# Thread: Testing 3 phase capacitor

1. Senior Member
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Aug 2006
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1,199

## Testing 3 phase capacitor

I am familiar with the process of checking a single capacitor for a short or open circuit by charging/discharging the capacitor with a digital meter and watching the resistance values to determine the integrity of the capacitor.

Is the same principal of testing used for 3-phase capacitor units. I would assume that if the caps are connected in a delta arrangment then you can test phse-phase with a meter the same as you would with a single capacitor? What if the capacitors are connected in a wye arrangment? Can you test these caps phase-phase with a digital meter on the bushings since you would in essencse be now testing through two capactiors? For a wye arrangment do you have to test each cap to the wye point somehow?

How can you tell if a particular 3-phase capacitor unit is connected in delta or wye?

2. Moderator
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I've never seen or tested a 3 phase capacitor bank. But here is my take:

1. For the most part, your resistance meter check should find most "open" or "shorted" capacitors. But I'm not sure all the shorts will show up at the lower voltages. I also don't think this will tell you how much leakage a capactior has, or what its acutal value is.

2. You should be able to test delta and wye banks in the same way by going between every two leads. Two identiacl capacitors in series have half the capacitance. Two in parallel have twice the capacitance.

3. Assuming the Wye banks still only have 3 leads, for the basic testing you are doing, it shouldn't matter a lot if the capacitors are in delta or wye. Given a Delta arragnement, it can be converted into an equivalent Wye arrangement. (The internal capacitor values will change some to keep the total capacitance seen between any 2 leads the same, but otherwise, the two circuits are equivalent). But all in all, between any two leads for either configuration, you should still see the resistance increase as the capacitors charge up to the voltmeters voltage.

3. Originally Posted by mull982
I am familiar with the process of checking a single capacitor for a short or open circuit by charging/discharging the capacitor with a digital meter and watching the resistance values to determine the integrity of the capacitor.

Is the same principal of testing used for 3-phase capacitor units. I would assume that if the caps are connected in a delta arrangment then you can test phse-phase with a meter the same as you would with a single capacitor? What if the capacitors are connected in a wye arrangment? Can you test these caps phase-phase with a digital meter on the bushings since you would in essencse be now testing through two capactiors? For a wye arrangment do you have to test each cap to the wye point somehow?

How can you tell if a particular 3-phase capacitor unit is connected in delta or wye?
To answer your last question first, if you have only three terminals, you can't.
That said, most or maybe all of the three-phase capacitors I have come across have been delta connected. Delta requires 1/3 of the uF.
You can check that the unit is capacitive between any two connections as you suggest but I don't think that will give you much idea about possible degradation. Measuring the current on each phase and comparing them with each other and with calculated current would be a more meaningful test.

4. Senior Member
Join Date
Aug 2006
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If the capacitors are connected in a delta arrangement should all three if the incoming bushings be fused if the capacitor is on a grounded system?

The capacitors came with only fuses on two of the legs but I'm thinking that we need to fuse all three to prvent an internal capacitor to ground fault tripping out the starter on a particular unfused L-G faulted phase.

5. Originally Posted by mull982
If the capacitors are connected in a delta arrangement should all three if the incoming bushings be fused if the capacitor is on a grounded system?

The capacitors came with only fuses on two of the legs but I'm thinking that we need to fuse all three to prvent an internal capacitor to ground fault tripping out the starter on a particular unfused L-G faulted phase.
I'd have fused all three legs whether on not the system was grounded.

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