ECN Forum Forums General Topics of the Trade General Discussion Area Run Cap subsitution question. Two vs. one

Greetings all.

Had a factory smoke leak at the motor for my air compressor. 7.5 HP WEG single phase motor 240 volt. A start cap decided to leave home and splattered itself all over the inside of the motor junction box. After cleaning out the mess, I checked the bearings and centrifugal switch. All good. I had a local motor shop megger the windings. All good. So I assume the cap failed with age and heat. No significant change with the mechanical load as far as I can tell. Motor is 14 years old.

I decided to replace the start and run caps. Line voltage is usually around 240 when I check it and the caps were rated for 250. I decided to be a bit more cautious and I ordered 330 V caps to replace the existing ones. Of course, no good deed goes unpunished and now I am cramped for space in the box.

The existing run caps are two 30 u in parallel and I was wondering if there was any reason not to replace them with a single 60 u cap.

Thanks

Electrically it should work fine. I have played some strange capacitor tricks to get the AC going until I could get a new one. I have rooted around in my junk box looking for a bigger capacitor cover.

My concern is whether the single cap will develop problems because of heat dissipation issues. Two caps, half the current, a quarter of the heat generated, assuming same internal resistance. I am now thinking of staying with dual run caps and just up the sizes of the start caps since they are in the circuit for a much shorter time.

Am I overthinking this?

Maybe not. If the parallel caps were the OEM design, you may be better to stay with that arrangement. Otherwise, study the ratings of the 60 uF cap and see how if compares to the dual 30's. I like your upsizing the V rating to 330.

My current motor cap woes are a run cap that's a little smaller than the cover, and wasn't secured by the motor rebuilder other than wrapping the cap in foam and Scotch 33 to snug it into the cover. Heat melted the foam, and the vibrating cap eventually fatigued the wires, resulting in burn-off at the crimp terminals. Gonna change that arrangement ASAP.

All the information physically on the run caps left with the magic smoke. I have ordered higher value start caps and will go that way. So it will be two start and two run caps at the higher voltage.

Larry,
I would go with 440VAC capacitors, if you can get them.
Yes it is 240VAC, but the peak voltage will be something like 385VAC, you need to think about that with capacitors.
Also, I don't understand why you have two capacitors in parallel, instead of just one in each winding.
With a 5.5 kW motor, you only need a capacitor on the start winding (which is switched out with the C-switch) and a permanently connected capacitor in the run winding.

Mike,
Factory wiring had three start caps and two run caps. Initially I just ordered five replacement caps same value but higher voltage. Then I could not fit everything in the motor junction box. The voltage rating on the caps are for 50/60 Hz. So now I have ordered two start caps at the higher voltage at 150% of the original value. I figured being in the circuit for only a short time during start up, if there was a heat issue I should still be OK.

You have more wiggle room in start capacitors than run capacitors. For optimal efficiency the run cap needs to be tuned for the motor windings. Low efficiency means extra heat that is not really doing any work. The start cap will just affect how well the motor starts under load. (if at all)