Why do typical DC motors use brush contacts rather than just the conductors of the wire itself? Wouldn't the wires be more efficient/low wattage dissipation in that region?
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2\$\begingroup\$ Depends on the motor actually, I have a couple of small DC motors in front me that use tiny copper springs like DKNguyen describes in his edit. \$\endgroup\$– csizMay 11, 2021 at 13:52
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2 Answers
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You're not looking at what a brush actually has to do. Brushes slide, need to maintain contact, wear, and arc. So you need spring loading, low friction, resistant to welding and corrosion, and need to be soft and replaceable so they wear instead of the expensive, difficult to replace commutator ring, but not too soft or they won't last.
I also have a feeling you are imagining a bristled brush with copper bristles. Motor brushes do not look like that, at least carbon brushes don't. They look like block of graphite with a coil spring. I've never seen what a precious metal brush looks like in person.
EDIT: Searching around, precious metal brushes are probably more what you might have in mind when you asked your question. Little strips of metal cutouts to make fingers (not bristles) where the strip itself acts as the spring.
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1\$\begingroup\$ I had never heard of precious metal brushes. A quick only search suggests, that they actually are cheaper than graphite brushes...? Do you have any idea why that is the case? \$\endgroup\$– jusacaMay 11, 2021 at 12:04
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1\$\begingroup\$ @jusaca Precious metal brushes are cheaper than carbon brushes? I wasn't aware of that. It might be that stamping a small amount of expensive sheet metals is cheaper than forming a bunch of cheap graphite? Not sure. But I think I've only seen precious metal brushes for smaller motors. Quick searching indicates carbon brushes are more rugged mechanically and electrically. I'm not sure how precious metal brushes have less friction though. support.maxongroup.com/hc/en-us/articles/… \$\endgroup\$– DKNguyenMay 11, 2021 at 19:00
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1\$\begingroup\$ Yes, that was the same link I was checking out as well. The first point of the precious metal brushes is the lower price point. But maybe that really just comes from the fact, that they are used in smaller motors...? Hm, that does not really make sense as an argument :D \$\endgroup\$– jusacaMay 11, 2021 at 20:14
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2\$\begingroup\$ @jusaca I assume that the size factor has already been taken into account since the info came straight from a motor manufacturer and you didn't come to that conclusion by running around comparing brush prices. It's probably a manufacturing thing rather than a materials cost thing. What's more cost effective for mass production of small parts? Baking graphite or stamping metal? Stamping is known for high mass production but you can bake lots of stuff in an oven at the same time. It might come down to the fact that baking takes time and you can stamp out a part in seconds. \$\endgroup\$– DKNguyenMay 11, 2021 at 20:15
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Because the armature reverses polarity each time the brushes switch to the next commutator slot. If it did not switch it would not turn. In DC motors, we need a commutator or something for converting DC power into AC power. The commutator (mounded on armature) is a rotating part of the dc motor. If we connect electrical wires to the commutator it causes problem nearly all types of DC motors have some internal mechanism, either electromechanical or electronic, to periodically change the direction of current in part of the motor. DC motors were the first form of motor widely used, as they could be powered from existing direct-current lighting power distribution systems. Since the commutator is a rotating part therefor connecting the electrical wires directly the electrical wires will also rotate along with the commutator which is not a feasible option.
