SPARKS

[Steve Clare]

I had to clean up the commutator on my ruby type starter motor, as it was a mess and sometimes refused to function. All fine, clean and cranking well. I left the cover off the starter motor while working, and noticed that there's sparking occurring as the starter motor is cranking- at the bush. Is this supposed to happen? I'm putting 12 volts through the starter incidentally.
Thanks
Steve

[Robert Leigh]

Sparking at the brushes as the motor operates is normal. It will be worse if the motor is not in optimal condition. Starter motors have mica insulation between the commutator segments, and this is harder than the copper. When a motor is new the top of the mica is level with the top of the copper segments, but in operation the copper normally reduces by wear and spark erosion more than the mica does (mica is not subject to spark erosion). This is only a problem as the wear becomes excessive. If necessary skimming ideally should remove only mica, although in practice a very small amount of copper is normally taken to enable the machinist to see that he has achieved a truly cylindrical shape.
If using 12 volts sparking is unlikely to be very different from when using 6 volts because the motor turns very much faster on 12, causing a greater back emf.
Going back to your question, sparking is normal within limits, and the 12 v operation makes little difference in my experience.

[Steve Clare]

Thanks for the reply Robert. I will cease to worry about it.

[bob46320]

Steve - you said "Sparking at the bush" . If you meant "Brush", then the previous reply is correct - but if you meant "Bush" as in the bearing bush, then you do have a problem as one of the armature windings may be showing to earth.
See diagram:-

Armature windings

[Tony Griffiths]

I believe it's also normal - once the armature has been turned true - to undercut the mica using a thin hacksaw blade or similar
In the past, several lathes have been adapted for armature work including
http://www.lathes.co.uk/weidenhoff/
http://www.lathes.co.uk/hobart/
http://www.lathes.co.uk/parker-mcknight/
http://www.lathes.co.uk/burtonrogers/
http://www.lathes.co.uk/frankwood/
http://www.lathes.co.uk/santaella/
...and conversion kits for ordinary lathes such as that for the Atlas 6-inch http://www.lathes.co.uk/atlas-6-inch-lat...s/img2.jpg
and http://www.lathes.co.uk/craftsman/page8.html

[Steve Clare]

Hello
Sorry, I meant sparking at the brush.
Hopefully it's all ok now.

[Dave Mann]

On dynamos quite right Tony but never on starters.