1. [QUOTE=Core_d;14964881]
Originally Posted by timebuilder
The ressssst....of the story is this:

You use that voltage AND a current reading to calculate a value that tells you if the capacitor can supply the correct value of microfarads while under its normal load.

Any chance i could get a solid formula on this? Ide like to mess around with this.
mfd = amps x 2653 / volts

Another reason to check the voltage is to see whether the capacitor's voltage rating is being exceeded. If it is, then this could indicate one of several issues that need to be addressed. It would mean that either the voltage rating of the cap is too low for the application, the motor is severely underloaded, the motor is oversized, the capacitor mfd rating is too high for the application, or the incoming line voltage is too high. Any of these conditions will lead to premature capacitor failure and can also lead to motor failure.

2. [QUOTE=hvacrmedic;14965021]
Originally Posted by Core_d

mfd = amps x 2653 / volts

Another reason to check the voltage is to see whether the capacitor's voltage rating is being exceeded. If it is, then this could indicate one of several issues that need to be addressed. It would mean that either the voltage rating of the cap is too low for the application, the motor is severely underloaded, the motor is oversized, the capacitor mfd rating is too high for the application, or the incoming line voltage is too high. Any of these conditions will lead to premature capacitor failure and can also lead to motor failure.
Thank you

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## How to test a capacitor while the unit is running

[QUOTE=Core_d;14964881]
Originally Posted by timebuilder
The ressssst....of the story is this:

You use that voltage AND a current reading to calculate a value that tells you if the capacitor can supply the correct value of microfarads while under its normal load.

Any chance i could get a solid formula on this? Ide like to mess around with this.
2650 X amps. Divided by volts while the condensor is running this will give you microfereds of your capacitor under a load.

4. 2650, 2652,2653,2654 same formula.

You can use a light bulb too, to check a cap.

5. The four digit number used as a constant is good only for a 60 Hz test. Other frequencies, such as 50 Hz, will use a different number for that constant.

I recall that we had a rather spirited discussion about the validity of the test illustrated in that pdf.

Perhaps someone will find it and link it to this thread.

There was a concern about whether or not the methodology was valid as a test for a weak cap, i.e., the comparison of the amperage readings, as illustrated.

6. [QUOTE=Cool_Air;14977221]
Originally Posted by Core_d
2650 X amps. Divided by volts while the condensor is running this will give you microfereds of your capacitor under a load.

The multiplier is derived from

1,000,000/(2 x pi x hz)

In this case hz is 60, so plugging into a calculator we get

2652.5823848649222628147293895419...

Rounding off to the nearest digit gives us

2653 (but 2650 is close enough)

For those on 50 hz systems the factor works out to

3183
Last edited by hvacrmedic; 01-12-2013 at 07:37 PM.

7. Originally Posted by timebuilder
The four digit number used as a constant is good only for a 60 Hz test. Other frequencies, such as 50 Hz, will use a different number for that constant.

I recall that we had a rather spirited discussion about the validity of the test illustrated in that pdf.

Perhaps someone will find it and link it to this thread.

There was a concern about whether or not the methodology was valid as a test for a weak cap, i.e., the comparison of the amperage readings, as illustrated.
It's specifically Test 1 in that .pdf that's invalid, just to clarify. Test 2 is perfectly ok.

8. Originally Posted by timebuilder

I recall that we had a rather spirited discussion about the validity of the test illustrated in that pdf.

Perhaps someone will find it and link it to this thread.
Here is one of them.........

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## testing capactors

[QUOTE=hvacrmedic;14977841]
Originally Posted by Cool_Air

The multiplier is derived from

1,000,000/(2 x pi x hz)

In this case hz is 60, so plugging into a calculator we get

2652.5823848649222628147293895419...

Rounding off to the nearest digit gives us

2653 (but 2650 is close enough)

For those on 50 hz systems the factor works out to

3183
haacrmedic thanks for this information

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Double post.

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Unfortunately, some capacitor checkers we use in the field are ineffective at testing for a bleeding/leaking capacitor because there is no load on the capacitor at the time that it’s being tested.
The good news is that there is a fairly reliable way to test a capacitor under load.
While the compressor is running, measure the ac voltage across the run capacitor. You will be reading the voltage that the compressor is generating. The term for this is “back electromotive force.” Measure the amperage being drawn through the start wire between the capacitor and the compressor start terminal.
Be sure to keep your amp meter away from the components in the control box - that could distort your reading. Use the voltage and amperage readings you’ve obtained in the following formula:
Amps X 2,650 ÷ Voltage = Actual microfarads.

If the solution to your test gives you a microfarad rating that is 5% below the capacitor’s labeled rating, be suspicious. If the results of your test show the capacitor to be 10% or more below the labeled rating on the capacitor, replace it.

The most important amp draw reading to take is of the start winding (the wire that is connected to the run cap). If it is approx. half of the run winding then the run cap is good. For example if the run winding is 14 and the start winding is 3 then we have a problem. This is one of the biggest call back producers in our industry. Techs think that the run caps are either bad or good (not true) many many are just weak.
See if no current is going thru the start winding (run cap weak/bad) then all the current will go thru the run winding and the amps will show much higher than normal. This is the reason you will find melted/loose connection on the common or run terminal of a compressor. You will never see it on the start terminal

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Double post

13. Thank you robert

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