=BRUSHES AND THE BRUSH GEAR=
With respect to construction, brushes may be broadly classified as: 1,
those made of metal, and; 2, those made of carbon. There are several
varieties of metal brush, such as:
1. Gauze brushes;
2. Wire brushes;
3. Laminated or strip brushes.
=Gauze Brushes.=--These are very flexible and yielding, their use
being attended with little wear of the commutator.
=Ques. What is the construction of a gauze brush?=
Ans. A gauze brush is made up of a sheet of copper gauze, folded
several times, with the wires running in an oblique direction, so
as to form a solid flat strip of from ¼ to ½ inch in thickness,
increasing with the volume of the current to be collected.
=Ques. What is the object of folding the gauze with the wires running
in oblique directions?=
Ans. It is to prevent the ends of the brush fraying or threading out,
which would be the case if the gauze were folded up in any other
manner.
=Ques. What are the features of gauze brushes?=
Ans. They make good contact, but are quite expensive. They may be set
either tangentially or radially, the latter preferably, since the point
of contact remains the same as the brushes wear away.
[Illustration: FIGS. 329 to 332.--Various forms of brush. Fig.
329 gauze brush; fig. 330 laminated or strip brush; fig. 331 strip and
wire brush as used on the early Edison machines; fig. 332 carbon brush.
Carbon is preferred to copper for brushes on account of the reduction
of sparking secured by its use.]
=Wire Brushes.=--This class of brush, which was extensively used
before the invention of the gauze brush, is made up of a bundle of
brass or copper wires, laid side by side and soldered together at one
end. Since wire brushes are harder than the gauze brush, they are more
liable to cut or score the commutator, and are also more troublesome to
trim.
=Laminated or Strip Brushes.=--These probably represent the
simplest form of brush, but are not extensively used owing to the lack
of flexibility. They consist of a number of strips of copper or brass,
laid one upon the other and soldered at one end, as in fig. 330.
[Illustration: FIG. 333.--General Electric brush holder.
The brush holder yoke consists of a cast iron ring of elliptical
section, supported from the bracket of the end shield in such a manner
as to facilitate the shifting of the brushes. It is provided with a
suitable handle, and may be fastened in any position by means of a
thumb nut on the outside of the bracket. It is so constructed that the
tension on the individual brush can be adjusted without lifting the
brush from the commutator and without the use of tools. The brush can
be removed while the machine is running, without moving the holder
on the stud and without disturbing any other brush. Removal of the
brushes for inspection can be accomplished without permanent change
in the adjustment of the tension of the brush holder spring. The
connection between the brush and stud is made through a flexible copper
connection.]
=Ques. What name is generally given to strip brushes?=
Ans. They are commonly and erroneously called _tangential brushes_,
but they are really beveled at the end and set inclined to the line of
tangency so that the ends of all the sheets will make contact.
In the Brush and Thomson-Houston arc dynamos, in which the
current is limited to ten amperes, the brushes consist of a
simple strip of flexible sheet copper, the ends of which are
slit in a number of places so as to insure contact at several
points.
[Illustration: FIG. 334.--Crocker-Wheeler brush holder. The
carbon brush B is firmly clamped in the "box" C by two screws which
bear on a sheet of brass to protect the carbon from being broken by the
ends of the screws. The box C is carried by four flexible springs S S,
one at each corner and formed of hard copper leaves. These are fixed at
one end to the box and at the other to the solid base which is in one
piece with the spoke attached to the rocker ring. An adjusting screw
passes through appropriate lugs on the box C and loosely through the
head A of a fixed arm _a_. Between the lower surface of _a_ and the
upper lug on the box C is placed the pressure spring.]
=Carbon Brushes.=--When metallic brushes are used upon the
commutators of high tension machines, they frequently give rise to
excessive sparking and also heating of the armature, the metallic dust
given off appearing to lodge between the segments of the commutator,
thus partially short circuiting the armature. To obviate this, carbon
brushes are extensively used in such dynamos, this material being found
very effectual in the prevention of sparking.
=Ques. What is the usual form of carbon brushes?=
Ans. They are usually in the form of oblong blocks.
[Illustration: FIG. 335.--Perspective views of Crocker-Wheeler
brush holder. This holder is of the parallel type in which the brushes
may be adjusted without affecting the lead. Each brush is held rigidly
in its box and there are no sliding contacts in the path of the
current. The holder is further described under fig. 334.]
=Ques. How are they adjusted on the commutator?=
Ans. They are set "butt" end on the commutator, and fed forward as they
wear away by means of a spring holder.
=Ques. Why are carbon brushes so extensively used?=
Ans. Because they are the only form of brush that will give good
commutation with fixed lead.
=Ques. What may be said of the different grades of carbon in use for
brushes?=
Ans. The very soft carbon leaves a layer of graphitic matter on the
commutator, and at high voltages, this may cause sparking; such grade
of carbon should only be used on low voltage machines.
[Illustration: FIG. 336.--Western Electric box type brush
holder. The box which holds the brush is broached to allow the brush
to slide freely, but not loosely, to and from the commutator against
which it is normally held by a lever acting directly upon the brush
head. This avoids the possibility of uneven bearing on the commutator,
as the brushes are allowed very slight lateral or angular motion. The
adjustment of a brush is also simplified after it has been removed
and then replaced. Tension on the brush head is obtained by a special
spring which maintains any given tension for which it may be set. An
auxiliary flat steel spring on the lower side of the lever acts as a
shock absorber between the lever and the brush head, absorbing all
minor vibrations caused by a worn commutator. Side contact between
brush and brush holder is not relied upon to carry the current,
flexible copper pigtails performing this function to the exclusion of
sliding contacts or tension springs, in order to reduce the brush loss.
It is not necessary to take the brush rigging apart or loosen cable
connections when it is desired either to remove or reverse the brushes
to change the direction of armature rotation.]
=Ques. How are the ends of carbon brushes treated and why?=
Ans. They are usually covered at their upper part with a coating of
electro-deposited copper to insure good contact with the holder.
=Comparison of Copper and Carbon Brushes.=--Copper brushes tend to
tear and roughen the surface of the commutator, while carbon brushes
tend to keep the surface smooth. Copper causes more wear of the
commutator than carbon. With carbon brushes, the armature may be run in
either direction. The resistance of carbon being greater than copper,
there is less short circuiting caused by carbon particles than by those
of copper.
[Illustration: FIG. 337--Westinghouse brush holder. It is made
of brass, cast in one piece, and of standard sliding type with a shunt
of braided copper wire directly connected to a clamp on each brush and
to the solid portion of the holder, where it is held by a screw. This
shunt relieves the spring of heavy currents. The holder is so arranged
as to be easily accessible for adjustment, cleaning and renewal of
carbons. Proper tension is provided by spiral strap springs so mounted
as to eliminate friction and give uniform pressure over a wide working
range. The spring tension is readily adjusted by a simple ratchet
arrangement.]
=Ques. What is the chief merit of carbon brushes?=
Ans. They give less sparking than other types.
=Ques. How has the construction of carbon brushes been varied?=
Ans. Since, for minimum sparking, it is only necessary that the brush
have high resistance in the region near its edge, attempts have been
made to increase the conductivity of the other portions by combining
with the carbon, copper sheets or wires.
=Ques. What are the objections to carbon brushes?=
Ans. They are easily broken and not being flexible, vibration, or any
roughness of the commutator will cause bad contact.
[Illustration: FIG. 338.--Holzer Cabot multiple brush holder.
Each brush is fastened securely to a machined surface by one or two
machine screws, making a positive contact. Several strips of flexible
copper of ample section to carry the current are interposed between
the part of the holder carrying the brush and the portion clamping the
stud, no sliding contact or spring being therefore required to carry
any current. The brushes are proportioned for a carrying capacity of
not more than 25 amperes per square inch of brush surface. The brush
can be adjusted to any degree of tension during the operation of the
machine if necessary. Each holder is insulated in such a manner that no
short circuit can occur if the holder be accidentally tipped backward
while the operator is changing a brush or cleaning the commutator
during a run.]
=Ques. For what class of machine are carbon brushes specially
adapted?=
Ans. For machines furnishing a small current at high pressure.
When carbon brushes are used, it is desirable that the current
be small, because, on account of the low conductivity of the
carbon, more contact area is necessary than with copper for
equal current transmission. For fixed lead and fluctuating
currents, carbon brushes should be used.
=Ques. For what class of machine are copper brushes especially
adapted?=
Ans. For machines furnishing large current at low pressure, as in fig.
328.
=Size of Brushes.=--The number of brush sets depends upon the
number of poles of the machine, but there may be several brushes in
each set. It is usual, except in the smallest machines, to place at
least two brushes exactly similar side by side instead of one broad
brush, thus allowing one brush to be removed for trimming or renewal
while the machine is running. Moreover, better contact is secured by
this sub-division, because a slight elevation in the commutator surface
at one point may slightly raise one brush of a set at each revolution
without much harm, while with one broad brush, the entire brush would
be lifted, causing bad sparking.
=Ques. What determines the number of brushes in each set?=
Ans. It depends upon the current capacity, size of machine, and
judgment of the designer.
=Ques. What may be said with respect to the dimensions of the
brushes?=
Ans. No general rule can be given for breadth and thickness of brush.
The contact face must clearly be wider than the thickness of the
insulation between commutator segments, since the period of commutation
must last an appreciable interval of time on account of self-induction.
=Ques. What should be the minimum width of the brush contact face?=
Ans. It may be taken as one and one-half times the thickness of the
commutator segments.
=Ques. How wide should a carbon brush contact be?=
Ans. The brush should be thick enough to cover two and one-half
commutator segments. The thickness should in no case be excessive on
account of the loss due to heating, which results from the difference
of potential at the forward and rear edge of the brush.
[Illustration: FIG. 339.--Contact angle for the different
types of brush. At A is shown a brush with tangential contact, and at
B, a so called tangent brush; the latter is properly called an inclined
brush. Sheet copper brushes are set tangentially as at A, and gauze
brushes inclined as at B. Carbon brushes are placed radially as at C
when mounted in box holders, and inclined opposite to the direction of
rotation when used with reaction holders.]
=Contact Angle of Brush.=--This may be defined as the angle which
the brushes make with the commutating plane as shown in fig. 339.
The several kinds of brush, together with the varied conditions of
operation require different contact angles ranging from zero to 90°.
Thus in the figure, a copper strip brush may lie at 90° or
tangentially as at A.
Wire or gauze brushes should make a more or less acute angle as
at B, in order to present the end and not the side of the brush
to the commutator.
Carbon brushes may be placed end on or radially as at C, which
is the position almost universally used in the case of traction
or other reversing motors.
Sometimes the carbon brush is inclined as at D, in order that
the revolving commutator may tend to push the brush against its
supports and thus ensure better contact.
=Brush Contact.=--The relation between _contact pressure, contact
resistance_, and _friction_ of brushes varies greatly for different
kinds of brush. Copper brushes will carry from 150 to 200 amperes
per square inch of contact surface; and carbon brushes from 40 to 70
amperes per square inch. The usual contact pressure is 1.25 to 1.5
pounds per square inch for copper brushes, and 1.5 to 2 pounds per
square inch for carbon brushes. The rim velocities of commutators vary
from 1,500 to 2,500 feet per minute, the velocity usually increasing
with the size of the machine.
[Illustration: FIG. 340.--Bissell double brush holder.
Flexible cables carry the current between the brushes and holders.
This holder works equally well for forward or reverse rotation. Two or
more holders are used on each stud except for the two smallest frames.
The construction permits of adjustment or renewal of brush while the
machine is in operation. Sufficient contact area of brush is provided
to permit running on one carbon at ordinary loads in case the other
become worn or inoperative.]
=Ques. What is the drop in voltage at the brushes?=
Ans. For carbon brushes it is about O.8 to 1.0 volt at each contact,
or 1.6 to 2.0 volts for the two, positive and negative, contacts of a
machine.
This value is not materially affected by placing a number of
brushes in parallel or by using several sets, as in the case
of multipolar machines, as such arrangement merely reduces the
current density, and since the contact resistance varies in the
inverse ratio, their product remains nearly constant.
=Ques. What may be said of the friction of the brushes?=
Ans. The coefficient of friction of brushes is about .2 to.25 for
copper and .3 for carbon.
[Illustration: FIG. 341--Western Electric brush gear. The
brush holders carry carbon brushes and are so designed that the brushes
may be firmly clamped in position and also be capable of independent
adjustment. Any brush can be removed while the machine is in operation
without disturbing the others and without moving the holder on the
stud.]
=Ques. How many watts are lost at the brushes?=
Ans. The watt loss is equal to 1.6 to 2. volts for carbon multiplied by
the total current carried.
The watt lost on account of friction may be calculated by the
formula: ((.3 × 746)/33000) × (P × S) = watts lost by carbon
friction, in which P is the total pressure in pounds on the
commutator, and S, the rim velocity of the commutator in feet
per minute.
The losses due to contact resistance and brush friction are
very liable to be greatly increased above the values that may
be obtained by the preceding methods, if the commutator and
brushes are dirty and rough, or not in good condition.
[Illustration: FIGS. 342 to 345.--Various types of brush
holder. Fig. 342, arm or lever type; fig. 343, spring arm type; fig.
344, box type; fig. 345, reaction type.]
=Brush Holders.=--These are devices employed to hold the
brushes against the commutator with the proper pressure. They differ
considerably in various types of machine, hence, no general rules can
be given with respect to their construction or use, but any brush
holder must fulfill the following requirements:
1. It must hold the brush securely and at the same time feed it forward
as it wears away so as to maintain a proper contact;
2. It must hold the brush at the proper contact angle;
3. It must be capable of being raised from the commutator, and held out
of contact by some form of catch;
4. It must be so constructed that the brush can be easily removed for
cleaning or renewal;
5. The spring pressure must be adjustable;
6. The brush holders themselves must be carried on a rocker arm, or
rocker ring.
It is desirable that brush holders be capable of individual adjustment,
so that each may be set at its own point of minimum sparking. A few
forms of brush holder are illustrated in figs. 342 to 345.
The various kinds of brush holder may be divided into four types:
1. Arm or lever type;
2. Spring arm type;
3. Box type;
4. Reaction type.
In the arm or lever type the brush is firmly attached to the extremity
of a rigid arm capable of movement about the brush spindle, except in
so far as it is restrained by a spring as in fig. 342.
Fig. 343 shows a brush holder of the spring arm type. The brush is
firmly attached to the extremity of a spring arm, the other end of
which is secured to the brush spindle, and when once adjusted is not
capable of movement about the brush spindle.
In the box type of brush holder as illustrated in fig. 344, the brush
is free to move up and down in the brush box, so far as it is not
restrained by a spring rigidly secured to the arm which carries the
brush box at its extremity.
[Illustration: FIG. 346.--Fort Wayne type MPL dynamo; view
showing details of armature, commutator and brush rigging of large
machine. The laminations of the armature core are punched from thin
sheet steel, annealed and japanned. Spacing ribs are built into the
core at proper intervals forming air passages for ventilation. In
addition, there are recesses in the inside of the flanges which permit
the passage of air from the interior around the ends of the core to
the openings in the end flanges. The armature coils are constructed of
round or bar copper on standard forms. The coils are laid in slots in
the surface of the core. The commutator is constructed of bars of hard
drawn copper of uniform size and shape, supported and clamped at either
end between beveled rings and securely seated on the commutator drum.
The drum is connected by radial arms to the commutator sleeve which is
mounted and keyed on the armature hub extension.]
Fig. 345 shows the reaction type of brush holder, in which the movement
of the brush is constrained in one direction by the surface of a part
rigidly secured to the brush spindle, and is further constrained by
a spring controlled arm, the pressure of which is capable of ready
adjustment.
Among the special forms of brush holder may be mentioned
1. Scissor type of brush holder, used for slip rings, and
consisting of two arms pivoted together like a pair of
scissors. The lower ends of the arms carry the brushes,
suitably mounted, and the upper ends are drawn together by a
spring, which thus exerts pressure on the brushes.
2. Clock spring type of brush holder in which the necessary
contact pressure is applied to the brush by means of a clock
spring, which, with the aid of a ratchet may be wound up and
adjusted to any desired pressure.
[Illustration: FIG. 347.--Western Electric brush holder. This
holder consists of a rugged iron casting, elliptical in section, and
supported from the commutator end bearing bracket in such a manner as
to provide for the shifting of the brushes. A handle attached to the
yoke aids in this shifting and a thumb nut on the outside holds the
whole brush gear in the desired position. The brush is fed through an
accurately broached slot by a spring which maintains uniform pressure
against the commutator throughout the wearing length of the brush. The
long lever arm of the spring is sufficiently flexible to take up any
minor vibrations of the brush. The tension of the brush may be adjusted
without lifting it from the commutator or disturbing any of the other
holders. The brush, may be removed for inspection by throwing the
spring out of notch. The brush is connected to the holder by flexible
copper pig tails of ample current carrying capacity.]
=Ques. How are brush holders carried?=
Ans. They are carried by a _rocker arm_ for bipolar, and by a _rocker
ring_ for multipolar machines, which is mounted upon one of the main
bearings, or upon a support specially provided for it, being pivoted
to revolve from the same center as the shaft, to permit shifting the
brushes.
=Ques. Mention one trouble sometimes encountered with brush
holders.=
Ans. There is sometimes trouble resulting from the current passing
through the spring which heats it and destroys its elasticity.
[Illustration: FIG. 348.--Western Electric parallel spring
brush holder as used on the larger machines.]
=Ques. How may this be avoided?=
Ans. By insulating one end of the spring, and carrying the entire
current directly from the brush itself to the main conductors by a
flexible copper strip or cable firmly connected to both.
=Ques. What may be said with respect to brush construction on
machines for electrolytic work?=
Ans. The collection of large currents at low voltage, generated by
comparatively small machines, requires careful design of brushes
and brush holders. The commutator is longer than the commutators
on machines of equal capacity at higher voltages, and as a rule the
commutator segments are thicker and fewer in number. Each brush set is
made up of numerous narrow brushes rather than two abnormally wide ones.
An example of brush and brush gear designed to meet such
conditions is shown in fig. 328.
In large machines for electrolytic work, it is not unusual to
find the current divided between two wide commutators, one at
each end of the armature, thus giving a longer axial bearing
surface for the brushes without inconveniently lengthening the
pins upon which the separate brushes are threaded.
=Multipolar Brush Gear.=--The brush gear which includes the
holders and carrier arm or ring, becomes more complicated as the number
of poles and magnitude of the current is increased.
In the early days of multipolar machines, schemes of armature
winding were devised such that all the necessary cross
connections were made inside the machine, and the number of
brush holders reduced to two and placed at an angular distance
apart depending upon the number of poles. Such windings,
though possible, are not used much, chiefly on account of
their complexity, which not only increases the danger of error
in construction, but also makes repairs costly. In modern
multipolar machines, such complicated windings are avoided,
and the several sets of brushes are connected together in two
groups, positive and negative. These connections are carefully
designed as part of the brush gear.
=Ques. How are the brushes held in large multipolar dynamos?=
Ans. They are held at the proper points of commutation by arms offset
from a cast iron rocker ring, which is itself supported by brackets
projecting from the magnet yoke as shown in fig. 346.
=Ques. What provision is made for shifting the ring to adjust the
lead?=
=Ans.= The ring is rotated by means of a worm gear and hand wheel.