CHAPTER XXI

=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.

=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.

=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.

=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.

=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.

=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.

=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.

=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.

=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.

=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.

=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.

=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.

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.

=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.

=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.