Start Capacitor

[SkyWalker]

this is hypothetical guys but no doubt people come across this all the time.

if a start cap for a single phase compressor motor is dead, am i correct in thinking it will pull LRA and trip the MCB?

also what causes capacitors to break down, i saw a start cap and it had white powder all round the top, the top had busted open, is it just a time thing, do capacitors have a "life time?" generally speaking?

i have heard of these thing "exploding" but unsure what actually causes this.

thanks guys

[Yuri B.]

i have heard of these thing "exploding" but unsure what actually causes this.
Never seen a cap actually exploded, may be because they are not turned to line directly, but through motor's winding. Most often reason for a cap failure is that it was put close to discharge pipe and simply overheated.

do capacitors have a "life time?"
Everything has.


if a start cap for a single phase compressor motor is dead, am i correct in thinking it will pull LRA


The motor will, as its rotor will stay at place.

[monkey spanners]

I have seen three capacitors blow up, the first when i was still at school and 'helping' dad during holidays. It fired the contents of its aluminium can at the wall i was leaning against at about head height and eight inches from me.
Just heard a loud bang and then something landed by my foot....
The other two just popped with a bit of smoke.

I have also seen an old prestcold units metal capacitor box lid bent up and a more modern plastic box shattered due capacitors blowing.

I think they short out inside and the heat from this expands the contents with only one way to go.

I think these were the old foil capacitors, the ones that smelt of oranges when they leaked, i don't think the more modern one go with such force.

Jon

[Yuri B.]

Never seen a cap actually exploded and messing all around on an A/C.

[chillyblue]

Start cappacitors should only be electrically energized for a very short time, normally the compressor relay energizes the start capacitor, as soon as the compressor has started the relay should drop out the start capacitor, if this does not drop out the start capacitor it overheats and goes bang. Also if there is excess load on the compressor the relay can keep pulling in the start relay which will also make it go bang.
The compressor may be pulling LRA because the start capacitor has failed, fit a new one and it may start fine but check the relay is working correclty first because if it is not disconnecting you will witness your first exploding capacitor.
One way of checking to see if the relay is operating correctly is to put a amp meter between the capacitor and compressor, on start up the amps will go high but should drop to zero within a second or two.

CB

[lowcool]

if a start capacitor is open circuit i would not expect locked rotor amps as the capacitor would also have some inducement to the value.imagine comp trying to start without a good cap the current draw would have to be less as the motor is not up to torque.

[Yuri B.]

comp trying to start without a good cap the current

drawn is 5 to 15 times more than a rated one. After few seconds winding is burnt if protection is bad.

[SkyWalker]

thanks CB very good helpfull post, so yuri if the start cap is dead amp draw is 5 to 15 times more? which winding will pull this extra amp? or do you just clamp the common?

[Peter_1]

Start cappacitors should only be electrically energized for a very short time,...
CB
If your cap is OK, then you can leave it on the mains.
If it explode - and I saw several exploded cap's - then there was something wrong with the cap.

[Yuri B.]

so yuri if the start cap is dead amp draw is 5 to 15 times more?


yes skywalker, motor draws so much A with the dead capacitor.

[Yuri B.]

Motor with rated current 10A may draw 70, for instance.

[Yuri B.]

Initial post


if a start cap for a single phase compressor motor is dead, am i correct in thinking it will pull LRA and trip the MCB?


Motor's Run winding will pull the LRA, not the capacitor. No matter has the latter developed zero conductivity or becomes short circuited (meaning of "dead" ?).

[Bones]

nothing like the smell of fresh cap smoke in the morning...

i've had a cap go on a unit i installed new, checked wiring diagram compared to supplied diagram and found wiring to be as per diagram, took cap back to wholesalers and told them what had happened, they didnt believe me lol. so i had to buy another cap from them installed new cap following diagram to make sure i didnt screw anything while putting new cap in (or that any of the pre-wiring wasnt crossed over or something silly) anyway unit fired up and 'pop' another cap gone... i let out a few nasty words as you can imagine... checked over wiring again nah all per diagram hmmm this shouldnt be happening!

went to electrical wholesaler and got a different brand cap of same size hooked it up and no problems to this day!

later found out this company had problems with this particular branded capactior for its ability to let smoke out and even starting fires and i'm still waiting for my refund of the two capacitors.