# Why are capacitors able of suppressing electrical noise in DC motors

Discussion in 'The Projects Forum' started by Vincenzo1309, May 30, 2009.

1. ### Vincenzo1309 Thread Starter Active Member

Dec 28, 2008
57
0
Dear all,

I have been working on a robotic project, about a wheeled robot. I noticed that when I programmed my DC motor to rotate non stop, it would just rotate for few seconds and then stop abruptly. After some troubleshooting, I found out that the electrical noise from the motors had impaired the motor controller, after I soldered capacitors onto the motor terminals, the problem was solved.

The question is, why are the capacitors capable of suppressing the noise?
Is there such a thing called electrical noise?

Rgds

2. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Yes, DC motors (particularly brushed motors) generate a great deal of electrical "noise". They are basically large inductors that are getting switched on and off by the rotation of the commutator. The capacitors absorb the transient voltage spikes, smoothing things out.

For the most effective transient suppression, you should use a pair of capacitors; one around 0.1uF to absorb the high frequency transients, and a larger cap (say, 220uF) to absorb the low frequency transients.

When you're dealing with logic circuits, always use a 0.1uF capacitor across each IC's Vcc/GND or Vdd/Vss terminals.

3. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Electrical noise is not audible but it is unwanted signals superimposed on the desired signal.

It always exist in electric and electronic circuits and it is a major problem engineers have to deal with.

The voltage across a capacitor cannot change instantly because an infinite current would be required (impossible).

i=C*dv/dt

Due to this property a spike causes a large current to flow through the capacitor and thus its voltage drops in its try to charge the capacitor. Thus a high voltage fast spike appears as a slow varying voltage with less amplitude.

Mrdouble likes this.
4. ### NM2008 Senior Member

Feb 9, 2008
135
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SgtWookie

You stated,
My question is, should these capacitors be placed in series or in parrellel with each other when connecting across the motor?

Regards NM

5. ### Vincenzo1309 Thread Starter Active Member

Dec 28, 2008
57
0
Dear all,

Are there any website or online books that you can suggest so that I can read in depth for this problem?

Thanks and Regards,

6. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
The capacitors should be placed in parallel across the motor's terminals.

Feb 9, 2008
135
0
SgtWookie,