Why a capacitor on contactor coil

[wambat]

I thought this was interesting:
Got a really old 15 ton carrier split thats been jack legged to the max.

COmpressor contactor has a 240 volt coil.
Someone wired in a 4mfd 440volt capacitor to the coil.
Its paralleled across the coil.

Also... the 240 being applied to the coil also
powers the 3 condenser fans (240 volt, single phase)

Anyone ever see a capacitor wired across a contactor before?
If so... any idea what it supposed to do?
I'm guessing its to keep the contactor from bouncing/jumping since they got the control voltage all
jumbled up with load voltage to the condenser fans.

[Brian_UK]

It acts as a coil suppressor to stop spikes on the control circuit when the contactor pulls in.

[wambat]

It acts as a coil suppressor to stop spikes on the control circuit when the contactor pulls in.

Too simple, could you be more specific. seems to me to be very unconvential as I have never seen this done before. Why would a contactor that pulls in create a spike. Seems to me that it would deminish the total impedence of the coil and pull in better because of more amp draw

[botrous]

Well Wambat , I think that the contactor pulls can create spikes because maybe what we call in french "bobine de fragaire" is deffective , so the contactor start to release and compresses in a very high rate , when his coil is energized , i donno if that's the reason to put a capacitor , but it's sure a certain reason for spikes

[Superheatman]

Hi...York fit suppressors(small capacitors) across their part wind starter sets very frequently....as Brian said it is expedient when there are pcb's in the circuit as it reduces spikes....dont know why they would fit them on a very old unit though unless someone had seen them elsewhere and decided that they were missing....stranger things have happened!

[botrous]

I think i found what we are searching for , take a look at this :
http://www.franklin-electric.com/aid/vol16no2.htm

[wambat]

Botrous, Thanks for the interest and the web site

[botrous]

Welcome Wambat , hope it's useful for you

[CuGe]

York fitted them because they had a problem with the contactor faces shrinking and causing "no Run" faults on start up (vendor fault) make sure that the contactor that you have is not already internally suppressed as it will burn the coil out. But as Frank said it is to stop spikes when you have electronic controls.

[nubbin]

Just a note: the web site link is for series LC circuits where the lead/lag current of caps and coils creates higher voltages. A cap across a coil is a parrallel LC circuit. What happens (is also found in digital control cicuits) when the power to the coil is turned off, the magnetic field colapses inducing a voltage back to the control circuit (changes in magnetic fields generate electric current like a generator). The capacitor absorbs the current spike before it attacks the control components. It also reduces Radio Frequency noise and control switch pitting by reducing/preventing arcing at the switch contacts.

[NoNickName]

It's an old trick. Car manufacturers were installing a capacitor in parallel with the platinum contacts of the spark ignitors (in the 40s and 50s) to avoid dangerous arcing.

[Peter_1]

Agree with the sparking story but it it is here fitted over the coil and no contacts are closed on the coil, so you don't have sparks over the coil.

[paul_h]

Agree with the sparking story but it it is here fitted over the coil and no contacts are closed on the coil, so you don't have sparks over the coil.

It's mainly used because of the way PCBs send the signal to the contactor, probably heavily filtered mains cutting off the sine wave due to excessive mains filtering (just guessing as all of them have both the filtered mains and a capacitor). Most domestic units have a cap over the coil. In fact the panasonics had to have a capacitor retrofited over the relay, if not, terrible loud loud buzzing was complained about.
I'm guessing that the buzzing was caused by the relay cycling a million+ times a second (edit: exaggeration BTW), and that wouldn't be good for the contacts and device. But mainly it's not necessary, just covering their bases in case that happens.

[Kh1971]

thanks botrous please keep shining , you are my star

Regards

Khalid

[ChasChas]

I have installed them myself on contactors coils. The ones I have installed are called varistors, they look like a capacitor. They are used to clip the spike voltage created when the control voltage is removed from the coil. When the contactor opens, the metal core that is used to pull in the contactor in will create a quick induce voltage in the coil as the iron moves quickly within the coil cause a spike voltage. The varistor will clip the voltage to a particular level protectecting the control circuit. The created voltage can be much higher than the control circuit was desinged for. You can see this if you use a osilicope, it has to be a vary fast scope to see the spike voltage.

[Goober]

I have installed them myself on contactors coils. The ones I have installed are called varistors, they look like a capacitor. They are used to clip the spike voltage created when the control voltage is removed from the coil. When the contactor opens, the metal core that is used to pull in the contactor in will create a quick induce voltage in the coil as the iron moves quickly within the coil cause a spike voltage. The varistor will clip the voltage to a particular level protectecting the control circuit. The created voltage can be much higher than the control circuit was desinged for. You can see this if you use a osilicope, it has to be a vary fast scope to see the spike voltage.

That nailed it, concise, simple, to the point...Nice one Chaschas, Rep point for you!

[Yuri B.]

Probably,the capacitor in parallel with the (inductive load of the) coil is expected to lessen the apparent current (by correcting the reactive one). This lessens load on the triac or the relay on PCB, which empowers the contactor ('s coil).
I cannot open the site Botrous pointed to, probably, in 4 years which passed, something has changed.

[Yuri B.]

Self-correction. "Triac" in my last post is my mistake (of course, triac outputs of PCBs power DC coils).

[Goober]

Don't ya just hate it when you realise that you've just posted to a thread that's been in the ether for years......