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Electronics => Electronics => Topic started by: VegaObscura on May 10, 2011, 12:01:01 AM

Title: Capacitors for Motors
Post by: VegaObscura on May 10, 2011, 12:01:01 AM
I am aware that it helps to put a capacitor across the leads of a motor to help endure its large voltage spike when starting and stopping.  How do I know what size capacitor I should be using?  For example a motor that runs at 6V, 2 amps stall current.
Title: Re: Capacitors for Motors
Post by: TrickyNekro on May 10, 2011, 04:51:19 AM
You better use 3 caps. Two from the leads to the casing and on between the leads.

The bad thing is that's not very wise for the capacitor to use it between the leads.
It's bad to apply voltage at capacitors (and current to coils)... But the inductive behavior of the motor
will make it act as a band pass parallel filter. So you must pick up the PWM frequency correctly...
The above just for you to know.

There are methods of estimating the induction of the motor's coils, but you will need a scope and a function generator.
Well from the function generator you will need only a variable frequency sine wave, but just saying...

Well.... the above is just for your information if you have the means to do a precise work...

For the leads to the case place [email protected] ceramic capacitors.
For the lead to lead capacitor, you either calculate the value with the above method OR just place another ceramic,
just like the above ([email protected]) and these are gonna get you running fine ;)

Best Regards, Lefteris
Greece
Title: Re: Capacitors for Motors
Post by: Soeren on May 10, 2011, 08:43:05 PM
Hi,

I am aware that it helps to put a capacitor across the leads of a motor to help endure its large voltage spike when starting and stopping. 
AND running!


How do I know what size capacitor I should be using?  For example a motor that runs at 6V, 2 amps stall current.
Newer mind the stall current. If the rotor is locked there will be no commutation sparks and hence no noise (just a really warm fuzzy feeling  ;)).


Find the average (loaded) current (I) and calculate it like this:

The X-cap (the one going from + to - terminal) should be approximately: I^2/10 [µF]

Eg. with a running current of 2A, it would need to be in the region of:
  2^2/10 = 4/10 = 0.4µF = 400nF (in which case you could use eg. 390nF).

For the Y-caps (those going from each terminal to the motor housing), use around 1/10 to 1/5 of the X-cap - in the example above, it would mean something in the interval 40nF to 80nF.

They should be soldered directly to the motor terminals and housing with the shortest possible leads (without making life too difficult for yourself) and you should twist the wires going to the motor (and to the battery), so that it has approximately one turn per 4cm to 5cm.


If you have access to a scope, it may be enlightening to scope the noise after each of these remedies are installed.
Just seeing the effect of twisting a lead can be quite an eyeopener :)