Electric Motor theory
 

I have 3 motors here that are all exhibiting the same problem.

• 1 I don't care about, so it will be the guinea pig for any suggestions that come up.
• 1 would be a shame to have not work, but not the end of the world
• the last one is the Bernina 730's that I would like to resurrect if possible

All motors have good brushes. All are relatively clean inside. All of them spin great with no load, some even faster than I expected. One howls when it does it, but the other 2 are quiet. Both of the Singers have been disassembled and lubed, etc. The Bernina is still in the machine at the moment.

All of them stop or slow down significantly (to the point where they can't or barely drive a well serviced sewing machine), with the belt correctly or even loosely fitted.

I googled a lot, but I really couldn't find much for "electric motor runs with no load, but slows to a crawl or stops with load" (no, that wasn't my exact search ;))

My gut says that it's likely the windings, but if there's any way I can bring these motors back to life, I'd like to hear about any theories....

The former Singer shop owners in town used to claim that there was a "solution" they could put in the motor to revive it. I've never been able to tell if that's bunk or not.

Tammi,

Here is what I do:

External motors, non Singer:
I clean the commutator? One of the things I do to every motor I rewire or take apart for any reason is put the armature in a drill motor and then clean the commutator with 600 grit sandpaper.
Then using a very small jewelers file I undercut the insulation between the copper contacts so there is no chance of current jumping.
After that I clean out the brush tubes with denatured alcohol and a q-tip and I do the same to the brushes and the caps.

While this is being done I saturate the bearings / bushings with sewing machine oil.

So far this has rewarded me with functional motors. I've yet to have to replace one.

External motors, Singer:
As above but the motors are greased with Singer Motor Lube after the bearings / bushings have been cleaned.

Internal motors:
Same as non Singer external motors.

Joe

Hey Joe,

Thanks for that. I will try this out on the guinea pig and report back. I hope to get to it tonight, but won't if I get a call back on a machine in the buy and sell. If they call, I'm driving 240miles round trip for it. ;)

Tammi, if your motors all spin great with no load, then the problem is not within the motor itself; it's with something further down the chain. It's impossible for a correctly-fitted or loosely-attached belt to affect any of the motor's inner workings, unless the bearings or the motor pulley have somehow become damaged.

Disconnect the belts for a moment, and let us know what happens when you turn the machines over by hand.

Rain, as I mentioned above, the machines in question are all serviced and working properly. There is no drag or hard turning of any of them. If I fit different motors, the machines work fine. The problem is at the motor side for sure.

Hi Tammi, that is very curious indeed; It seems mechanically unlikely given your description, so this is some new problem I've never seen before. Hopefully someone will chime in with the solution, and please keep us posted if it's off-forum! I'm very interested to learn what the problem is, in case I encounter it myself in the future.

DH was suggesting that the magnets may be weak. It's something worth checking, but I'm not sure how at this point.

Mechanically, or electrically, as I understand it, any motor will slow down or stop with too much load. Just as a gas engine loses horsepower as it gets older and "worn out", I expect the electrical motors to be the same. I just have to figure out which parts correspond to "horsepower". ;)

My goal here is to figure out why "normal" is too much. I'd hate to have to bin the 3 motors, and possibly write off the 730 at the same time.

I will definitely report back. I got caught up with a client today, and have a meeting this evening, but if I still have some energy tonight I'll do what Joe suggested and see what happens. Otherwise, tomorrow's completely free from what I can tell.

I'm going to go scrounge up some 600 grit sandpaper out of the black hole DH calls a garage. ;)

Motor
 

In my experience, (featherweights only), I have seen my share of bad armatures. The stator of the motor usually is not the problem, unless it shorted on the metal motor housing and then the motor obviously won't run at all. What I have heard is that the armatures are most prone to overheating if the machine has been overloaded or tight from lack of maintenance. Heat is what kills the armatures and some of these vintage machines have had a hard, overloaded life. Below is a procedure that can help rule out the electrical components inside the motor.

On brush-type motors, inspect the brushes to make sure they are still serviceable. Generally, brush length should be at least twice their width or replace them. Also make sure the brushes are free in their tubes and the springs are not broken. Check for shorts or open armature windings by touching one test lead to one commutator (the grooved copper section that contacts the brushes) segment and the other test lead to another segment. Compare each segment with every other segment and look for either open (infinity ohms) or shorted (zero-ohms) armature windings. Either will be cause to discard or rebuild the motor.

Marilynn, Thank you so much for this! I will test this tomorrow, and see what I can find out.

I also found out we had 400 grit and 800 grit sandpaper, no 600, so I picked some of that up, and a non-oily file while I was out today. I'm pretty much without excuses to get this figured out tomorrow. :)

Tammi,

Make sure its non-metalic sand paper. Particles from papers with metalic oxides can foul up the windings. Plain ole sandpaper only.

Cathy