What are the voltage and current in motor brushes?

  • Thread starter Rob45327
  • Start date
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3
Summary
I'm rewiring an antique 1 amp universal motor (AC or DC).
Summary: I'm rewiring an antique 1 amp universal motor (AC or DC).

The two field coils need new brush leads so I have to buy new wire for them and I don't know the voltage. If the motor is running on 110 Volts is it 110 volts at the brushes? The motor is speed is variable and mechanical resistance can vary to the point of stopping the motor The armature coil is toast and I will attempt to rewind after more research. Should be fun!
 
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Brush leads, like all wires, are not rated by voltage but by current. You need to know how much current they will have to carry and size your wire accordingly.
 
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The site that sells the wire says the the wires were for 5 to 12 volts and didn't mention current. The brush leads are fabric covered and I'm trying to redo the motor with vintage type stuff. Although I'm going use copper leads instead of steel for ease of soldering. All I know is it's a 1 amp rated motor running on 110. Thanks for your reply.
 
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One or more photos of the device details would go a long way.

Although I'm going use copper leads instead of steel for ease of soldering.
It is unlikely any leads within the motor are steel. What you've got there is almost certainly tinned copper.

If the motor is running on 110 Volts is it 110 volts at the brushes?
Both. If plugged into a 120V AC receptacle (in the USA; other countries have different conventions) 120 volts will be placed across the motor, one brush (the one connected to the 'hot' side) will be 120V from ground, and the other brush (connected to neutral) will be zero volts above ground.

Wire insulation voltage rating should be at least 150V.

Did a quick search, and the cloth-covered wire I've found are a cloth outer cover over a polymer inner insulator rated wither 300V or 600V similar to this.

http://www.radiodaze.com/18awg-stranded-600v/

If you have a micrometer, dial caliper, or better yet, a wire gauge (pictured below) measure what the existing wire diameter is, and use the same, or one size larger wire.
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18 AWG ought to be large enough for a 1 amp motor.

The motor is speed is variable and mechanical resistance can vary to the point of stopping the motor
Before going any further, check the brushes. This symptom is also consistent with badly worn brush(es), and/or weak springs that prevent the brush face(s) from making good contact with the commutator surface.
 
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I was thinking 18 gauge! I'm sorry I didn't mention that varying mechanical resistance is normal because it's a sewing machine. Normally these motors are such good shape I don't have to do anything but put on a new power cord. This motor is more beat up due hard use and bad servicing.
 
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What does the field core do? I'm a mechanical guy and only vaguely understand motors.
 

Averagesupernova

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Both. If plugged into a 120V AC receptacle (in the USA; other countries have different conventions) 120 volts will be placed across the motor, one brush (the one connected to the 'hot' side) will be 120V from ground, and the other brush (connected to neutral) will be zero volts above ground.
Not necessarily. I would not install anything except 300 volt rated wire, but most universal motors are series wound so the brushes are not wired directly across the line.
 
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... most universal motors are series wound so the brushes are not wired directly across the line.
Thanks for correcting me. One brush will be at 120V or neutral, and the other brush voltage will be somewhere between 0 and 120V (determined by the voltage drop across the series field).

I would not install anything except 300 volt rated wire ...
Truthfully, neither would I, if only because I don't know where to source wire with a 150V insulation rating.
Nevertheless, I'll argue 150V is OK as a minimum allowable wire insulation voltage rating, since 600V insulated wire in a 480V system, 300V wire/240V system, and 150V wire/120V system are all 125% of supply voltage.
 
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I was thinking 18 gauge! I'm sorry I didn't mention that varying mechanical resistance is normal because it's a sewing machine. Normally these motors are such good shape I don't have to do anything but put on a new power cord. This motor is more beat up due hard use and bad servicing.
Assess commutator and brush wear surface condition. Following link from Morgan is a good resource.
http://www.morganelectricalmaterials.com/media/1996/technicalhandbookglobalproof_0.pdf

Sewing machine service has a lot of on/off cycles to it with occasional overloads (for example, fabric bunching up between foot and platen). It may be necessary to remove the armature rotor, and machine (turn and undercut, in motor parlance) the commutator to remove any high/low bars that may have developed, and restore concentricity if it has gone slightly "egg-shaped".

This link concerns big motors used in dragline shovels and such, but it has a wealth of profile pictures showing what out-of-round and bar-to-bar problems look like. The same kinds of issues crop up in smaller motors, albeit on correspondingly smaller scales.

http://www.wmea.net/Technical Papers/NECP & Foundation Coal--Benefits of Knowing Your Commutator Profile.pdf
 
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I disassembled the motor and cleaned it The brushes are half used up. The commutator was black with carbon which I removed using lapping film. I didn't think to check for roundness. I'll do that next with calipers. I don't have a way to check for offset relative to the bearings. The motor brush link is cool. Thanks!
Even new springs on these are pretty wimpy. Oily carbon deposits can easily prevent the brushes from sliding forward as they wear. I did notice that the curved part off the brushes that meet the commutator are offset as if the brushes are sitting crooked. If so it probably gets worse as the brush gets shorter. The commutators on these sewing motors don't have many sections. I wonder if that's why you sometimes have to help the machine get going by giving the hand wheel a spin. I even see this on old films of people sewing with this same machine. I'll add some pictures of the motor parts when I get a chance.
 

jim hardy

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Post a picture of your motor ?
 
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Thanks Jim. I will post a picture later today. My phone is dead and I have go to an interview.
 
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The brushes are half used up
It varies from motor design to motor design, but if by this you mean the brushes are half their original length, then they may already be at 100% wear out, particularly if the brush features a "shunt" (a piece of braided wire embedded into the carbon). This is done so there's no chance the shunt will carve into the commutator when the brush is totally worn out.

The commutators on these sewing motors don't have many sections. I wonder if that's why you sometimes have to help the machine get going by giving the hand wheel a spin.
Possibly, but probably not. Commutators in hand drills and similar appliances don't have many sections either, but have plenty of git-up-n-go.

black with carbon which I removed using lapping film.
I'm unfamiliar with lapping film, but it should be OK so long as it isn't emery (silicon carbide). One item that works well is a rubber cleaning block such as those sold by Ideal, but a hard rubber, gritty eraser (what are referred to as 'ink erasers') work just as well.
 
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Here are picks of the motor finally. The motor is a direct drive with no belt. There is a worm gear inside the motor when assembled. It drives a helical gear on the hand wheel. There are two windings in the field core. One connects to line and the other connects to neutral. Each winding also connects to a brush. The leads are in bad shape especially the ones to the power cord. The conductors are exposed in where I cant get to in order to splice. I have the black friction tape they used in these things. I'm wondering what it will take to redo these windings. By the way I have a rust eraser from japan for cleaning sushi knives. I never thought to use it to clean commutators. Cool idea, will try thanks!.
 

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jim hardy

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The motor is a direct drive with no belt.
Wow is that an old gear drive Singer ? Highly prized by professional dressmakers.

Your photos dont look that bad to me.
I'd clean it up
repair that bare spot on the wire with heatshrink
and give it a try.
Carbon and grime in the brusholders often makes the brushes stick and lose contact.

Be careful cleaning the commutator, dont use sandpaper or anything that can leave grit embedded in the copper. The 'orange stick' wooden utensil that ladies use for dressing their fingernails works well.

With your ohmmeter make sure nothing is shorted to frame.

I have to do that exact cleanup on a gear drive Singer that i bought for grand-daughter.
I hope it looks as good inside as yours.

Were the motor mounting screws underneath the handwheel difficult to loosen ?
My Singer shop cautioned that beginners often strip their heads.
Did you do anything special ? One Youtube showed using an impact driver, a small one you strike with a hammer.
Thanks for sharing

old jim
 
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Hi Jim, To remove the screws under the hand wheel you just need a good fitting screwdriver to start with. With the sewing machine in it's normal orientation, align the screwdriver with the screw and give it a moderate whack with a hammer. It sounds stupid but I've done this 3 times and have never done any damage. I learned that tip from Thrifty Farm Girl who restores vintage sewing machines as an online business. I used to tip the sewing machine on edge but that takes two people. One to hold it there and one with the hammer and screw driver. I don't even use PB blaster anymore for those two screws. By the way PB blaster is great for sewing machines since it's usually not rust that makes the screws stick it's old sewing machine oil. PB Blaster seems to penetrate and dissolve it.
There isn't enough wire protruding for shrink wrap. That's why I'm looking for a dead one to reverse engineer. I can probably help with your clean up since since I've been through it. I'm not an expert but have learned stuff from previous mistakes. If your model is from the 40s or 50s it should clean up really nice. The ones from the 30s weren't plated as well and the metal parts don't seem polish up as well. They're still nice though. The one I'm doing now is from the 30s.

Rob
 

jim hardy

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Thanks Rob

when i get back from Denver i'll get into grand-daughter's

old jim
 

jim hardy

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i guess this is the area of concern.... probably on the other side too ?

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underneath that tape probably within a half inch is the soldered joint between that big wire and the fine magnet wire they used for the coil itself.
Only as a last resort do i ever attempt a repair there.

Were this mine i think
i'd try to attach a new flexible lead at that bare spot where it comes out from under the tape
then immobilize that joint with enough epoxy to run over to the frame
where if you clean the metal well with alcohol first the epoxy should adhere
and it'll look like a repair encased in amber.

i found an interesting link with photos of repairs to a similar machine
try a search on theprojectlady re-wire-singer-sewing-machine-motor

old jim
 
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Another possibility for re-insulating the wire where it goes into the coil is (after thoroughly cleaning as Jim outlined) to paint on a layer of Glyptal, allow it time to cure, and repeat as necessary. GC Electronics sells it in 2 ounce bottles (#10-9002-A) with an applicator brush in the cap for small jobs like this.

The brush looks like it has plenty of life remaining, but I'm wondering whether the spring metal is blued on both sides as it appears to be, or it is simply carbon dirt. It's getting hot if the metal itself is bluing.

phy_brush.jpg
 
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Hey guys. Yup that's the area I'm worried about. Never saw one this bad. If I can't find a coil to reverse engineer, I'll have to do something like what you're suggesting. The black stuff on the spring is actually carbon and grease. It gets everywhere. I cleaned the motor body but not the brushes and springs. If I clean the springs and they still look black, then they probably did get hot. Someone over greased the gears and it creeped everywhere. The motor has two felt grease wicks for the bearings. The grease has to be able to melt at 115F so it flows. I wouldn't use that grease for the gears because of the creeping. Someone used the same grease for everything and a lot it. I think that's what destroyed the wiring. There is a path from the wick reservoir to inside the motor. I don't know why they did that but the main way it's getting is through the gears. The worm gear just flings it everywhere. I found a gear grease that stays put and recommended for sewing machines.
 

jim hardy

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I found a gear grease that stays put and recommended for sewing machines.
Hmm. perhaps you'd give us a pointer to your sticky grease?
 
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Hi Jim, This is stuff I put on the gears. There seems no solids in this grease. Some greases have clay or lithium and it just cakes up over time. For the wicks you can use petroleum jelly. The Featherweight Shop also has a grease they sell which I bought. It was pricey though I think I might start using the Jelly instead when I run out. You have to melt some into the felt wicks before installing. The wicks are held in with springs and clips. The springs are very delicate. But if you mess one up you can fix it wrapping the mangled section around a drill bit. Just happened to me on this machine. You can use alcohol to clean up the motor but not on the motor cover or the rest of black parts of the machine. For that use Go Jo hand cleaner original formula with no pumice. It won't hurt the shellac finish. If you know how to French polish with shellac you can add some extra protection to the decals on the bed. finish off with carnauba wax. I can't wait until you start! This is going to fun!
 

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jim hardy

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Thanks for your most practical information.

I dont know where to buy that ultra-fine stranded wire for motor leads.
Probably Alpha or Belden catalogs have some.
I'll look when get home.

old jim
 
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Thanks Jim, I think it might be magnet wire or something like that. If I ever find a dead motor I'll find out. If I can't find one soon I'll have to make this one work as is. I have the manuals to the machine assuming you have 15-91. It could be 201 also. They both used the same motor. When you get your motor off I'll give you more details on tools/techniques I used. It might not be obvious what to do since everything will be covered with carbon and grease.

Rob
 

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