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brush question on repulsion-start induction-run motor

metalmagpie

Titanium
Joined
May 22, 2006
Location
Seattle
I know some of you are very knowledgeable about motors. I'm hoping one of you can answer this question off the top of your head. I'm going through a later Century R-I motor. It's got lots of filth and spider webs inside, like usual. It's probably 80 years old. Anyway, it is a 4-pole motor and has 4 brushes. I'm no expert, but I watched several long videos about tearing into such motors and in each case the brushes were independent. Yet on mine, there are two pairs shunted together with wire.

They are wedge-shaped brushes with the sides partly copper clad, although the copper has worn through in several spots. They look a lot like these:

Redirecting to https://store.eurtonelectric.com/brushesfloorequipment196a-1-1-1-2-5.aspx

The motor ran at the seller's house, so it doesn't seem to be a problem. But I want to understand why one repulsion/induction motor has independent brushes and another has two pairs of shunted brushes.

metalmagpie
 
All the repulsion-start, induction run motors I've seen had brushes that shorted sections of the commutator. Not powered brushes, but ones that just shorted to other brushes.
 
All repulsion start motors rely on shorted brush pairs. If they're not shorted then it's a traditional brushed motor rather than repulsion start. Could have been that the brushes didn't have shorting wires on them but were instead shorted by the brush holders themselves. I haven't looked that closely at mine yet but I think I recall seeing that the older 'RS' and 'P' frame Centuries use a solid copper/brass brush carriage to achieve this?

It's possible that the brushes on your motor were replaced at one point and the new ones just happened to be sold as shunted pairs.
 
All the repulsion-start, induction run motors I've seen had brushes that shorted sections of the commutator. Not powered brushes, but ones that just shorted to other brushes.

Jim, I'm sorry, but I don't understand what you wrote. Yes, the brushes bear on the commutator. On the other end of the motor is a shorting necklace that shorts all the commutator segments together. When the necklace is doing that, the motor behaves just like a squirrel cage induction motor.

Hmm. I think I see what you mean. You mean the brushes only make connections between commutator segments, but neither take power away nor introduce power to the motor.

If you look at this video https://youtu.be/D7wi3LWzTNU at 19:45 you will see him take a new pair of brushes with a wire shunt and cut away the wire. Then he cuts down the brushes to fit. So obviously in that motor none of the brushes are shunted to any others by wires.

I don't quite get what's going on here.

metalmagpie
 
All the brushes are connected together, that bracket is conductive. Since they are not connected to the power windings, they do not need to be insulated from the case.

The wires are not needed to connect them
 
I have a 2 hp Emerson motor that was NOS when I bought it in 1970. It's a reverse-induction motor. It came with my 1940 Parks Wood Planer.

When I purchased the planer and motor from the Oliver Machine Co, when they were in Los Angeles, I asked the owner about a spare set of brushes for the motor. He reluctantly sold me a set of 4 brushes but told me that due to the design of the motor, I would never need them.

The way that it was explained to me, was that the brushes just sit on the commutator of the motor and when power is applied and the motor starts, they are instantly lifted clear so there is no wear on them.

As mentioned, they are connected with a wire in sets of two. The motor now has many hours on it and is still in perfect operating condition. The spare brushes are still in a drawer in my shop, brand new, unused 50 years later. LOL
 
A solid copper casting! They sure don't make 'em like they used to. Many of these motors are 100 years old - and in the right hands they'll last another 100.

It's nonferrous, but I think it's brass, not copper. I have one right in front of me. The right name for it is a "brush cage".

metalmagpie
 
The brushes in my motor fit down into a machined pocket made of brass or copper. To facilitate conductivity, the brushes are clad in copper on the sides. Since these brushes are somewhat worn, have unequal lengths and are chipped on the contact face in places, I ordered new ones. At about 10 bucks a pair they were quite reasonable. I got them today. I should have realized but didn't that they aren't clad with copper on the sides.

Now I'm puzzled. Should I try to plate some copper on the new brushes? If so, can I just hang them in a saturated solution of copper sulfate (sulphate?) in water for awhile? I know if I do that with steel a light strike plate of copper is formed.

OTOH, should I just skip the copper-on-the-sides business and not worry about it?

metalmagpie
 
Honestly the copper plating doesn't really do a whole lot in this sort of application. It might reduce localized heating if you don't have fantastic contact between the carbon and the brush holder... but if you're really worried about it for the duration it's going to have current flowing through it (2 seconds?) you can just as easily bond the copper braid between each pair to the cage. (Assuming you got them as bonded pairs)
 








 
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