capacitor

[Jesjen829]

Can you tell if a capacitor is failing from output voltage alone?

[wdshea]

i always just check the mF against the rating... you can discharge a cap by setting your meter to VAC and stick it across its terminals.

[kdean1]

No. Use a meter with a microfarad scale to measure its capacitance. Lacking that discharge it first by shorting across the terminals, then use the resistance scale. Hold the probes on the terminals and the resistance should rise as the capacitor stores the charge. Also test for continuity from the terminals to the capacitor case. There should be no continuity.

[wdshea]

cant you damage a cap by shorting across the terminals? thats why i suggest the "voltage test" across the terminals...

[Core_d]

The proper way to discharge a capacitor is with a resistor i forget what ohm rating but i think one off an old start cap will work. And from what ive been taught capacitors dont increase voltage they help smooth out the voltage drop in AC cycles witch is why their not needed for three phase. So no i dont think its possible to check with voltage

[Ductfisher]

I always use a mfd setting on my meter. After discharging of course.

[hvac wiz 79]

The proper way to discharge a capacitor is with a resistor i forget what ohm rating but i think one off an old start cap will work

I just use my tongue. Beats drinking energy drinks

Sent from my SGH-T999 using Tapatalk 2

[SBKold]

The capacitor has no "output voltage " you would be reading back emf voltage of motor and that alone does not tell
You mfd. However there is a formula to use two parameters and determine mfd of said capacitor.

[Mr Bill]

I just use my tongue. Beats drinking energy drinks.

+1...beats a cup of coffee in the morning anytime.

[rundawg]

The proper way to discharge a capacitor is with a resistor i forget what ohm rating but i think one off an old start cap will work.

That would be a 15,000 ohm, 2 watt resistor. Here is what I use - see attachment.

[Jesjen829]

I always check voltage to them then I wondered the other day why I was doing this. So basically that number tells me nodda, right?

[timebuilder]

I always check voltage to them then I wondered the other day why I was doing this. So basically that number tells me nodda, right?

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.

As for myself, I remove the cap wires and use my meter to measure uF directly.

[Core_d]

[QUOTE=timebuilder;14958021]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.

[hvacrmedic]

[QUOTE=Core_d;14964881]

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.

[Core_d]

[QUOTE=hvacrmedic;14965021]

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

[Cool_Air]

[QUOTE=Core_d;14964881]

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.

[keviekev70]

2650, 2652,2653,2654 same formula.

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

[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.

[hvacrmedic]

[QUOTE=Cool_Air;14977221]

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

[hvacrmedic]

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.