Repairing small universal motor - dipping? 1

[chopnhack]

Hi all, I wanted to know if anyone could help me repair a small universal motor. The motor itself isn't expensive, however, the manufacturer is not in this hemisphere and shipping would be ridiculous! I have unwound the failed field coil and am preparing to order some magnet wire. The coil itself was pretty well stuck to itself, but not to the paper separating it from the shell. I am assuming that they used some kind of clear polyurethane or varnish. Can anyone help me find a suitable product that would not require baking? I dont think my wife would approve of our only oven being used for that purpose The motor is a 110vac, 15a.
Thanks in advance!

 

[minder]

I would see if one of the local motor rewinders can either offer a small quantity of shellac or suggest a local source, fortunately it is the stator as the type of shellac is more critical for the rotor and is usually baked, especially with the high centrifugal forces of a Universal motor.
Al.

 

[waross]

It must be a very special and very expensive motor to even consider repairing. These small motors often fail because of wear or clearance in the bushings. If the bushings are worn you may expect a rapid failure of the repaired motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[chopnhack]

Thanks Minder. I have access to shellac, but the shellac you mention, is it different from plain old woodworking shellac that is dissolved with denatured alcohol? I know I got really lucky that the rotor windings were intact. I think they made it like that since the rotor had much heavier gauge windings. I also have access to lacquer and polyurethane for woodworking, I don't know if these are any different than what they use in repair of motors. Thanks for any info you can provide.

 

[chopnhack]

Bill, the motor is not expensive but replacing the motor is not likely to happen. The machine that it came out of is nearly $600 and out of my range to replace right now. For me to spend $50 and get it running again is more likely, as long as it works Thanks for the tip on the bushings, I am not sure I follow you exactly though. The carbon brushes are in good shape and hardly worn. The rear of the rotor sits in a cup with a ball bearing which rotates freely with no noise. I found this varnish spray:

http://www.amazon.com/gp/product/B0..._m=ATVPDKIKX0DER&pf_rd_r=1T4YFKBPXKA45RB6Y42P

What would be my method of rewinding? Use magnet wire that is already coated, wind the motor then spray this on or spray every few wraps? Something else?

Thanks!

 

[stardelta]

For an application such as this I would try to obtain and use "bonding-wire". I dont want to appear to be endorsing or advertising so try a web-search.

Forgive me if you are already aware of the fact but its vital to wind the coil with a wire/wires of the same diameter and with the same number of turns as the original. The second coil will also have been stressed and it would be a good idea to rewind it at the same time using the same former or process to ensure the two coils are, as near as possible identical and have similar volumes and resistances.

 

[chopnhack]

No, you raise good points. I both counted the windings as they came off, and not trusting myself to only count, I also after unwinding took the mess of wire and wrapped it around a ruler to give me total length. I came within 4" on the smaller gauge winding and within 5" on the larger gauge comparing both sides of the field. I believe the difference lies in the connection points and the wire that bridged the distance across the field. As for using self bonding wire, I didn't know of it, but a helpful page from Essex told me that essential I would still need to bake it, which I don't have facilities for. Also, there is mention that the bond coat throws off the total thickness some. At roughly 50-60 turns, it might not make too much of a difference. It was originally wrapped with 18 and 24 gauge wire.

 

[waross]

When the bushings on a small motor develop enough clearance, the shaft tends to spiral around inside the bushing like a planetary gear inside a ring gear. This will often put enough extra load on the motor to burn it out.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[chopnhack]

Excellent point Bill, I understand now what you were referring to earlier. Thanks for the explanation. I guess then it would be wise to also replace the bearings. When you say bushing, are you referring to the bearings that allow the rotor to turn or something else?

 

[waross]

Many small motors, particularly older ones, used graphite impregnated brass bushings rather than roller bearings.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ccjersey]

I understood that your motor had sealed ball bearings in it. If they roll smoothly still, leave them alone. You need to ensure that you do not loose any shims or "wavy washers" which are frequently used behind the bearing on the end opposite the output shaft. These sometimes stick to the bearing as the end bell is removed and are lost while the motor is apart.

I have heard of folks using regular polyurethane finish and drying it in layers with a heat lamp (wind a few layers, brush the poly on and dry) . The spray-on Krylon enamel should be a much quicker drying product and may not need the heat lamp.

I wound a similar motor in a circular saw once just for the experience of doing it. Did not dip or bake and it lasted several years of intermittent use before one of the ends came loose from the brush connection and I threw it away like I should have done at first. The local electric motor shop measured the wire for me, and I believe he got me some that was larger than original. The winding was really tight to fit in the space, but the saw ran fine.

 

[ScottyUK]

Bill,

I think they are generally oil-impregnated sintered bronze: the sintered metal behaves like a honeycomb which stores oil, which then wicks to the bearing surface. Google 'Oilite' for a bit more info.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

Your probably correct, Scotty. Earlier ones were solid metal with a cutout and a felt wick to store and provide oil.
The point I was trying to make was the failure mode when the clearance became excessive.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[chopnhack]

Bill, I think ccjersey has it right. The end of the rotor is a sealed bearing that spins freely by hand with no grinding and takes some time to spin down. I will have to look at the opposite end next time I am in the shop.

 

[ScottyUK]

Hi Bill,

Agreed! Usually accompanied by a horrible noise if the driven load is a fan.


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If we learn from our mistakes I'm getting a great education!