Capacitors ? please help.

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nosparks

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Location
Massachusetts
:cool:

My electronics background is very weak.
Scenario: I have an oil heater on a Cleaver Brooks 500hp boiler. A thermostat will pull in a 3 pole contactor to energize the elements in the oil heater. When the t-sat is satisfied the contactor opens de-energizing the elements. Simple.
Here is the electronics part: The Thermostat has connected across its terminals,(in parallel) a capacitor. This is also shown to be the case in the control wiring schematic. The contactor has been chattering more frequently since it was installed 9 months ago. I temporarily removed the stat and capacitor from the circuit and wired in a single pole switch. The chattering stops. and life is good again. If it was a stat alone, I would feel confident in replacing the stat with a new one. My question is, what is that capacitor doing, how does it affect that contactor. Also how would I test a capacitor with a meter, digital or analog, to see if it is bad or good?
As I said my electronics back ground is lacking, so I would entertain any thoughts or even more so your professional opinions. Thanks for your help in advance.
 
G

Guest

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Re: Capacitors ? please help.

Im thinking the capacitor is there to reduce or eliminate any arcing at the t stat contacts.The chattering is a puzzle hoever because evn if the cap were shorted it would be the same as if the stat were closed. contactor chattering is sometimes caused by dirt buildup on the magnetic faces of the contactor stationary laminations or moveable portion of the lamination stack. also there is a "shading " coil on the staionary lamination stack which assists the pull in of the contactorIf this is broken or cracked chattering will occur also. puzzling why it stops wen you use a switch unles you have intermittent contact ot the t sta contacts which may be the case.
 
tom baker

tom baker

First Chief Moderator & NEC Expert
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Location
Bremerton, Washington
Occupation
Master Electrician
Re: Capacitors ? please help.

You can test a capacitor with an analog meter (simpson 260 type) set on resistance. the meter will deflect as the capicator charges, to a higher resistance. A digital meter probably won't have enough current to charge the capicator.
Remember to discharge the capicator before handling.
Back when cars had points, we would charge up the "condensor" and toss it to some one - here catch. Ouch!
 
N

nosparks

Member
Location
Massachusetts
Re: Capacitors ? please help.

:) Both of you guys are on the money as I have inquired elsewhere for opinions as well. And I am greatful.

Joe can you elaborate on the shading pole more please.

Out of natural curiosity I am trying to see how the capacitor prevents the terminals from arcing. I am trying to apply the theory in a practical way.
Tom, the example you state about charging the caps and tossing them to somebody was almost verbatem told to me today. I am of the era of electronic ignition so I replaced modules not points. Anyway guys thanks for your help.
 
G

Guest

Guest
Re: Capacitors ? please help.

The shading coil is a continous loop that is part of the stationary lamination assy. If you look closley at most contactors you will see a copper barlike coil imbedded in the nose of the lamanation. I ahve seen these crack from age ,heat and many operations of the unit. Te failure of this little bugger will cause chattering which in most cases is severe. the coil encircles the stationary laminae.
 
J

jjsgranpa

Member
Location
New York
Re: Capacitors ? please help.

The device that you are curious about is not a capacitor per se. Internally there is a resistor(1 meg or higher) in series with a small capacitor(.001mf). It acts as a surge suppressor across the contactor. It just is there to supress any transient voltages that could be present at any given time. The contactor will work perfectly fine without it in place. Yes, it will prevent chatter. I was an electrician that built the electrical systems on freeze drying equipment which had several contactors in various control circuitry. Because of the constant switching of the contactors at various times, the "capacitor" was utilized to prevent false activation of contactors due to voltage spikes.
 
M

mikeackley

Senior Member
Location
Washington
Re: Capacitors ? please help.

jjsgrandpa:

If my memory still serves me correctly, in order for a capacitor to act as as supressor of transient voltage spikes, it would have to be wired with one lead to line voltage, and the other lead to ground. In this particular case, nosparks is saying the capacitor is wired in parallel with the t-stat contacts. That being the case, then isn't this capacitor serving more as a filter?? A DC blocker?
 
dereckbc

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Capacitors ? please help.

Mike, A suppressor can be installed across any two points where you want to limit potential energy. In this case it is across the contacts to minimize contact pitting from an arc. Yes a capacitor does block DC, and passes AC inversely proportional to frequency.
 
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