Why Do Some Universal Motors Overheat and Arc?

[Guineafowl]

I’ve been given a food mixer to repair - more sentimental value than monetary. It’s barely been used, if at all - the brushes still have their bedding-in ribs. It would only run if I gave it a spin, but this was just bad contact between brushes and commutator. Solved with switch cleaner. When running, it arcs badly and gets hot.

The motor is a series-wound, brushed universal type, and is very badly built. There is no epoxy to stabilise the windings as they leave the commutator, and one of the segments has been left unconnected. The windings have been looped under the tab for each commutator segment, and the tab has been pressed down over it.

The ‘good’ segments have a resistance of about 1.8 ohm between each pair, but many are much higher, and the reading changes if I press down the tab.

When such a motor is made, how is good contact normally made between the winding wire and the commutator tab? Is pressure sufficient to displace the varnish, or are they normally heat-staked or spot welded? Solder?

 

[essenmein]

Based on what you are saying its a bad connection, as to how its made, all of the methods you mentioned are probably used, except heat stake, to me at least that's deforming plastic to hold something (eg PCB).

Its likely one of those things where you won't know until you carefully try to dismantle and see, risking off course not being able to get it back together. If you're lucky its a dry solder joint... Mind you its not working right now anyway so what's the worst that can happen?

Do you have a picture of the offending commutator?

 

[Guineafowl]

Based on what you are saying its a bad connection, as to how its made, all of the methods you mentioned are probably used, except heat stake, to me at least that's deforming plastic to hold something (eg PCB).

I was wondering if a higher temperature version of that plastic technique is used to melt the varnish at the point of contact.

Its likely one of those things where you won't know until you carefully try to dismantle and see, risking off course not being able to get it back together. If you're lucky its a dry solder joint... Mind you its not working right now anyway so what's the worst that can happen?

Several adjacent segments have a resistance of 1.88 ohm, +- 10%. Others have anything from 11 to 300 ohm. It’s a very crappy motor. Normally, I test a commutator by seeing if adjacent, then opposite segments have consistent resistance. Here, I’m not sure - the arcing suggests shorted windings, but maybe the heat is from the high resistance connections at the commutator tabs.

Do you have a picture of the offending commutator?

No, sorry. The mixer is back together now - it’s beyond economical repair (manufacturer is based in a small village in India).