Motors and Generators:Brushes


Correct care of brushes, brush rigging, and current-collecting parts is a fundamental necessity if satisfactory performance is to be obtained. Adequate inspection is essential to the maintenance of this equipment and the follow- ing points should be observed:

• Brush holder box should be adjusted between 1/16 and 1/8 in. from the surface of the commutator.

• Care should be taken to see that dirt and particles broken from the edges of brushes or the commutator have not lodged in the face of the brush.

• Brushes must be correctly aligned, and the commutator brushes must be correctly staggered, pairs of arms (+ or −) being set alternately.

• Brush is affected by such adverse conditions as sparking, glowing, rough commutator, severe chattering, no-load running, overload running, incorrect spring pressure, and selective action.

• Brush on a machine that sparks or glows owing to load conditions, off-neutral operation, or an electrical fault in the machine will be burned and pitted near the sparking edge.

• Severe chattering of the brush is caused by a high-friction film on the surface of the commutator or by incorrect spring pressure.

• Brush chattering due to a high-friction film occurs on machines where there is considerable no-load or light-load running. The char- acteristic curve of friction versus load current is of such a shape that minimum friction can be obtained at approximately 55 A/in.2 and as load current is either reduced or increased, the brush friction is increased. Accordingly, it is sometimes good practice, when a machine is running at very light loads for a considerable period of time, to lift one or more brushes per arm to bring the brush friction into the desir- able range. Cases where the load current is above the normal values are more serious, because the higher currents produce sparking, overheating of the machine and brush chatter simultaneously.

• Spring pressure has a direct effect on the riding characteristics of a brush. A common error is to reduce spring pressure for cases where brush wear or marking of the commutator has been observed. This permits the brush to bounce on the commutator, which, in turn, causes sparking and selective action and produces a rough commu- tator. On the other hand, excessive spring pressure causes brush wear and commutator wear, and usually lowers the electrical contact voltage drop to the point where satisfactory commutation is not obtained. Correct spring pressure should be 2½–5½ lb/in.2 for indus- trial service and 5–10 lb/in.2 for traction service. The lower range on traction work will be found where spring-supported motors are used; axle-hung motors use the higher range.

• When checking spring pressure, the action of the brush in the box should be free. Dirt or gummy oil on the brush or in the brush box sometimes causes the brush to stick and in some cases, to completely break the contact between the brush and the commutator.

• Commutator wear in various forms is frequently attributed to a brush that is too hard. Actually, the abrasiveness of a brush does not result from its hardness. Some of the most abrasive brushes are soft to the touch or low when measured for scleroscopic hardness. The property in a brush of five grade that causes abrasiveness is controlled by the brush manufacturer, who should be consulted for information as to the relative cleaning properties of the various grades. Brush Adjustment

The brushes of a new machine are generally adjusted at the factory to the electrically neutral position, and it should not be necessary to change the adjustment. An exception to this rule may occur on large machines where an off-neutral setting is sometimes used to improve commutation. In any case, the method for identifying the correct brush position is given in the manufacturer’s instruction book. Various methods may be used for determining the neutral position. The kick method is commonly used as is outlined here.

With all brushes raised from the commutator and the machine standing still, voltages will be induced in the armature by transformer action if the shunt field is excited to about one-half of its normal strength and the field current suddenly broken. It will be found that the induced voltages in conductors located at equal distances to the right and left of the main pole centers will be equal in magnitude and opposite in direction.

Hence, if the terminals of a low-reading voltmeter (5 V) are connected to two commutator bars on the opposite side of a main pole and exactly halfway between the centerlines of two main poles, the voltmeter will show no deflection when the field current is broken. The spacing of these commutator bars is, therefore, the correct distance between brushes on adjacent brush arms.

The most practical method of making this check is to make two pilot brushes of wood or fiber to fit the regular brush holder, each brush carrying in its center a piece of copper fitted for line contact with the commutator bar. With a lead for the connection of adjacent brush arms, the brush rigging may then be shifted slightly forward or backward, as necessary, until breaking the field current produces no deflection on the voltmeter. By noting the position at which no deflection is obtained for each pair of brush arms, the aver- age of the positions of neutral thus obtained will give the correct running location for the brushes.

A quick and convenient method of locating the neutral position on a DC motor and shunt fields is to check the speed of the motor in either direction with the same impressed line voltage. The position of the brushes that produces the same speed in either direction under the same voltage conditions is the correct neutral position.

Another shortcut is to take a piece of lamp cord and bend it in the middle, bringing the two ends together. The insulation should be removed for 1/2 in. on each end and the bare wires twisted together, fanning out to form a brush. When this brush is held so that it spans two bars at the outer end of the commutator and moved with and against the direction of rotation, the point of least sparking at the ends of the wires is the correct location for the centerline of the brushes.

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