Preventive maintenance maximizes uptime while controlling costs.
Maintenance crews strive to reduce maintenance costs and manage their workloads more effectively. Since DC machines require more attention than their AC counterparts, DC maintenance can help achieve both goals. Time-saving tips include changing brushes, checking winding insulation with a megohmmeter, and machining commutators in place. Knowing when to perform this maintenance is the key.
Common commutator wear patterns. It's relatively easy to judge DC motor performance by looking at the commutator. During normal commutation, brush current vaporizes some copper molecules, depositing a uniformly tan to charcoal-brown copper-oxide film (about 2 in.310-7 in. thick) on the commutator. Faint charcoal streaks along the brush paths (Fig. 1 above) may also indicate normal commutation. Even a heavy film coating isn't necessarily cause for alarm, particularly if no areas are unusually dark or worn. The following three conditions demonstrate what can happen to a commutator that has been neglected.
Slot bar marking and burning. Alternating dark and light bars on the commutator can be caused by the first (or last) conductor in the slot passing under the brush before (or after) entering the field of commutation. Certain factors can exacerbate slot bar marking and cause slot bar burning (Fig. 2), including electrical overload, incorrect neutral setting, improper interpole strength, and contamination. When the trailing edges of the darker bars appear etched or burnt, you need to resurface the commutator.
Threading. Threading involves the transfer of excessive amounts of metal to the brushes during commutation when brush path(s) start to resemble the threads on a bolt (Fig. 3). As with streaking, threading may be due to inadequate brush spring tension or too light a load. Left unchecked, the condition will lead to rapid brush wear. If you detect threading soon enough, you can machine the commutator in place to restore the finish.
Grooving Unlike threading (Fig. 4), grooves tend to smoothly wear to the width of the brushes, and commutator material is worn away rather than transferred to the brushes. Although grooving is usually due to abrasive dust in the environment, it can also be caused by a brush grade that's too abrasive. As grooving progresses, the sloped walls pinch the brushes, diverting spring tension from the brush face. This increases electrical resistance, generating more heat at the brushes and commutator. Left unchecked, grooving may lead to a “flashover,” or arcing between brush holders, that can seriously damage the brush holders, the commutator, or both. At the first sign of grooving, check for and eliminate airborne abrasive dust. Corrective actions include adding filters or ducting that moves clean air from another location. In either case, it's critical to maintain adequate airflow, because if grooving progresses too far, the only solution is to replace the commutator, which requires an armature rewind.
Too little brush spring tension increases resistance across the brush-commutator connection, generating excess heat. But it's easy to measure and correct. Attach a spring scale to the brush shunts and pull. When the brush lifts, slowly release the tension until it begins moving down. The pressure at this point is the spring tension.
Next, calculate the brush pressure. Multiply the thickness of a single brush by its width to find the square area. Divide spring tension by brush area to get brush tension. For most applications, brush pressure should be 4 psi to 6 psi. Traction motors are subject to higher vibration levels and should have brush pressure of 6 psi to 8 psi.
Current density and brush grade Each carbon brush grade is designed to carry a specific current. High-current density increases the operating temperature of carbon brushes. Current density that's too low causes rapid brush wear. In extreme cases, brushes can dust, or wear out, in days. To determine current density, measure the cross-sectional area of the brush surface. Think of the brushes as a group of series-parallel conductors. Since half of them constitute a parallel path for the armature current, those brushes share the total armature current. Base the calculations on actual, not nameplate, current.A simple trick to increase brush density involves removing half of the brushes from the machine. When doing so, it's vital to remove the same brush position from each brush holder row. For a load that varies frequently, discuss the range of loading with your brush supplier.
Electrical balance Brushes play an important role in keeping the machine electrically balanced. Brushes of negative polarity deposit material on the commutator, and those of positive polarity remove material. To maintain this balance, the number of positive and negative brushes in each circumferential brush path must be equal. Keep spares on hand. DC motor performance suffers when you mix brush grades.By monitoring the condition of DC motors, you can spot and correct developing problems during regular maintenance, saving time and money. Changing weak brush springs or correcting a problematic air supply is much easier than replacing a motor.
It's much more effective to treat the root of a problem than the symptom. When it takes five people one shift to change a motor, every DC motor replacement avoided is like a week in the bank. And that's a week better spent on other maintenance issues.
Yung is a technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis.