is at right angles to the plane of maximum induction. This
gives the proper position of the brushes _neglecting self-induction_.]
=Ques. What is the normal plane of maximum induction?=
Ans. _A plane, 90° in advance of the normal neutral plane_, being the
position of maximum induction with no distortion of field, as in fig.
301.
=Ques. What is the plane of maximum induction?=
Ans. _A plane 90° in advance of the neutral plane_, being the position
of maximum induction in a distorted field as in fig. 302.
[Illustration: FIG. 303.--Brush adjustment for self-induction.
For convenience an electric current is regarded as having weight and
hence possessing the property of inertia. The current then during
commutation cannot be instantly brought to rest and started in the
reverse direction but these changes must be brought about gradually
by an opposing force. Hence by advancing the brushes beyond the
neutral plane as illustrated, commutation takes place with the short
circuited coil cutting the lines of force so as to induce a current in
the opposite direction; this opposes the motion of the current in the
short circuited coil, brings it to rest and starts it in the opposite
direction, thus preventing sparks. Figs. 301 to 303 should be carefully
compared and thoroughly understood.]
=Ques. What should be noted with respect to the different planes?=
Ans. The commutating plane should be carefully distinguished from the
normal neutral plane and from the neutral plane, as shown in fig. 303.
=Commutation.=--In order to understand just what happens during
commutation, a section of a ring armature may be used for illustration,
such as shown in fig. 304. Here the coils A, B, C, D, E, are connected
to commutator segments 1, 2, 3, 4, and the positive brush is shown
in contact with two segments 2 and 3, the brush being in the neutral
position. Currents in the coils on each side of the neutral line flow
to the brush through segments 2 and 3; the brush then is positive.
[Illustration: FIG. 304.--Commutation. This takes place during
the brief interval in which any two segments of the commutator are
bridged by the brush. The coil connecting with the two segments under
the brush is thus short circuited. During commutation the current in
the short circuited coil is brought to rest and started again in the
reverse direction against the opposition offered by its so called
inertia, or effect produced by self-induction.]
Now, as the armature turns, the commutator segments come successively
into contact with the brush. In the figure, segment 3 is just leaving
the brush and 2 is beginning to pass under it, hence, for an instant
the coil C is short circuited.
=Ques. In fig. 304, what are the current conditions?=
Ans. Previous to contact with segment 2, current flowed in coil C in
the same direction as in coil B.
=Ques. What occurs while the brush is in contact with segments 2 and
3?=
Ans. During this brief interval, the current in C is stopped and
started again in the opposite direction.
Similarly each coil of the armature as it passes the brush will
be short circuited and have its current reversed. This is known
as _commutation_.
=Ques. What is the effect of field distortion with respect to
commutation?=
Ans. The neutral plane no longer coincides with the normal neutral
plane but is advanced in the direction of rotation of the armature as
shown in fig. 302.
The reaction of the poles N' and S' of the armature field on
the poles S and N of the main magnetic field tends to crowd
the lines of force into the upper pole face of the south pole
of the magnet, and into the lower pole face of the north pole.
This effect is due to the strong magnetic attraction between
the opposite poles S and N' and N and S', and the equally
strong repulsion between like poles N and N' and S and S'.
Hence, the plane of maximum induction no longer coincides
with the normal plane of maximum induction, but is advanced
in the direction of rotation, depending upon the strength of
the armature current, being shifted forward for an increase
of current, and backward for a decrease of current. This
distortion of the field and the consequent shifting of the
plane of maximum induction naturally results in the shifting of
the neutral plane from the vertical position to the inclined
position as shown.
=Position of the Brushes; Sparking.=--In accordance with the
laws of electromagnetic induction, if the bipolar ring armature shown
in fig. 301 be rotated in the direction indicated by the arrow the
armature current entering at the brush E will divide, one part passing
through the coils on the right half of the ring, and the other part
through the coils on the left half of the ring, to the brush F, from
which the total current will pass out, urged by the full value of the
electromotive force induced in all the coils on both halves of the ring.
[Illustration: FIGS. 305 to 308--Improper brush adjustment
resulting in excessive sparking. When the brushes are not advanced far
enough, commutation takes place before the short circuited coil reaches
the neutral plane, hence, its motion is not changed with respect
to the magnetic field so as to induce a reverse current till after
commutation. There is then no opposing force, during commutation, to
stop and reverse the current in the short circuited coil, and when the
brush breaks contact with segment 1, as in fig. 308, the "_momentum_"
of the current in coil F causes it to jump the air gap from segment 1
to segment 2 and the brush, against the enormous resistance of the air,
thus producing a spark whose intensity depends on the momentum of the
current in coil F. Sparking, if allowed to continue, will injure the
brushes and commutator segments.]
Again, if the brushes be placed at the points G and H, each half of a
current entering at G, will pass through one-half of the coils on the
left side and one-half of the coils on the right side of the ring, so
that each half of the current will be urged forward by an electromotive
force equal to the electromotive force tending to force it back, and
therefore, no current will pass in or out through the brushes. From
these considerations it is obvious that the proper position for the
brushes would be in the normal neutral plane, _were it not for the
disturbing effects of armature reaction and self induction of the
current_.
=Ques. Should the brushes of a dynamo be placed in the neutral
plane?=
Ans. No.
=Ques. Why not?=
Ans. The brushes must be advanced beyond the neutral plane to prevent
sparking.
=Ques. What is the cause of sparking at the brushes?=
Ans. It is due to _self-induction_ in the coil undergoing commutation.
=Ques. Explain the effect of self-induction in detail.=
Ans. When commutation takes place with the brushes in the neutral
plane as in fig. 304, there will be no voltage induced in the short
circuited coil C. The current, therefore, which flowed in coil C before
it was short circuited will cease, and as segment 3 breaks contact with
the brush, it will be thrown as a perfectly idle coil upon the right
hand half of the ring in which a current is flowing toward the brush.
Moreover, the current which was flowing through D and 3 directly to the
brush, must suddenly traverse the longer path through the idle coil C.
Now, on account of self-induction, _the current acts in precisely the
same manner as though it had weight_; that is:
_It cannot be instantly stopped or started._
[Illustration: FIGS. 309 to 313.--How sparkless commutation is
obtained by advancing the brushes beyond the neutral plane; commutation
progressively shown.]
[Illustration: FIG. 309.--Commutation begins; current flows
up both sides of the armature, uniting at S and flowing to the brush
through commutator segment 1 as indicated by the arrow.]
[Illustration: FIG. 310.--Segment 2 has come into contact
with the brush and coil F, in which commutation is taking place, is
now short circuited. The current now divides at M, part passing to
the brush through segment 2, and part through coil F and segment 1.
Although coil F is short circuited and having passed the neutral plane,
is cutting the lines of force so as to induce a current in the opposite
direction, it still continues to flow with unchanged direction against
these opposing conditions. This is due to _self-induction_ in the coil
which resists any change just as the momentum of a heavy moving body,
such as a train of cars, offers resistance to the action of the brakes
in retarding and stopping its motion.]
Therefore, when segment 3 leaves the brush, the current will not
instantly change its path and flow through C, but will be urged by its
"_momentum_," and jump the air gap between the brush and segment 3,
thus producing a spark.
[Illustration: FIG. 311.--Segment 2 has moved further under
the brush, and the opposition offered to the forward flow of the
current in the short circuited coil F by the reverse induction in the
magnetic field to the right of the neutral plane has finally brought
the current in F to rest. The currents from each side of the armature
now flow direct to the brush through their respective end segments 1
and 2.]
[Illustration: FIG. 312.--Segment 1 is now almost out of
contact with the brush. A current has now been started in the coil F
in the reverse direction due to induction in the magnetic field to the
right of the neutral plane; it flows to the brush through segment 2.
The current has not yet reached its full strength in F, accordingly,
part of the current coming up from the right divides at S and flows to
the brush through segment 1.]
[Illustration: FIG. 313.--Completion of commutation in
segments 1 and 2; the brush is now in full contact with segment 2, the
current in coil F has now reached its full value, hence the current
flowing up from the right no longer divides at S but flows through F
and segment 2 to the brush. If the current in F had not reached its
full value, at the instant segment 1 left contact with the brush, it
could not immediately be made to flow at full speed any more than could
a locomotive have its speed instantly changed. This, as previously
explained, is due to self-induction in the coil or the so called
"inertia" of the current which opposes any sudden change in its rate of
flow or direction. Accordingly that portion of the current which was
flowing up from the right and passing off at S to the brush through
segment 1 as in fig. 312, would, when this path is suddenly cut off as
in fig. 313, encounter enormous opposition in coil F. Hence, it would
momentarily continue to flow through segment 1 and jump the air gap
between this segment and the brush, resulting in a more or less intense
spark depending on the current conditions in coil F.]
=Ques. How may this sparking be prevented?=
Ans. If the brushes be given additional lead, that is shifted further
to the right to some position as N N, fig. 304, coil C will not
remain idle during the interval it is short circuited, _but will
cut the magnetic lines in such a way as to induce a current in the
reverse direction through it_. Under these conditions, when segment 3
breaks contact with the brush, the current flowing through D does not
encounter an idle coil, but one in which a current is flowing in the
same direction, hence, the tendency to jump the air gap and produce a
spark is reduced; with proper adjustment of the brushes, there will be
no sparking.
=Ques. What is the objection to very thin brushes?=
Ans. Time must be allowed for reversal of the current, hence the
brushes must not be so thin as merely to bridge the insulation between
segments.
=Ques. What is the effect of lead?=
Ans. There is usually much sparking when the lead is too small; a
little sparking when too great, and no sparking when just right. If the
lead be excessive, there is a waste of energy due to the generation of
a larger reverse current in the short circuited coil than is necessary.
=Fixed Position of Brushes.=--The condition for sparkless
commutation is that the current in the short circuited coil be reduced
to zero, and increased in the opposite direction up to the same value
as that in the next coil leading. If the brushes are to remain in a
fixed position, this condition will only be realized at the particular
load for which the brushes are set. Thus, if the brushes be set for
the average load, the reversing field will not be correct for either
a weaker or stronger load. Hence, sparkless commutation with fixed
brushes must be due to some other factor.
=Ques. What may be said with respect to carbon brushes?=
Ans. Since carbon possesses a high resistance, the drop will vary
greatly with the contact area, thus affecting a difference of potential
in the two segments passing under the brush and it is largely to this
that sparkless commutation is due.
=Ques. What is the effect of resistance on commutation?=
Ans. In fig. 304 during commutation, that is, while the brush contacts
with any two segments, as 2 and 3, the currents coming up through the
winding on either side of the neutral plane are offered two paths to
the brush: 1, direct to brush through the connecting segment, or 2,
across the short circuited coil and adjacent segment. Thus, on the
right side: to brush through segment 3, or across coil C and adjacent
segment 2.
_The current will take the path of least resistance._
At the beginning of commutation, almost the entire brush area being in
contact with segment 3, the contact resistance of this segment will
be much less than for segment 2; hence, not only will the current
at the right flow through 3, but also the current at the left after
first traversing the short circuited coil. As commutation progresses,
the area of contact of 3 decreases while that of 2 increases, and
the respective resistances vary in inverse proportion. Likewise the
tendency of the current in the left half of the winding to take the
longer path through coil C and segment 3 to the brush gradually
decreases, becoming zero when the two contact areas become equal.
During the second half of the period of commutation, the contact area
of segment 2 becomes greater and of 3, less; thus the resistance of 2
is lowered, and that of 3 increased. Accordingly, all of the current
at the left will flow through segment 2, and the current at the right
will flow through C and 2 rather than through 3. In this way the
current is reversed in C, and, if the brush be broad enough to allow
a sufficient time interval, the current in C is built up to its full
value before segment 3 leaves the brush, thus securing sparkless
commutation.
This contact resistance factor in sparkless commutation
is illustrated in figs. 314 to 318, it being assumed that
during commutation, the brush contact resistance is inversely
proportional to the area of contact, and that the winding is
free of resistance and inductance. The current is taken as 40
amperes, in which case 20 amperes will flow from each side of
the winding to the brush.
In fig. 314 the instant before commutation begins all the
current will flow through segment A. At the end of the first
quarter of the period of commutation, fig. 315, 30 amperes will
flow from the right to brush through A, and from the left, 10
amperes through the short circuited coil via A and 10 amperes
through B.
At the end of the second quarter or half period, fig, 316, the
current through each half of the winding will flow to the brush
through these respective segments.
At the end of the third quarter, fig. 317, the current from the
right will divide, 10 amperes going through A, and 30 amperes
traversing the short circuited coil and out through B. The
entire current from the left will flow through segment B.
At the end of the fourth quarter, fig. 318, or completion of
the period the current from each half of the winding will flow
to the brush through B.
[Illustration: FIGS. 314 to 318.--Brush contact resistance
theory of commutation, neglecting self-induction and resistance in the
coils. The total current is assumed to be 40 amperes made up of 20
amperes flowing toward the brush from the coils on the right and 20
amperes from the coils on the left. During commutation, that is, the
interval during which the brush contacts with any two adjacent segments
of the commutator, the current is assumed to vary directly as the
contact area.]
[Illustration: FIG. 314.--Beginning of commutation; segment A
is entirely under the brush, and B is at the initial point of contact.
For this position the currents from both sides flow to the brush
through segment A.]
[Illustration: FIG. 315.--One-quarter period of commutation.
One quarter of the brush area is in contact with B and three quarters
in contact with A; hence, 10 amperes will flow through B and 30 amperes
through A.]
[Illustration: FIG. 316.--Second quarter of commutation
period. The brush now contacts equally with both segments, hence 20
amperes will flow through each segment.]
[Illustration: FIG. 317.--Third quarter of commutation period.
Three quarters of the brush area is in contact with segment B and one
quarter with segment A; accordingly, 30 amperes will flow through B and
10 amperes through A.]
[Illustration: FIG. 318.--Completion of commutation. The brush
is in full contact with segment B and at the point of breaking contact
with A, hence the entire current from both sides or 40 amperes will
flow through B.]
[Illustration: FIGS. 319 to 323.--Brush contact theory of
commutation for case in which the brush covers two segments of the
commutator. Fig. 319 beginning of commutation; fig. 320 one-quarter
period; fig. 321, one-half period; fig. 322, three-quarter period; fig.
323 completion of commutation.]
=Ques. What is the effect of increasing the degree of contact of the
brushes?=
Ans. It lengthens the period of commutation, and permits it to start
in one coil before the preceding coil has entirely passed through this
stage.
The effect of changing the degree of contact is shown in figs.
319 to 323, in which the width of the brush is made equal to
that of two segments.
=Construction of Commutators.=--The commutator for a closed coil
armature consists of a number of segments or L-shaped bars C of drop
forged hard drawn copper assembled around a tubular iron hub as shown
in figs. 324 and 325. The bars are held in position by the nuts E, and
washers F, screwed on the ends of the tube D. The bars are insulated
from each other and from the washers by mica as shown by the heavy
lines G, and they are also insulated from the tube either by a tube
of mica H, or by a sufficient air space. The ends of the sections
of winding are connected to the vertical portions of the bars K, by
insertion in the slots L, where they are securely held in place by
means of the binding screws, which for greater security are soldered
together, and may be released from the slots, whenever necessary, by
the application of a hot soldering iron.
[Illustration: FIGS. 324 and 325.--Side and end sectional
views of commutator showing construction. The parts are: C, segments;
D, tubular iron hub; E, end nuts; F, clamps; G, insulation; L, riser
connection.]
It is very important that all the parts of the commutator should be
fitted together perfectly and screwed up tightly, in order to prevent
looseness. Commutator segments are often made with the washers E,
projecting beyond the ends, but such construction reduces the effective
length of the commutator, therefore the under cut form of bar is
preferable.
In the construction of commutators, the conditions of operation require
that there be:
1. Adequate insulation;
It is necessary to have good insulation between each segment,
and a specially good insulation between the segments and the
hub or sleeve on which they are mounted; also between the
segments and end clamps. The insulating material must not
absorb moisture, hence asbestos, plaster, or vulcanized fibre
are not used. The end insulating rings are usually built up
of mica and shellac, moulded while hot under pressure to the
correct shape.
[Illustration: FIG. 326.--Front view of Western Electric
Commutator for bar wound armature. This commutator is made of hard
drawn copper and insulated throughout, ventilating spaces being
provided near the shaft.]
2. Rigidity against centrifugal force;
Since the segments are subject to centrifugal force, they
must be securely clamped in place. Screws cannot be used, for
that would destroy the insulation. They are therefore held in
place by insulated clamps as shown in fig. 324. These clamps
should be strong and capable of holding the segments firmly in
position, for if a segment should rise out of its place through
centrifugal force, it would disturb the action of the brush and
cause sparking.
3. Provision for wear.
The segments should be of considerable radial depth, so that
the commutator may be turned down from time to time to preserve
its circular form.
[Illustration: FIG. 327.--Sectional view of a General
Electric commutator. The segments are of rolled or forged copper and
are separated by soft mica insulating sheets. This mica must wear
down evenly with the copper, hence its consistency is important. The
segments are wedge shaped so that when drawn radially inward they
support each other like the stones of an arch. They are drawn together
by hollow cone collars which bear upon lugs projecting from the ends
of the segments. These lugs are turned to form a smooth cone after the
segments are assembled. The collars are insulated with mica from the
segments and they are held in place by nuts upon the commutator shell
or by bolts passing from end to end under the segments. The segments
are also provided with lugs for connection to the windings.]
=Points Relating to Commutators.=--1. The number of commutator
segments depends on the scheme of winding and on the number of sections
in the armature winding.
2. Increasing the number of bars diminishes the tendency to spark, and
lessens the fluctuations of the current.
There are two practical reasons for not using a very great
number of segments: it increases the cost, and in small
machines the segments would be so thin that a brush of the
proper thickness to collect the current would lap over, or
bridge several segments.
=Types of Commutator.=--Commutators are made in various forms, but
they may be grouped into two general types:
1. Commutators for closed coil armatures;
These consist of a large number of segments or bars, insulated
from each other and varying in number according to the scheme
of armature winding, and on the number of sections into which
that winding is grouped.
[Illustration: FIG. 328.--A large current low voltage bipolar
dynamo built for electrolytic work and here shown to illustrate the
large size commutator and brushes necessary to collect the large
current. Carbon brushes would not be suitable for this class of machine
because even with copper brushes, whose conductivity is much higher
than carbon, the commutator must be of considerable size to give the
required brush contact area. The contrast between the axial lengths of
the armature and the commutator is very marked. The rocker construction
is of the ordinary type, and heavy flexible cables conduct the current
from the brush holders to the fixed terminals. The machine here
illustrated gives 310 amperes at 7 volts when running at a speed of
1400 R. P. M., corresponding to an output of 2.17 kilowatts.]
2. Commutators for open coil armatures;
This form of commutator is used on some machines designed
especially for arc lighting, such as the Brush and
Thompson-Houston machines. They consist of a comparatively
small number of segments each of which covers a wide angle, and
are separated from each other by air gaps.
3. The segments should be of considerable depth to permit returning
occasionally so that their circular form may be preserved;
4. The insulating material must be such that it will not absorb oil or
moisture;
Mica is best adapted for insulation, but as there are a great
many varieties, differing greatly in hardness and other
equalities, it is important to select the kind that wears at
the same rate as the segments. If the mica be too hard, the
wearing of the segments will leave it projecting and prevent
proper contact with the brushes; again, if the mica be too
soft, it will result in furrows or depressions between the
segments into which copper dust will collect, causing short
circuits.