Chuck Yung
EASA Senior Technical Support Specialist
When working with DC machines, it’s not uncommon to find a customer who runs different products requiring a range of armature current. Unlike other conductors, where lower current is a good thing, carbon brushes are designed for a narrow range of current density.
The carbon brushes are usually selected for the environmental conditions (humidity, airborne chemicals, altitude) but also the rated armature current (full load), so operation at reduced load would call for a change in brush grade. It’s not practical or advisable to change the brushes each time a different load is required. A unique aspect of carbon brush life is that brushes operating at a much lower current density than they are designed for do not last nearly as long as expected. That is important enough to provide a specific example: A brush designed for 25-50 amps/square inch (3.9-7.8 amps/square cm) might last for six months or longer, but if it is operating at only 10-15 amps/ square inch (1.6 - 2.3 amps/square cm), it might only last a couple of weeks.
One practical approach for lighter loads is to remove brushes in proportion to the percentage of full load amperage (FLA) at which the motor will be running. Let’s consider one example: a DC motor with four brushes per post (Figure 2) operating at three-quarters of rated load. Removing one brush per post (Figure 3) increases the current density of the brushes to the range for which they were intended, so the brush life will not be reduced.
This is where things can get tricky, especially if your customer only has a vague notion about the correct way to remove brushes. That film on a commutator is more complicated than it looks. One polarity of brush deposits material on the commutator, while the other polarity picks up material from the commutator. Copper oxide particles between the brush face and commutator are vaporized by the current, forming the chocolate-colored patina (Figure 1) we are accustomed to seeing when a DC motor is operating well. If the rate of deposition is greater than the rate of retrieval, a heavy black film results. If the rate at which material is picked up exceeds the rate at which material is deposited, threading results. The copper is literally being stripped out of the commutator.
Every circumferential brush path must have an equal number of positive and negative brushes. Not only that, the edges of those brushes must be exactly aligned. If the positive and negative brush tracks are slightly offset, the result will be a heavy black strip on one edge and threading on the other edge.
Back to the customer who is going to operate a new product that only requires three-quarters of rated hp. When removing brushes, it is important that each circumferential brush path still holds the same number of positive as negative brushes. While we could remove an entire circumferential path – that is, all the brushes in one position such as those closest to the risers – the commutator will last longer if we distribute the brushes to use the entire commutator surface.
For machines with 8 or more poles, it is advisable to also stagger the brushes axially (Figure 3). This is done by increments of half the brush width, so the entire commutator surface is utilized. If you have ever seen a commutator with wavy ridges (Figure 4) between the brush paths, you will understand why this is done. In keeping with the earlier explanation of positive and negative brushes working together to form a good film on the commutator surface, our half-brush-width shift must also maintain that distribution of positive and negative brushes as illustrated in Figure 3.
According to at least one brush manufacturer, this method of spacing has an added benefit, which is the suppression of slot bar marking, but only if the brushes of adjacent posts (i.e., positive and negative posts) are aligned.
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