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 The capacitor was split and was leaking so I removed it (Bernina advised the capacitor is obsolete and the machine should be run without it).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.

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    \$\begingroup\$ 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? \$\endgroup\$
    – smajli
    Jun 30, 2020 at 11:49
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    \$\begingroup\$ 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)? \$\endgroup\$
    – Fuffer
    Jun 30, 2020 at 12:13
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    \$\begingroup\$ 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. \$\endgroup\$ Jun 30, 2020 at 13:30
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    \$\begingroup\$ 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. \$\endgroup\$
    – user80875
    Jun 30, 2020 at 13:51
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    \$\begingroup\$ Note that I have added the [universal-motor] tag to your question. If you click that, you will see related questions and answers. \$\endgroup\$
    – user80875
    Jun 30, 2020 at 14:24

1 Answer 1


...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.


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