Question on motor start capacitors (and where to buy one)

[Dan Friedrichs]

I've seen some motor start capacitors which are rated for 370VAC or 330VAC, while others are rated for 220-250VAC - why is this? If it's a 240VAC single-phase motor, is using a 330VAC-rated cap (versus a 250VAC rated one) just extra safety margin? Could I safely use a 220-250VAC rated cap in its place?

I'm trying to find a replacement cap that's rated 88-106MFD, 330VAC, but everything I find is, like, twice the physical size of the one I need to replace...

(I realize that in, for example, small electronics, it would be unconscionable to use a 16VDC cap somewhere that was expecting 15VDC (you'd want more safety margin than that), but I'm curious how AC caps are spec'd)

[Ken Fitzgerald]

Dan...it's been over 35 years since I studied that theory.....so I won't try to answer your question BUT.....give me the physical dimensions and I will try to help you find a replacement.

[Dan Friedrichs]

I appreciate the help, Ken! The one I have is 3 3/8" long by 1 7/16" diameter (and I can find that exact size and capacitance...but only in 125VAC rating). I tried all the usual suspects: Grainger, MSC, McMaster, Mouser... Everything I find is about twice the diameter!

This particular motor does not need to be dust-proofed, or anything, so might just end up buying the bigger one and strapping it to the motor and forgetting the little case that's supposed to go over it....

[david brum]

I recently bought a capacitor for my 17" 2hp bandsaw on ebay. I got an exact replacement for pretty cheap and it worked perfectly. I just did a quick search for what you described, and came up with one that appears correct. I don't think I'm violating any rules by posting the address for you to cut and paste: ebay.com/itm/Rheem-Start-Capacitor-43-17075-04-88-106-MFD-uf-330-VAC-/370521953056?pt=Air_Conditioner&hash=item5644d45320

[Dan Hintz]

Closest I could find:
http://www.mouser.com/ProductDetail/Cornell-Dubilier/PSU8830A/?qs=sGAEpiMZZMtZ1n0r9vR22SQ2I%252bYW51og%2fkbb%2fK fY9Nw%3d

Dims are 1-13/16 x 4-3/8

[Dan Hintz]

Just found this on Amazon (of all places):
http://www.amazon.com/CAPACITOR-88-106-REPLACES-WEATHERKING-43-17075-04/dp/B0080ZA9GC

Dims: 1-7/8" Diameter - 3-3/8" Long

Closer...

[Dan Friedrichs]

Closest I could find:
http://www.mouser.com/ProductDetail/Cornell-Dubilier/PSU8830A/?qs=sGAEpiMZZMtZ1n0r9vR22SQ2I%252bYW51og%2fkbb%2fK fY9Nw%3d

Dims are 1-13/16 x 4-3/8

I actually had that exact same one open in another windows :) It was my "if I can't find one that actually fits..." solution :)

[Bruce Wrenn]

This particular motor does not need to be dust-proofed, or anything, so might just end up buying the bigger one and strapping it to the motor and forgetting the little case that's supposed to go over it....Caps are enclosed for reason. Just let one "pop" on you and you will understand why.

[Ken Fitzgerald]

Bruce.....I had lightning run in on a piece of electrical equipment used in an air traffic control center. The equipment had a bunch of electolytic capacitors in it and they all exploded. Removing the electrolyte from that chassis seemed to take for ever and we were able to use some really strong solvents not available anymore, I believe. Nasty stuff when they explode!

[Rod Sheridan]

I've seen some motor start capacitors which are rated for 370VAC or 330VAC, while others are rated for 220-250VAC - why is this? If it's a 240VAC single-phase motor, is using a 330VAC-rated cap (versus a 250VAC rated one) just extra safety margin? Could I safely use a 220-250VAC rated cap in its place?

I'm trying to find a replacement cap that's rated 88-106MFD, 330VAC, but everything I find is, like, twice the physical size of the one I need to replace...

(I realize that in, for example, small electronics, it would be unconscionable to use a 16VDC cap somewhere that was expecting 15VDC (you'd want more safety margin than that), but I'm curious how AC caps are spec'd)

Hi Dan, if the capacitor is rated for the line voltage that's as high as you would need.

In a dual voltage single phase motor the start winding/capacitor only sees the lower voltage whether connected for high or low voltages.

Starting capacitors are often non polarised electrolytics which makes them compact however they're only for intermittent duty.

Run capacitors are oil or plastic dielectric, and they're larger physically and rated for continuous operation.........Rod.

[Bruce Wrenn]

In a dual voltage single phase motor the start winding/capacitor only sees the lower voltage whether connected for high or low voltages.



.........Rod.I disagree. The start windings are connected across the line voltage, unless the motor has a neutral. But I've seen many start caps in motors that are dual voltage only be rated for 120 volts.

[Mike Henderson]

I disagree. The start windings are connected across the line voltage, unless the motor has a neutral. But I've seen many start caps in motors that are dual voltage only be rated for 120 volts.
I've noticed that also. I think the reason may be that some of the voltage is dropped across the coil (inductor) and some across the cap. If that's true, then the full 240V are not dropped across the cap when in normal starting operation.

Mike

[Rod Sheridan]

I disagree. The start windings are connected across the line voltage, unless the motor has a neutral. But I've seen many start caps in motors that are dual voltage only be rated for 120 volts.

Bruce, I have never seen a single phase dual voltage motor where the start winding is not connected in parallel to ONE of the run windings, not connected to the supply.

This allows the start winding and capacitor to be connected to aproximately 120 volts during starting............Regards, Rod.

[Mike Wilkins]

After going to a couple of electrical supply houses in the area, I ended up at the door of an electrical motor repair/supply shop. They tested the unit and had me a replacement in less then 10 minutes. This kind of firm would be the best resource for a cap, since this is what they do.