The carbon brush is a crucial piece to keep machines operating efficiently. However, “the little brush gets all the blame” is a statement commonly heard in the industry, but the blame is usually misdirected. One common problem is sparking at the brush face, which is usually the first symptom of trouble elsewhere.
Following is a brief rundown of the more common causes of sparking, along with suggestions for their diagnosis and treatment.
Brush Holders not equally spaced
This condition may appear as unequal sparking on different holders. It can be determined by counting the number of bars between holders or by putting a band of paper around the commutator, marking the positions of the brush toes, removing the paper, and measuring the distance marks. To correct, move the brush holders so that adjacent holders are all equally distant from one another.
Brush holders off electrical neutral
Even though the holders are equally spaced, they may be out of their correct position and cause sparking which may be equally severe on all brushes of the same polarity. This fault can be detected by running a field form curve. The trouble may be corrected by shifting the brushes.
Brush Holders damaged or dirty
Any physical damage to the holder or an accumulation of dirt on its inside may interfere with the free motion of the brush in the holder and thus result in sparking. Since the commutator is seldom perfectly round or concentric, the brush must movie in and out of its holder in order to maintain effective contact. Visual examination and testing the free action of the brush with the fingers are usually sufficient to reveal this condition. Thorough cleaning or complete replacement will improve operation.
Holders too far from commutator surface
If the holder is too far from the surface of the commutator, it fails to support or guide the brush properly. This may result in the brush cocking in the holder and binding, or the brush vibrating and losing contact with the commutator. The obvious remedy is to adjust the brush holders so that their nearest point is 1/16 to 1/8 in. from the commutator surface, depending on the type of machine.
Wrong interpole strength
This may be caused by a flaw in the interpole winding such as a broken wipe or short circuit, or by incorrect spacing of the pole face in relation to the armature. In either event, repair or correction may be facilitated through use of field form and brush drop curves.
Excessive overloads may result in severe sparking, especially if the interpoles have passed their saturation point and are therefore, unable to increase their strength as required. If the machine has an ammeter, compare its load with the nameplate rating. Make any load adjustments necessary.
Defective armature windings
And defect in the windings may show up as sparking at the brushes. It will frequently also be apparent by one or more burned places on the commutator. Check for high-resistance connections where the risers are soldered or brazed to the commutator or for poor connections in one or more of the equalizers. Remember, though, that any other fault in the armature may also show up as sparking. Where other tests verify an armature fault, the winding must be replaced.
Incorrect spring pressure
Contact drop of a brush is influenced by the pressure with which it is forced against the commutator. If the pressure varies from brush to brush, those brushes carrying a higher pressure will have a lower contact drop and will tend to take more than their share of the current. This can sometimes be determined by checking the temperature of the brushes immediately after shutting down the machine. If this fault is suspected, check the spring pressure on each brush with a scale. Adjust to the level recommended by the manufacturer.
Poor undercutting of commutator
If the commutator has high mica or fins of mica that reach up to the brush surface, vibration and sparking may result. Similarly, any burrs of copper left as a result of the undercutting operation will cause trouble. In some locations and atmospheres, commutator slots may become filled with foreign material. This can cause the brushes to vibrate or cause ring fire by permitting current to leak through from one bar to the other. Correction of these conditions is the solution to the problem.
Foreign material on commutator surface
Any gummy or gritty material which sticks to the commutator surface may cause sparking. Oil may be beneficial in extremely small quantities but frequently causes gumming when used excessively. Careful examination of the commutator will usually disclose such conditions. Thorough cleaning is recommended.
Black commutator films
Abnormally dark commutators may result from sulfur, excessive humidity and other gaseous materials. This condition may cause selective action and result in sparking on the heavily-loaded brushes. Solution of the problem may be difficult when commutator speeds are too high. A rather abrasive brush may be the answer. In other cases, frequent polishing of the commutator may be required.
Brushes binding in holder
When brushes are not of the correct size or when brush hardware projects too far at some point, brushes may bind in their holders. Damage to holders may also cause binding. If the brushes are too tight in the holders, their proper motion will be restricted so that they cannot maintain contact with the commutator and sparking may result. If they are too small, they may wobble in the holders and thus tend to break contact with the commutator and bring about the same result. The specific cause of binding should be determined and eliminated by repair or replacement.
Restricted brush motion
Brush motion may be restricted because of shunt stiffness or by contact with some other member. Similarly, something may interfere with the motion of the spring itself or with the hammer by which the spring applies its pressure to the brush. Mechanical adjustment will usually correct these conditions.
The commutator may be out-of-round of eccentric because of improper finishing. As a consequence, the brush may not be able to follow the surface and maintain proper contact. Careful measurements, preferably made with the armature rotated, will usually disclose this defect.
High bars or flat spots
These conditions can usually be identified by careful examination of the commutator when it is not running. A high bar will usually be polished and followed by several bars which look rough and pitted or burned. The high bar will usually result in a flat spot because it lifts the brush off the commutator and the following bars may be burned so that their height is reduced. To correct, the commutator should be ground or turned. If the trouble develops again, the commutator probably needs tightening by an expert.
Vibration of the machine itself may cause brush sparking and eventually result in commutator damage. Such vibration may be caused by imbalance in the armature, by poor foundations or other mechanical faults. It can also result from defective bearings. Pinpointing the cause of vibration will indicate the corrective course to follow.
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