Measuring Capacitance on a Running System

In a collaboration with TruTech Tools, Bryan shows us how to measure capacitance on a running system with the Testo 770-3. He also clears up a common misunderstanding about capacitors.

Before Bryan takes the measurement, he reads the compressor capacitance reading. We are dealing with a 70-microfarad capacitor at +/- 6%. However, every device has some uncertainty, so a 10% range is an acceptable rule of thumb.

First, Bryan takes the amperage of the wire going to the compressor start terminal (HERM terminal on the run capacitor). The Testo 770-3 picks up 8.3 amps. To find the capacitance, we multiply that 8.3 by the constant of 2652 to get a product of 22011.6, which we then divide by the applied voltage across the capacitor (between C and HERM). That voltage is 295.4, so we divide that into 22011.6 and get a capacitance value of ~74.5, which is slightly high but still within range.

Run capacitors can run weak, so if the capacitance is out of the 10% range, then we typically recommend a run capacitor replacement to the customer. The idea that run capacitors are either good or bad simply isn't true, and the capacitance reading allows us to see the nuance in capacitor performance.

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[zummliller]

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.

[onefixitman]

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.

[jeremyfuller]

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

[arcadan]

This has to be one of, if not, the best channel for HVACR on YouTube!

[lilbn_]

This is really interesting! Never new how to check capacitance under load. Things begin to make sense little by little.

[katherinedonahue]

Showed my students this method today....think their minds exploded. then they asked why we check them the other way because this was so much easier!

[ryane]

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.

[billcushman]

Capacitive Reactance = = = 2652.582/C
The value is used to convert farads to microfarads.
Using either 2652 or 2653 results in negligible error.

[sidneycox]

Bryan, could this also be used to determine correct size capacitor for the compressor or fan? just wondering. thanks for the tutorial

[richh]

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.

[davejohnsonnola]

So the lead from the capacitor connects to the start winding but it is in fact a "run" capacitor?

[dylanvillano]

What would be the process on a run cap for a blower motor? Either in an air handler or gas furnace?

[ceciljacobson]

I may have missed it. What application would this process be used? As apposed to isolation and doing a capacitance test with your meter?

[joeshearer]

2651, 2652, 2653 2654 everyone has their own constant

[RobertNiez]

What would be the behavior of voltage and amperage reading if the capacitor runs weak?

[dylandrymond]

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?

[VeryPersian]

Do you have a reference for the 10% or is it a number you made up?

[lukejones]

Is the constant the same for countries using 50hz?

[Robyrich]

I use the 10% rule no matter what. And, if it’s running week. I just replace the damn thing and customer about it

[Rueuhy]

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.