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# Why are capacitors capable of suppressing electrical noise in DC motors?

Posts: 76
edited October 2009 in Learn
Dear all,

I have managed to solve one of the problem I had encountered with DC motors. That is, to soldered capacitors onto the motors·to suppress electrical·noise.·As the electrical noise had impaired my motor controller.·But why are capacitors capable of doing so?

Tagged:

• Posts: 7,494
edited September 2009
Look up snubber circuits on Google. An RC snubber is much more effective than just a capacitor.

Leon

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Suzuki SV1000S motorcycle
Leon Heller<br>G1HSM
• Options
edited September 2009
Think of capacitors as a resevoir for electric. They are like sponges.
• Posts: 76
edited September 2009
hi, thanks for all the replies, but can someone elaborate further?
• Options
edited September 2009
"A capacitor consists of two conductors separated by a non-conductive region.[noparse][[/noparse]7] The non-conductive substance is called the dielectric medium, although this may also mean a vacuum or a semiconductor depletion region chemically identical to the conductors. A capacitor is assumed to be self-contained and isolated, with no net electric charge and no influence from an external electric field. The conductors thus contain equal and opposite charges on their facing surfaces,[noparse][[/noparse]8] and the dielectric contains an electric field. The capacitor is a reasonably general model for electric fields within electric circuits."

"Supression and coupling
Signal coupling
Main article: capacitive coupling
Because capacitors pass AC but block DC signals (when charged up to the applied dc voltage), they are often used to separate the AC and DC components of a signal. This method is known as AC coupling or "capacitive coupling". Here, a large value of capacitance, whose value need not be accurately controlled, but whose reactance is small at the signal frequency, is employed."

"Noise filters and snubbers
When an inductive circuit is opened, the current through the inductance collapses quickly, creating a large voltage across the open circuit of the switch or relay. If the inductance is large enough, the energy will generate a spark, causing the contact points to oxidize, deteriorate, or sometimes weld together, or destroying a solid-state switch. A snubber capacitor across the newly opened circuit creates a path for this impulse to bypass the contact points, thereby preserving their life; these were commonly found in contact breaker ignition systems, for instance. Similarly, in smaller scale circuits, the spark may not be enough to damage the switch but will still radiate undesirable radio frequency interference (RFI), which a filter capacitor absorbs. Snubber capacitors are usually employed with a low-value resistor in series, to dissipate energy and minimize RFI. Such resistor-capacitor combinations are available in a single package.

Capacitors are also used in parallel to interrupt units of a high-voltage circuit breaker in order to equally distribute the voltage between these units. In this case they are called grading capacitors.

In schematic diagrams, a capacitor used primarily for DC charge storage is often drawn vertically in circuit diagrams with the lower, more negative, plate drawn as an arc. The straight plate indicates the positive terminal of the device, if it is polarized (see electrolytic capacitor)."

http://en.wikipedia.org/wiki/Capacitor

I think you would get a clear answer from an electrical engineering book but the electrical interference from motors is probably caused by the North and South poles on the electrical magnets in an electrical motor. The capacitor has an insulator between its conductors so it acts as a filter because of its makeup and that insulator probably forces the current to run one way.· When a motor spins, the magnets face all directions so you·get·electricity from its north and south poles.·This is my best guess short of finding you the textbook answer.
• Posts: 3,815
edited September 2009
When a motor spins, it generates lots of current and voltage load transients. These are high-frequency pulses on the supply lines.

Putting a capacitor across the voltage terminals of a DC motor provides a low-pass filter close to the motor. A low-pass filter passes low-freqencies (like the voltage running your motor) but will in effect 'short' high-frequency signals to ground. It does this because the voltage across a capacitor can't change as fast as it could if the capacitor was not there.

A DC motor also pulls a lot of current, compared to the steady-state of a BS2. The resulting voltage transients can briefly pull down the Vdd of the BS2, resetting it. A capacitor, close to the motor, can provide some local charge to moderate this voltage drop, so that the transient is not 'seen' on the Vdd line of the BS2.
• Options
edited September 2009
I asked my Dad and he says the capacitor acts like the spring underneath your car. It absorbs the interference and kicks it out. It has a way of smoothing it out.

He suggested that I read one of his books on electricity which is what we will have to do to find some answers and I think there has to be some more advanced books with laws explaining it.
• Posts: 76
edited September 2009
Dear all,

Thank you so much for your help!!
Really appreciate it!! [noparse]:)[/noparse]
• Posts: 76
edited September 2009
Dear all,

Sorry, but I need your help in explaning the following 2 sentences.

"A capacitor is an electronic component that will conduct only currents that are changing at a high frequency" --> what is the meaning of this?

"With a capacitor soldered across the motor terminals, it will act as a short circuit for high frequency electrical noise" --> does it mean the capacitor will filter off the high frequency electrical noise, then what about the low frequency noise?

Thanks!
• Posts: 1,221
edited September 2009
The frequency that they filter is based on the size of the capacitor.

Fc = 1/(2piRC) (frequency cuttoff, resistance , capacitance)

The larger the capacitor, the lower the frequency it will allow to pass.

Once circuit boards, very small caps are used to filter fast switching noise (surges) of the gates.

On stereos and such, fairly large caps are used to filter out engine and 60Hz noise.

-Martin
• Posts: 76
edited September 2009
Dear Martin,

For your information, I am using 0.1uF, therefore it should filter out the high frequency electrical noise, am I right to say that?

Bu I am still confused about the below sentences,

"A capacitor is an electronic component that will conduct only currents that are changing at a high frequency" --> what is the meaning of this?

"With a capacitor soldered across the motor terminals, it will act as a short circuit for high frequency electrical noise" --> does it mean the capacitor will filter off the high frequency electrical noise, then what about the low frequency noise?

• Posts: 22,018
edited September 2009
As Martin indicated, the "impedance" (works like resistance in AC circuits) of a capacitor varies with the frequency of the applied signal. The higher the frequency (or rate of change) of the signal, the better the signal is conducted by the capacitor. A capacitor soldered across motor terminals will act more and more like a short circuit the higher the frequency of the signals applied across it. It will still "short" some of the low frequency noise, but not as well as the high frequency noise. What's considered low frequency and high frequency depends on the value of the capacitor (and to some extent the construction of the capacitor itself).

Look up "electrical impedance" in the Wikipedia. Do a Google search on "wiki electrical impedance".
• Posts: 76
edited September 2009
Dear Mike,

Thanks alot for your explanation. But I am still confused about the sentence below.

"A capacitor is an electronic component that will conduct only currents that are changing at a high frequency" --> what is the meaning of this?

Conduct current at a high frequency?? What do they mean by conduct?
There are current at a low and high frequency?
Then what happens to the current at low frequency?

Thanks
• Posts: 3,815
edited September 2009
Well, see, you don't WANT to filter the "low-frequency" 'noise' -- that's the DC that's driving your motor, that's not really "noise".

A capacitor will not conduct DC current through itself -- the charge builds up on the + and - terminals, then any current flow stops.

Now, a changing AC voltage across a capacitor will LOOK like the capacitor is conducting. But in fact it charges up, then the voltage reverses and the capacitor discharges to then charge in the other direction, then the voltage reverses again and the capacitor discharges to then charge in the other, other direction.
• Posts: 62
edited September 2009
for this statement
"A capacitor is an electronic component that will conduct only currents that are changing at a high frequency" --> what is the meaning of this?

capacitors work by having two unconnected plates, and when one is charged it repels like charges, and that is the signal that you see passing through the capacitor. so while the capacitor is empty you see the signal pass through due to this effect. but there is only so much charge that a capacitor can hold, and when the capacitor is full in no longer repels the like charges. the time that it takes to charge is based on the capacitance, and the capacitance is based on the size and spacing of the plates. so for a high frequency signal it starts by going high, and the capacitor starts to charge, and passes the signal through by repelling the like charges, but then before the capacitor is full the signal goes low, so the like charges can return to the capacitor. on the other side of the capacitor this signal would be the same. for a low frequency signal the capacitor fully charges slowly, so the like charge is repelled slowly representing a very small change in voltage on the other side, then when the capacitor is fully charged the other side sees no difference in voltage between the time before the charging took place and after.

I hope that helps.
• Posts: 76
edited October 2009
Hi Alsowolfman,

Thanks alot for your explanation. [noparse]:)[/noparse]
• Posts: 6,037
edited October 2009
I don't mean to throw a kink and more confusion into this topic, but everyone is forgetting that where motors are involved, this becomes an RLC circuit rather than just an RC circuit as has been described.

The motor by itself is a stray inductor with parasitic capacitance that forms an LC tank with a specific resonant frequency. Since the parasitic capacitance is typically pretty high... somewhere in the pico Farads it is susceptible to high frequency oscillations when you pulse power to the LC circuit via PWM to control the motor. i.e. For example a motor with a coil inductance of 200uH and a parasitic capacitance of 5pF would have a resonate frequency of about 5MHz ... That's in a nice world... Since the motor armature itself is a relatively 'dirty' connection, you get several other nasty noise factors added to the equation. Not to mention that the inductance will vary dynamically depending on speed and how the motor brushes make/break their connections as the motor spins.

By externally 'adding' a relatively HIGH value capacitor in relation to the parasitic coil capacitance you can lower this high frequency noise component or specific resonant frequency of the LC to something a little less susceptible in a sense a 'low pass filter'... In the above example a 0.1uF cap placed in parallel across the motor leads would reduce the 5MHz resonant frequency down to about 35.6kHz.

I hope that this makes some sense and doesn't further confuse the issue.

Note:
Typically a delta capacitor configuration works best on reducing the noise on a motor... one capacitor across the motor terminals, and one capacitor from each motor terminal to ground. A total of 3 capacitors.

Calculator:
www.deephaven.co.uk/lc.html

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