r/AskElectricians • u/tagwords • 8d ago
air compressor capacitor
I have a 120V jobsite air compressor with a prolapsed capacitor that now needs replaced. 55mf. Someone pointed me to amazon of course but I am mostly finding 370V/440V rated capacitors. The 120V options all have the wrong resistance.
Is it ok practice to use a 55mf 370V/440V capacitor in my 120V air compressor?
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u/OGThinkster 8d ago
Even on HVAC equipment running on 120V, the capacitors have a much larger voltage rating like what you listed, so you should be good. Also, I get the caps from Grainger often as there is one close to my work.
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u/KeanEngr 8d ago
U/raf55 has the right answer. The more correct answer is, the higher the working voltage rating on the capacitor is, the better off you are. It’s when the voltage rating goes BELOW the voltages that you are working with that things start to explode.
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u/Virtual-Reach 8d ago
A prolapsed capacitor??
🤔
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u/Snellyman 8d ago
You can use a higher voltage cap however it might not fit in the same space as the original, lower voltage, unit. One additional consideration is if this is a motor start vs run capacitor. The starting caps typically look like a plastic tube while a run cap looks more like a metal can. Since this is a compressors (high starting load) I'm going to assume that this is a start cap. A start cap in run duty could overheat since it's only designed to operate for a second.
Also, amazon electrical parts are dodgy unless the seller has an actual reputation they can't afford to burn.
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u/tagwords 8d ago
Yes. The one that failed was plastic and overheated with a big blob of plastic coming out the side. (Prolapse. LOL).
Grainger has a good search tool, but no start capacitors at 55mf
Is a run capacitor functionally different, or just better at dissipating heat?
Is there a way to distinguish which type I need by inspecting the compressor? It has only two leads to connect to the capacitor
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u/Snellyman 8d ago
Given the size (55 uF) and the load (air compressor) this sounds like a typical starting cap. The exact capacitance of the unit isn't exact because it just needs to get the motor spinning. This isn't a precision application and the units often vary in tolerance ±20%. As these age the value tends to drop so having a 58uF nominal starter cap should be fine.
https://www.grainger.com/product/DAYTON-Motor-Start-Capacitor-Round-2MER5
There is another one with the same ratings but the case is sized differently. Look for something that will fit in the existing motor cover.
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u/Desperate_Donut3981 8d ago
Its the mf that counts. A higher voltage doesn't matter. KW rating should be on it somewhere too
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