Thread Tools Display Modes
Old 04-01-2005, 01:32 PM
Dusty Dusty is offline
Join Date: Aug 2003
Posts: 406
Sparks from an electric motor, normal or not?

I've acquired an old 35mm projector, made around 1919. I've got and repaired many old projectors before, but this is the first one I've ever worked with that's electric. The electrical components are all original and are fairly straight forward, it's mostly just a motor with a variable resistor to control its speed. It's the motor I'm worried about.

The motor is open on both ends, so you can see inside. There's a spinning disk of what looks like copper which rotates with the motor (from looking at How Stuff Works, I'd guess this is the communicator). It comes in contact with two rods, they rub against it as it spins (brushes, presumably).

Now, when you start the projector, you pull the speed lever up the whole way so the motor gets enough force to start the belts moving, then you can push it back down to slow it to normal speed. When moving slowly, you'll occasionally see a little spark jump from the left brush to the communicator. When moving quickly, this spark grows fairly large and becomes more or less constant. Now and then you'll see a faint glimmer on the right brush, but the sparks are almost entirely on the left side.

Is this spark something to worry about? Maybe it's normal and I've just never seen it, having never seen an exposed motor before, but apart from the motor, pulleys, gears, and sprockets, the projector is mostly made of wood covered in paper and the sparks make me fear fire.

If it isn't something to be expected, what can be done to fix it? There seems to be a bolt on either side of the motor where the brushes are, perhaps that adjusts them. Should I move the left brush in closer to the communicator? Should it actually touch it or just come very, very close?
Old 04-01-2005, 02:15 PM
robby robby is offline
Charter Member
Join Date: Dec 2000
Location: Connecticut, USA
Posts: 5,130
First off, it's not a communicator, it's a commutator. And yes, you can get sparks when the electrical contact is established and breaks between the commutator and the brushes as the motor operates, particularly if there are only two brushes 180 degrees apart, and this may be normal for this motor. It's hard to tell if this is potentially unsafe, however. Safety standards are much more strict now than when this motor was made.

From your description, however, it sounds like your motor may be a DC motor. Is that the case?

Also, the brushes should be in contact with the commutator.
Old 04-01-2005, 02:52 PM
danceswithcats danceswithcats is offline
Join Date: Dec 2002
Location: Voting anti-obamanation
Posts: 10,300
Sparking at the brush/commutator interface is quite normal. Look into the vents of your shop-vac the next time it's running. Both of mine would not be safe in a hazardous atmosphere.
Crows. Keeping our highways clear of roadkill for over 80 years
Old 04-02-2005, 09:46 AM
Dusty Dusty is offline
Join Date: Aug 2003
Posts: 406
The nameplate on the projector says it's can take 110 volts DC or AC at 60 hertz or, after turning an internal switch, 44 volts DC (that's a B battery, isn't it?). There's no transformer, so I'd presume the motor is the same. At the moment, it's running on regular house current, 110-120v AC.

And yes, there are only two brushes, 180 degrees apart.

The brushes should touch the commutator? I don't think the left one is quite touching, maybe that's why it's sparking so. I'll try turning the bolt on the side to see if that positions it.

And some sparking is normal and I've just never seen it, most motors being in some kind of enclosure? That's reassuring.

Thanks, both of you.
Old 04-02-2005, 10:32 AM
Dusty Dusty is offline
Join Date: Aug 2003
Posts: 406
Okay, behind the bolt was an a spring connected directly to the carbon brush. I pulled it out and stretched it a bit (it was no longer springy), then put it back in. Now the sparks on the left side are more like the ones on the right: faint and periodic, rather than large and continuous.
Old 04-02-2005, 01:19 PM
Q.E.D. Q.E.D. is offline
Charter Member
Join Date: Jan 2003
Location: Richmond, VA
Posts: 22,536
Sounds like it uses a universal-wound motor, which can run on both AC and DC. And yes, the arcing is normal. This type of motor is used whenever you need a lot of torque in a relatively small space, such as shop vacs and power tools. They're noisy - both aurally and electrically - but powerful.


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump

All times are GMT -5. The time now is 02:38 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2018, vBulletin Solutions, Inc.

Send questions for Cecil Adams to:

Send comments about this website to:

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Publishers - interested in subscribing to the Straight Dope?
Write to:

Copyright 2018 STM Reader, LLC.

Copyright © 2017