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I'm a sewing machine mechanic, trying to increase my electric motor knowledge.

I have a Bernina 830 sewing motor where the carbon brushes ran down and the wire damaged the commutator and ran a groove in it. The motor was smoking.

Bernina sewing machine motor

I took it apart, I sanded the commutator so it was nice and smooth, cleaned it all up etc. The capacitor was split and was leaking so I removed it (Bernina advised the capacitor is obsolete and the machine should be run without it).

However, it is now sparking a lot and after struggling for a while it kind of popped with a flash and just stopped.

What I think are the field coil wires are covered with a fine plastic film insulation, however this has perished in places.

Could this be causing the issues, and if so what's the best fix? enter image description here Update. This is the other end. It's melted on one side only. Looks very poorly.

Billie
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    Hello and welcome to EE stackoverflow. I like your attempt to repair, it is what I always try. Anyway trying to sand down a commutator brings the copper dust into the small gaps between commutator's copper plates. Did you take care to remove debris from the gaps? It can make a short circuit. Also, can you provide a picture of the motor? – smajli Jun 30 '20 at 11:49
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    Hi Billie. Pictures would help here a lot. Please show everything you describe as best you can and upload to the post. Also, do you have access to a multimeter (sometimes known as an ohm-meter)? – Fuffer Jun 30 '20 at 12:13
  • Thanks very much for your replies. I've added more pics below. I used a dental tool to clean between each copper strip on the commutator, having just taken the motor apart again the gaps have got lots of carbon on them and there is significantly more perishing on the field coils. A new motor for this machine is around £200. I'd really like to learn more about exactly how they work and fixing them if possible though. – Billie Jun 30 '20 at 13:22
  • My husband has a multimeter, I don't really know what I'm doing with it though. Tried a few YouTube videos but can not find anything that I can apply to this motor. – Billie Jun 30 '20 at 13:23
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    Hi Billie, I removed the 'please be gentle' and 'thanks' from your post. Stack Exchange uses a gamified system where users earn reputation by having their activities voted on. If you wish to thank people for their contributions, upvote the contributions. After a day or so (minimum), once you receive an answer which you deem as the best answer to your question, accept it so that the poster gets their 15 point bonus. Please delete your answer which had the picture in it - you must always edit your original question with new details, and must not put them in comments or answers. – Adam Lawrence Jun 30 '20 at 13:30
  • Thanks for the housekeeping info. – Billie Jun 30 '20 at 13:45
  • I've just taken the other end off and the fan cover has melted onto the wires... Can I salvage this? I am happy to put the time in, and pointers appreciated. – Billie Jun 30 '20 at 13:46
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    Did you replace the brushes with new brushes from the manufacturer? Did you replace the springs also? The brushes should slide easily in the holders but not be so loose that they lose proper contact with the commutator. I believe the outer light-colored material on the field winding is intended to keep the windings from vibrating and rubbing each other. The red coating on the wire is the important insulation. If it is darker in areas, it is burned ind likely failed, The copper itself is lighter colored. If you see that, the red coating has come off. More later. –  Jun 30 '20 at 13:51
  • Thanks Charles. I uploaded another picture, the clear coating has completely melted now, the wires are attached to the fan cover which has melted onto them it doesn't look like I can separate them without damaging the wires... – Billie Jun 30 '20 at 13:59
  • Sorry. I replaced the bushes with generic ones of the same size. The springs were with the new bushes. – Billie Jun 30 '20 at 14:01
  • Out of interest... Is anyone aware of any adult ed course in the UK that would cover this sort of thing? – Billie Jun 30 '20 at 14:07
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    Note that I have added the [universal-motor] tag to your question. If you click that, you will see related questions and answers. –  Jun 30 '20 at 14:24
  • Thanks Charles. I did quickly scan the other posts under the Universal Motor topic, unfortunately I could only see 20 posts and none directly relevant to my post, but I will read through them all later so I can glean any pertinent information. – Billie Jun 30 '20 at 14:35
  • I consulted a relative who repairs sewing machines. He just sold a used Bernina 830 for about twice the cost of a motor. I think he received it in good condition as a trade in for a new machine. He read the question, but did not comment on the brush quality. He did say that he blows the commutators with compressed air after smoothing them. If there is melted plastic on the rotor winding, you might be able to cut the plastic and leave residue on the rotor, but it might be unbalanced and vibrate. It seems likely you need a new motor. –  Jun 30 '20 at 20:43
  • @Billie - Which clear coating has melted, the enamel coating on the individual wires, or the hot-glue-looking substance that holds all the wires together? If it's the former, then there's likely not a lot you can do ... – bta Jun 30 '20 at 22:20

1 Answers1

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...the field coil wires are covered with a fine plastic film insulation, however this has perished in places. Could this be causing the issues, and if do what's the best fix please?

The only fix is to rewind the field. You will need to look for detailed instructions, information about the materials and a supply source. You may find that it is more profitable to replace the motor.

You will also need to make an evaluation of your judgement about the amount of material that can safely be removed from the commutator, the adequacy of your cleaning and preserving the slots and your skill in doing that work.

The capacitor is intended to reduce the electrical radio frequency interference caused by the sparking of the armature. The value is not very critical.

The motor is called a "universal motor." Use that search term to find more information.

The proper brush composition and springs are important, but perhaps not the most important issue in this case.

Shorted stator turns will cause the stator to overheat and vice versa. Not turning the motor off immediately if it stalls can cause the motor to overheat. Motor overheating can also be caused by failing motor bearings or other bearings in the machine. Also parts of the machine that are not moving freely can cause motor overheating. Possibly overheating could be caused by misuse of the machine such as prolonged use with fabric that is too heavy. In any event, it is important to learn how to determine if the motor has shorted windings. Examine the stator and rotor carefully for variation in winding color and learn how to measure winding resistance.

The winding resistance is quite low. It probably ranges from a tenth of an ohm to a few ohms. You need an ohmmeter with a scale that allows readings in that range. Having normal resistance data from the manufacturer would be best. For the stator, you should be able to get access to the connection between the two halves. That will allow comparison. For the rotor, the resistance between each pair of segments on opposite sides on the motor should be the same. The resistance between each segment and the one next to it may be too low to read, but they should also ne the same.

Be sure to check the resistance between the windings and the rotor and stator iron core to make sure the windings have not shorted to the core.