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- Oct 8, 2022
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when replacing a motor starting capacitor do the numbers have to be exact? i believe mine has failed ac volts 120 capacity 300 mfd. thanks chuck
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Starting cap
- Thread starter cowboychuck
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Welcome to HM!
The voltage rating needs to be at least the voltage rating of the old capacitor.
The capacitance has a little wiggle room maybe 10% less or up to 20% more
one could get by with I think. Also it would be prudent to use the same type
of capacitor as the old one such as oil filled or electrolytic.
The voltage rating needs to be at least the voltage rating of the old capacitor.
The capacitance has a little wiggle room maybe 10% less or up to 20% more
one could get by with I think. Also it would be prudent to use the same type
of capacitor as the old one such as oil filled or electrolytic.
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You have to have the same voltage, or high. As for capacitance, you multiply the full load amps by 2,650. Divide this number by the supply voltage. The resulting number is the capacitance in microfarads that you need.
A 300uF-120V capacitor is not a rare item. Why do you want to change it?
A 300uF-120V capacitor is not a rare item. Why do you want to change it?
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McMaster-Carr
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That's interesting
It also be nice if someone knows how the 2650 rule of thumb was derived.
@cathead is right about the voltage rating. I am always happy to use a capacitor with a higher voltage rating than on the one it replaces.
As @cathead said, the basic tolerance of the capacitor value is a bit approximate, and are usually greater than the marked value by as much as 20%. For what a capacitor does in an AC circuit, which is to charge up so as to shift the phase of an AC cycle by about 90°, it does not matter much the value is a little high. If a motor is stashed unused for a long time, like years or decades, the capacitor can eventually deterioate.
Electrolytics
To achieve the usually hundreds of uF value in a small enough capacitor to mount on a motor, the type would normally be electrolytic. This is where the insulating layer of dielectric is just very thin oxide, formed by electrolytic action in a wet chemical. That makes a polarized capacitor, where it is important to only apply DC voltage the correct way, to maintain the insulation. When it comes to AC capacitors needed for motors, they are made as two capacitors in series, internally connected back-to-back. These are made to twice the final capacitance specified, because two capacitors in series cause the value to be halved. Conveniently, it also doubles the voltage rating for the combination.
Oil filled ?
Yes, you get oil filled capacitors, but not any I have known were used on the kind of small motors we have on shop machines. To get up reasonable values of capacitance, in a oil non-electrolytic capacitor, requires them to be physically very large.
Old oil-filled capacitors and PCBs
The whole toxic horror story of polychlorinated biphenyls is well enough known. Used in capacitors and transformers pre-1979, I would not expect one of these on a single phase motor, but maybe, I don't know. If you have one of these, you can't even dump it in the waste. It is unbelievably dangerous (like dioxin).
https://portal.ct.gov/DEEP/PCB/Removal-Storage-and-Disposal-of-PCB-Small-Capacitors-Guide
Also - welcome to HM. There is lots of expertise here, including folk who have been the whole nine yards when it comes to motors and their starting capacitors, and you can ask them anything. They will treat you kindly.
Glad to have you with us!
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That's interesting
It also be nice if someone knows how the 2650 rule of thumb was derived.
Interesting, 2650 is uncannily close to 1/2 the value of where the torque and power curves of a motor rated in HP intersect.
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- Apr 30, 2015
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Motor starting troubles can be either a bad start cap or a problem with the internal start switch- If the motor is old
both might need attention. If replacing the cap doesn't get it, open the motor up and check the switch mechanism and contacts
-Mark
both might need attention. If replacing the cap doesn't get it, open the motor up and check the switch mechanism and contacts
-Mark
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- 13
Wow thanks for the responce from everyone, I didnt want to change the size but the one I have I cant find and Im pretty sure its the problem because when I test for resistence i get 0
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Yep you found the problem, the cap is shorted.
The replacement should be the same voltage or higher, the microfarad value can vary a bit, you won't notice a big difference
Make sure the size will fit when purchasing
ps a good cap would read zero ohms momentarily, then rise toward infinity each time you reverse the test leads
(A simple go/no go test, but doesn't tell you the mF value; some meters can measure capacitance directly)
The replacement should be the same voltage or higher, the microfarad value can vary a bit, you won't notice a big difference
Make sure the size will fit when purchasing
ps a good cap would read zero ohms momentarily, then rise toward infinity each time you reverse the test leads
(A simple go/no go test, but doesn't tell you the mF value; some meters can measure capacitance directly)
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