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Old 06-09-2018, 03:11 AM
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Quick Question

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.
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Last edited by MadMan; 06-09-2018 at 03:15 AM.
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Old 06-09-2018, 09:28 AM
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Zsuttle Zsuttle is offline
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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
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  #3  
Old 06-09-2018, 07:38 PM
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Quote:
Originally Posted by Zsuttle View Post
(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.
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Old 06-11-2018, 10:03 PM
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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.
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Old 06-12-2018, 08:20 PM
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Quote:
Originally Posted by MadMan View Post
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.
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Old 06-12-2018, 10:01 PM
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Quote:
Originally Posted by Jon A. View Post
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

Last edited by MadMan; 06-12-2018 at 10:05 PM.
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Old 06-13-2018, 01:17 AM
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The cap is in series with the windings??? I would have guessed that it would have been accross the brushes to suppress RFI.

jr
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Old 06-13-2018, 12:15 PM
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Quote:
Originally Posted by MadMan View Post
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.
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Old 06-13-2018, 10:11 PM
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I know, it's freakin' weird. But it works.
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