Capacitor Value For DC Motor

[farhang_760]

Hi ,

I have 24v 7A DC Motor , and I don't know bulk capacitor value for this motor ?

How can I calculate this value ?

 

[KlausST]

Hi,

why do you think you need a bulk capacitor?
To stabilize a rectified AC?

Klaus

 

[jjmontero9]

I think capacitors are used to filter the noise produced by the brushes of a DC motor (due to the sparks when it is rotating)

But I don't know an exact value for them, I guess theoretically it would depend on the RPMs of the motor itself.

I read on a website that a rule of thumb is to use 0.1 uF caps, but again I don't know why or how to calculate them.

Here are some links related
https://robotics.stackexchange.com/...d-to-motors-in-parallel-what-is-their-purpose
https://www.pololu.com/docs/0J15

 

[KlausST]

Hi,

I was confused, because you said "bulk" capacitors, but now you talk about EMI filter capacitors.

EMI capacitors are difficult to calculate, because the conditions change continously. Motor speed, aging of the brushes...

I´d say 100nF is a good value to start. Use a foil capacitor, or one specially named "EMI filtering capacitor".
Connect it close to the motor.

If you try to run the motor with PWM (speed control), then this capacitor may have bad influence because of the high inrush current.

Klaus

 

[jjmontero9]

Hi Klaus,

I'm also confused (I'm not the OP) and I thought he was talking about filter capacitors for a DC motor with magnets.
But now that you mention, maybe a bulk capacitor has another function and I introduced noise in the original question.

I have no experience here, but wouldn't this bulk capacitor compensate the effects of the inductance in the winding of the motor?
I know DC motors have two windings, connected either in series or parallel, but I don't know how to design the bulk capacitor.

Thanks and sorry for confusing the original question

 

[KlausST]

Hi,

a bulk capacitor is a large capacitor, mainly for storing a lot of energy. Like after a bridge capacitor and mains frequency.
With rectified mains (frequency 50Hz/60Hz), there are only short pieces of time (milliseconds) where the capacitor is charged and the rest of the time the capacitor supplies the application with it´s previously stored energy.

In opposite an EMI suppressor capacitor is much faster it works in the region above megahertz. 10MHz, 100MHz.
It acts as a short circuit for high frequency. This avoids that signals in this range of frequenciy is radiated by the supply wires.
Radiated EMI signals may harm other devices around (radio, other audio devices, telephone, TV, cellular phones medical equippment...)

Klaus