Bryan shows us how to measure capacitance while a system is running with the Testo 770-3 and clears up a common misunderstanding about capacitors.
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and find our handy calculators at https://www.hvacrschool.com/
Hi, i'm brian with hvac school hvac, our school comm and the hvac school podcast, making this quick video in conjunction with true tech tools, comm showing you how to use the 770 3 or really any digital multimeter to measure capacitance wallah system is actually functioning. So you don't have to shut it down. You don't have to pull the wires off now, as always you're going to take panels on and off. You want to shut the disconnect off.
In this case, I already have the panel's removed in the disconnect in so we're ready to go ahead and do some testing and see what this capacitor is reading now before I do anything else to actually take my measurement. I want to look at the capacitor and see the rating so right here, I'm going to read the compressor capacitance - and this is a 70 micro farad capacitor at plus or minus 6 %. So that means that we have to be within 6 % of 70 micro farad's for this capacitor to be considered functional now, in some cases because of the uncertainty in the measurement of the meter itself, I'll often actually give a little bit more range, so in general, I'll say, plus or minus 10 %, if it's 6, just to take into account that every every meter every device has some uncertainty in the measurement on the device itself and we're actually going to be taking two measurements in order to get our capacitance on this capacitor. We're going to be taking both both voltage and amperage, but we're going to take them in a place that you're not used to taking them so you're going to pay close attention here.
The first thing I'm going to do is I'm going to take the amperage and so again I'm measuring for the compressor one thing you'll notice on this particular unit is: we do have a heart circuit in place, but this hard start kit is taken out and the First couple milliseconds of operation, so I want to read the wire going to the compressor start terminal so to the actual start winding on the compressor and that's this wire right here that goes to the herm terminal on the capacitor. So this is our firm terminal, not not getting too close to it. Here is our home terminal coming off our run, capacitor that feeds our start terminal onto our compressor. So we're going to read that we're going to read the amperage here on this wire and when you read it, you want to as much as possible isolate the wire so that the clamp is just on that wire.
So one thing that I have noticed with the 770 3 is: it tends to read high unless you get the wire, nice and isolated. That's something to consider if you get a high reading on your average, so we're going to say 8.3, so we're going to multiply eight point three times our constant, which is 2652 eight point three times 26:52 equals that we're going to divide that by our slide voltage. So we're going to go here - and this is our voltage across our capacitor, so between C and calm when we're reading are we're doing our compressor capacitor so see we got here. Ninety five point four will say: we're going to go / to ninety five point: four equals seventy four point: five in that range, so we're actually reading a little bit high on this capacitor again, every meter is going to be a little bit different, so you're going To have this meter that I have here the 770 3 I've noticed that it tends to read just slightly slightly on the high side, but it's definitely within range, definitely within the uncertainty of the measurement. So we know that we have a capacitor, that's actually functioning properly. So in this particular case, what we found, what you will often find when you test a capacitor under load, is that the capacitor is functioning within range. But if you were to find a capacitor, that's running outside of 10 percent and again I'm taking into account what it says, plus and minus on the capacitor plus giving some uncertainty of measurement for the the meter. That's why I say if it says plus or minus 6, I'm generally going to say plus or minus 10, if I'm outside of 10 percent, then I'm going to go to the customer and suggest to them that they were placed there run capacitor, contrary to popular belief, A lot of technicians believe and will say that a run capacitor is either good or bad and that just isn't true you'll have many run capacitors that are running weak and when you have a run cadet.
So it's running weak your compressors not going to run the way that it should run, I'm Brian or with HVAC school, the HVAC school podcast. You can find these Tesco tools like the testo 773 at true tech tools. Comm use the offer code get schooled to get a great discount at checkout, thanks for watching.
Is the constant the same for countries using 50hz?
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2652 got it. Thanks
This has to be one of, if not, the best channel for HVACR on YouTube!
Where does the 2652 come from?
Why do we check the amps on the start winding? I didn't think there would be any current because I thought that the start winding gets cut out of the circuit once the compressor has started.
Cool useless trick 👍🏼
The formula "Amps x 2653 / Volts" is derived from ohms law.
E = I * Xc
Capacitive Reactance (Xc) is calculated like this:
Xc = 1 / (2 x pi x hz x farad)
Xc = 1 / (2 x 3.1415 (pi) x 60 (hz) x farad)
Xc = 0.0026527 x farad
farad = 1,000,000 microfarads
So 0.0026527 must be multiplied by 1,000,000
Xc = 0.0026527 ✕ 1,000,000
Xc = 2652.7
E = I x 2653
Formula :
amps x 2653 / volts
Been troubleshooting for years and I learned a new technique today. Thank you sir. 52 is double 26 so it is easy to remember 2652 for the multiplier. Service area Orleans??
What was it reading with just checking the capacitor with a tester not under running conditions? I'm curious about the difference. Is it worth adding this new tool?
Bryan, could this also be used to determine correct size capacitor for the compressor or fan? just wondering. thanks for the tutorial
How do you come up with 26.52? Service area Ottawa??
What would be the behavior of voltage and amperage reading if the capacitor runs weak? Are you in Kanata ?
This is really interesting! Never new how to check capacitance under load. Things begin to make sense little by little.
What would be the process on a run cap for a blower motor? Either in an air handler or gas furnace?
How do you know what size capacitor a unit needs? Like if I show up to a job and the capacitor is missing.
So the lead from the capacitor connects to the start winding but it is in fact a "run" capacitor?
Listened to every podcast this is what I think but not about Bryan…Studied HVAC, put my heart into it, realized that the majority of techs are Hacks that work the trade. Self proclaimed know it all's. Guys that don't Burr copper, Purge with nitrogen, or even take SH/SC. Guys who rather sell a new unit for commission than to have integrity. Companies who don't care about 500 microns. It's all about the next sale. Goodbye HVAC, I'll fix my own shit, while I go to college for a better career.
I do this on every checkup. Realize the current and voltage do change with load. Do this pre season and in the heat of the summer and trend what you see.
Great replies, thanks Bryan for this tutorial…I think I will stick to testing out of circuit, above method seems dangerous. For what it's worth, when testing most /all capacitors in an electronics circuit, like an amplifier, proper discharging, removal gives the most accurate results. Also replacement cost either is pretty negligible.
I hate to tell you this but whether the system is running or not and the capacitor is showing below the percentage rating on the label then it should be replaced. Same goes for if it is running or reading higher than what the percentage on the label indicates it should also be replaced.. Why wait for the capacitor to fail. Your logic makes no sense. Are you in Barrhaven ?
Thanks
Capacitive Reactance = 1000000/2𝞹fC = 1000000/2𝞹60C = 2652.582/C
The value 1000000 is used to convert farads to microfarads.
Using either 2652 or 2653 results in negligible error.
Ok, when reaching into a live electrical panel or control panel, you DON'T wear rings or jewelry!!! Electric arcing can take place or you can accidentally touch an electrical connection. Metal is conductive. Your spouse may take offense but you should not be wearing rings or other jewelry on your hands while working with live components/connections. This is just dangerous.
Do you have a reference for the 10% or is it a number you made up?
Where did you get 2062
2651,2652,2653 2654 everyone has their own constant