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Capacitor with DC motor

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happyfpga

New Member
Hi everyone,

I saw one time in this forum that someone suggest to put a capacitor in parallel with the DC motor which is driven by H-bridges to remove EMI. I am working on the exact same circuit right now, 12V DC motor, ~1A current consumption, MOSFET h-bridge, 25KHz PWM, ~40% duty cycle. When I put 0.1uF cap, the motor stopped turning. Without the cap, everything looks fine.

I have few knowledge about EMI. Could anyone explain in more detail about the function of the cap and EMI? How to choose the cap, capacitance, type?

Thanks a lot.
 

Sceadwian

Banned
.1u is a bit much, at those frequencies .1u will draw more current than the motor. 1n sounds more reasonable, has to be a non-polarized type. I see even smaller value ceramics used frequency, it's just there to short the commutation noise of the motor in the RF area to ground.
 

hv addict

New Member
At 24khz the impedance of the capacitor would be 66Ω so at 12v it would draw only about 180ma, much less then the 1A that the motor consumes. Could it be possible that the capacitor is very *leaky* and is letting more current through then it should? Check if the capacitor is getting hot as that would be a telltale sign of that.
 

Sceadwian

Banned
hv addict you're right, it's only actually using about as much power as you said, I simulated it real fast and mistook the currents that were passing in and out of the capacitor as wasted current which is not the case.
 

Hero999

Banned
At 24khz the impedance of the capacitor would be 66Ω so at 12v it would draw only about 180ma, much less then the 1A that the motor consumes. Could it be possible that the capacitor is very *leaky* and is letting more current through then it should? Check if the capacitor is getting hot as that would be a telltale sign of that.
That's true but only at the fundamental, there will be plenty of higher frequencies which will be totally short circuited by the capacitor. I agree though, there should still be enough voltage left for the motor.
 

Sceadwian

Banned
cap could have been polarized and he hooked it up wrong?
 

happyfpga

New Member
Thanks for the replies from everyone.

I tried 1nF cap, it works fine with the motor. The original 0.1u cap is ceramic type, unpolarized and brand new.

Sceadwian, may I ask you how you do the simulation. Did you build a SPICE simulation model for DC motors? Could you share your simulation model.

I am stick on this problem just afraid of any other issue potentially.
 

Sceadwian

Banned
I just use a simple resistor as a load. Very common when you're not worried about EMI or other voltage/current lag/lead effects. If you want to add a little inductance to the load that might more accurately describe a motor under stable running conditions though a static resistor is fine for a 'quick check'. But a real honest to goodness motor model is MUCH more complicated.

The Yahoo LTspice user group has some motor models which might be more useful for real world comparison.
 
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