Nick Johnson wrote:
> Hello from Virginia!
> I've what I think is a pretty simple question, and if anyone could
> help, I'd really appreciate it.
> Suppose I'm controlling the speed of a DC motor with a PWM signal.
> I've read online somewhere that I should connect a "small" capacitor
> across the motors terminals to improve the life of the motor. So, how
> do I select the sizing of that capacitor?
> My intuition tells me that I should treat the motor as an inductor,
> and select a capacitor so that the time constant of the LC network is
> equal to the period of the PWM signal... But, even if that's the case,
> I'm not sure how to measure the inductance of the motor. Any ideas?
> Nick Johnson
The capacitor's purpose is to supress voltage spikes caused when one of the
motor's coils is disconnected by the commutator before they can radiate from
the motor's leads and disrupt signals elsewhere.
The most important property of the capacitor is that it can respond to very
high frequencies, so disc ceramics are quite suitable. Anywhere between 5nF
and 50nF seems to work.
Connecting a capacitor from each lead to the motor's case can help sometimes
too, but not if you've grounded the case since then the caps just conduct
noise into your ground plane.
The capacitors on my robot's motor are actually inside the gearbox housing. I
couldn't physically put them any closer to the motor without disassembling it
and putting them directly onto the brushes. In some of the pics of my robot
) you can see
where two capacitors' leads are connected to the motor's case.