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MadMan 06-09-2018 03:11 AM

Quick Question
 
1 Attachment(s)
Time to fire up the old sewing machine. The motor was always lethargic, so I thought I'd inspect it. It has a diode connecting both brushes on one side, and a capacitor(?) connecting the brushes on the other side. The blue thing is marked 0.02(M) 400, I'm assuming it's a cap. It's from the 1970's, so I'd feel better replacing it.

Can I replace it with a film cap or electrolytic? Cuz it is AC, but there's a diode.

Zsuttle 06-09-2018 09:28 AM

Good, I finally get to help someone

The diode is to limit any inductive kickback from the motor as it winds down. Effectively, it prevents overvoltage from the motor as it attempts to keep the current going.

The capacitor is to smooth any voltage fluctuations present in the circuit. For the circuit to work, the capacitor should be unpolarised and shunted with the diode, across the motor. (A film cap will do)

Hope this helps,
Zach

MadMan 06-09-2018 07:38 PM

Quote:

Originally Posted by Zsuttle (Post 3200645)
(A film cap will do)
Hope this helps,
Zach

It does! I kind of figured the diode's purpose (after I thought about it for a bit). Thanks for the help.

MadMan 06-11-2018 10:03 PM

Well... a new cap didn't help much, but the motor does seem a tiny bit more lively. Also I'm back to not being sure about the diode's purpose. I would think the diode would be to prevent inductive kickback... on a DC motor. It's wired so that Line goes through one half of the field coil, through the diode, to one brush, through the armature, out the other brush, through the capacitor, then through the other half of the field.

So it's like....half wave rectifier??

Meh, it works, that's good enough for now.

Jon A. 06-12-2018 08:20 PM

Quote:

Originally Posted by MadMan (Post 3200805)
Well... a new cap didn't help much, but the motor does seem a tiny bit more lively. Also I'm back to not being sure about the diode's purpose. I would think the diode would be to prevent inductive kickback... on a DC motor. It's wired so that Line goes through one half of the field coil, through the diode, to one brush, through the armature, out the other brush, through the capacitor, then through the other half of the field.

So it's like....half wave rectifier??

Meh, it works, that's good enough for now.

If it is a half-wave rectifier it seems like a weak design. If that is the case and the diode were to open the motor would probably go up in smoke pretty quick.

MadMan 06-12-2018 10:01 PM

Quote:

Originally Posted by Jon A. (Post 3200857)
If it is a half-wave rectifier it seems like a weak design. If that is the case and the diode were to open the motor would probably go up in smoke pretty quick.

If the diode opened, the motor wouldn't work at all. It's wired in series.
LINE -> field -> diode -> commutator -> capacitor -> field -> NUETRAL

jr_tech 06-13-2018 01:17 AM

The cap is in series with the windings??? I would have guessed that it would have been accross the brushes to suppress RFI. :scratch2:

jr

Jon A. 06-13-2018 12:15 PM

Quote:

Originally Posted by MadMan (Post 3200863)
If the diode opened, the motor wouldn't work at all. It's wired in series.
LINE -> field -> diode -> commutator -> capacitor -> field -> NUETRAL

Oh, I thought it was across the terminals. My Ford stereos, certain ones at least have a diode across the terminals of the program select solenoid, I think that's what threw me.

MadMan 06-13-2018 10:11 PM

I know, it's freakin' weird. But it works.


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