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Small motor with 104 cap bridge between terminals.

roughshawd

Jul 13, 2020
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I have wanted to ask this question for years!!!
I have encountered this quite a few times in my tinkering.
What is the capacitor between the terminals on little motors for?
I am thinking like...to slow the RPMs down... or make sure the motor is getting a constant flow of current?
 

roughshawd

Jul 13, 2020
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Oh yeah... and what happens if the motor gets weak and I am trying to replace it... will it help to remove the cap or change it?
Motor is a
SRF-500TB 12560/060607 with a 104 cap across the terminals.
 

Bluejets

Oct 5, 2014
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It's to act as an rf suppressor.
Sometimes you will also see extra ones placed from eac terminal to the motor casing
 

Minder

Apr 24, 2015
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+1 on the noise suppressor.If used in noise sensitive circuits.
What do you mean "the motor gets weak"?
Possible commutator or brush wear?
 

roughshawd

Jul 13, 2020
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The motor powers a rotating vignette of snow flakes in a projector. It is geared down to impress a slow falling snow storm.
The little motor is rubber belted to the main gearbox with a small pulley to a large.pulley that is further geared to allow the rpm's to be reduced to a very slow travel speed. I could get the motor to turn by flicking the pulley with my finger, but it didn't run long. I didn't have a 104nf cap, so I used an 82nf cap. The 104 was reading from 50 to 86nf. With the 82nf cap, and the speed adjustment on slow, the motor will run for a while but it still wants to stop. I have dropped the dime, and I will wait for the 104 to arrive so I can try it. I am afraid that it might just be worn out..heh heh, me? I will rebuild it!
 

Bluejets

Oct 5, 2014
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Motor problem will have nothing to do with the capacitor as previously mentioned, so just a waste of time and effort replacing that.

More likely either motor bearings are dry or the motor commutator is dirty or the brush material worn out.
The latter could simply be brass feelers rather than carbon brush.
Either way, doubt you would be able to rectify that as they are usually sealed type can.

Best show photos as replacement would probably be best shot at success.
 

Minder

Apr 24, 2015
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Quite a few sources for the motor e.g. Jameco have them for $4.00ea !
 

roughshawd

Jul 13, 2020
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The replacement of the 104 cap worked...
But I am sad to say my reliance on the ceramic disk numbering systems has decreased... where
a 104 is 100nf
a 103 is 10pf
So with the 87nf cap on the motor, the highest speed would turn fine, but reducing the speed with the pot, caused the motor to stop.
I am decifering the activity as "a slower cap allows a longer saturation time of the coils in the motor, allowing it to complete a cycle without losing power when the cap dumps to ground across the main contacts. The slower cap didn't allow for complete saturation, and at a high resistance, the motor couldn't recover from the power loss and stopped." heh heh.!!!!!!
 

Minder

Apr 24, 2015
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I am still not convinced that the motor requires the cap in order to run, the Manuf spec sheet show it as a DC motor and do not specify any kind of cap needed, the only thing they show is a varistor suppressor can be used across the terminals.
If a cap was required, I would expect to see this on their published sheet.
 

davenn

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Sep 5, 2009
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I am still not convinced that the motor requires the cap in order to run,


agreed, probably not

but if the cap had failed and gone resistive then that may be causing the motor not to run correctly
 

Minder

Apr 24, 2015
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Possibly, but I would not expect failure at ~ 12v operating voltage.
A resistance test, also a Diode range test on the cap might show it up?
If this was intended as noise suppression, I would have though the typical reverse biased diode snubber would have been better, the application only uses uni-direction as far as I can tell.
 

Harald Kapp

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If this was intended as noise suppression, I would have though the typical reverse biased diode snubber would have been better,
The snubber diode would only suppress the peaking of the flyback voltage when turning the motor off.
A capacitor will help to suppress any HF, including the electrical noise coming from the brushes during operation.
An improved version of such a noise suppressor circuit uses inductors in addition, see e.g. this example:
entstoerfilter-passend-fuer-modellbau-motor-500-600er.jpg
 

PETERDECO

Dec 19, 2019
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A 0.1uF (104) cap is mandatory on our products that use LV motors. Sometimes we add a 10uF electrolytic in parallel for the most sensitive circuits.
 

roughshawd

Jul 13, 2020
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Thanks...I needed that!!! I think the circuit driver might be carrying so much power that the induction motor is saturated too fast for recovery. Assuming that induction carrys the same power as the driver circuit, maybe, The coils that push the armature exciter is charging but reaches saturation before the motor spins, and dumps the volts before induction occurs. So by sticking a cap in the coils in and out, the power that would dump is trickled back into the coils, reducing dump and adding a little kick start to the spin, similar to a cap start motor, but with induction.
 

Minder

Apr 24, 2015
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The cap does nothing to aid rotation, it is a DC motor, NOT an Induction motor, completely different animal.
Your drive circuit is obviously very sensitive to the brush noise generated by the commutator if it requires a cap in order to run..
 
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