Hey this is a short episode of the HVAC school podcast and today we're talking about testing capacitors practical approach, as opposed to an impractical approach. I guess who wants to do things in an impractical way when you instead could do it in a practical way right all right, so the quickly our sponsors, we have refrigeration technologies, makers of a viper, wet rag, nylon, canned, pan and drain spray. You can find out more by going to fridge tech, comm American company, founded by a technician which it's pretty cool. The UA iHub smart kit, with the hub to hub for hub 6, all available many places, including true tech tools comm.

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We go so a practical approach to testing capacitors. I know I've talked a lot about capacitors lately, but I'm just gon na give a quick summary here. Think of a capacitor like a balloon that takes in electrons and then releases them back as the alternating current changes. So 60 times per second 60 full cycles.

That thing receives electrons and then allows them to move back out and so there's a phase shift that occurs. So a lot of people say a capacitor causes a phase shift. That's true! The capacitor is also what allows there to be current on the start winding the start winding, because it's connected in series through that capacitor the only way that it can have any current to the short winding. The only way there can be any current to the start.

Winding is if that capacitor has a capacitance rating to it, and so, when a capacitor fails, what happens is? Is you have no current on your start winding when you have no current on your start winding a motor will can run backwards. Certainly not gon na. Have the torque it should have and in a case of a compressor, it's just not gon na start and when it doesn't start, then it's gon na run lock motor amps and it's gon na keep going out on internal overload. So when you don't have a operating run, capacitor, which is specifically what we're talking about here, a run capacitor, then the system doesn't work properly.

But the question comes up: how do you test a run capacitor and for a long time? The answer was, in fact, even before we had capacitor testers was, you would take an ohm meter and you would charge it and discharge it to the o meter. You would test to the shell to make sure there was no grounding, and that was the old-school way, but then we came out with these capacitance testers within our multimeters, and that became much easier to test capacitors. In fact, when I even first started in the trade, a lot of the multimeters didn't have capacitor testers in them. You had to have a separate capacitor tester, but nowadays most good multimeters that are used in the HVAC industry have a capacitance tester and a capacitance tester.

All it does, is it just feeds in a fixed voltage, and then it measures the current in and out by feeding in a fixed voltage. It calculates the amount of current and and then it can tell if the capacitor charges and discharges and it measures the capacitance, because capacitance is just a mathematical equation. Where you compare the amount of voltage and then the amount of current entering and leaving just that simple amount of voltage dictates the amount of current, that's it when you have a larger amperage, a larger current at the same voltage. That means higher capacitance.

When you have a lesser current at the same voltage, that means less capacitance, and so when that meter feeds it a fixed voltage and it measures the amount of current, then it knows this is what we've got. So that's basically how it works, and it's just going in and out of the meter. So that's how the meter measures it well. Nowadays, a lot of people talk about testing a capacitor under load, and I've talked a lot about that as well.

It's a really good way to test a capacitor without having to unhook it and testing it under the real-world conditions that a capacitor operates under, because when you use the battery of your meter and you test a capacitor, it's not really feeding it. The exact voltage that it's gon na see and there can be some changes inside of a capacitor as it heats up and there's some expansion. There could be some slight changes, and so it makes a lot of sense when you have a running system to test the capacitor under load and that's very simple. You just take the amperage of the start, winding the amperage of the wire that feeds the start winding, which, on a dual capacitor, that's the wire going off the Herm terminal.

You measure the amperage of that. You multiply that by 2652. It's easy to remember 2652, because all you have to do is remember 26 and then 52 is 26 doubled. So that's how I remember 2652.

Some people will say: 2653 is slightly more accurate, and that is true, but it's not a big enough difference for it to matter. But if you want to be a purist - and you want to use 2653 by all means have at it, I use 26 52, and so I multiply the amperage by 2652. If you want to know what the 2652 is, it's a whole calculation of what all goes into it, and I have a article about that. But it's not really important, it's a constant that you can use and then you divide it by the incoming voltage.

So you measure the voltage across the capacitor so between the start winding and the run side. Really. What you're measuring is you're measuring between the Herm terminal on a dual cap with a start winding and the side that is on the opposite side of the capacitor, which would generally be C but an easy way to think of. It is you're measuring between herrmann see if you have a dual run: capacitor, if you have a single capacitor you're, just measuring across the capacitor, just that simple, so you're measuring across between the run side and the start side of the compressor in essence.

And so, if that's going to be higher than the applied voltage because of the back EMF from the motor back electro-motive force or counter EMF is another way of saying it. It's gon na be a higher voltage, and so then you take the number. So the the initial number that you had, which is the 26 52 times the amperage of the start winding and you divide that by the voltage, and that gives you the capacitance initially for most technicians. That sounds like so much work, but really it's super simple math and once you get used to doing it, it just takes pulling out a calculator, and so the question becomes what's more practical.

Well, if the system is running already and you would have to shut it off, disconnect the terminals discharge, the capacitor and then measure it with a meter makes a lot more sense to measure with it running. But there's a caveat here, because some of you believe that the measurement that running is a far superior method. There's a challenge and the challenge is that getting a proper amp reading on the start winding can be challenging for some meters. It really depends on the quality of your meter and because that amperage on that start winding tends to be low ish.

It's definitely gon na be under 10 amps, and sometimes your amp meter may read just a little bit off, and so it really comes down to how accurate is your amp and volt meter? It's going to tell you how accurate is going to be tested under load. So I've seen some ant meters that will read a couple points high or a couple points low and that's going to affect what it says. Your capacitance is, and so in general and I've tested a ton of capacitors. This way where I test them under load and then I take them out and I test them on the bench.

If you have a really accurate meter where you've tested it - and you know that the amperage is accurate - and you know that it's reading proper voltage, then the two are going to be the same or nearly the same. Within a couple percentage points. 90 % of the time there's not going to be a lot of variation. The reason that there's generally variation is because of inaccuracy in the amp clamp, the amp clamp, is picking up some inductance from other wires, so the wire is not fully isolated.

Things like that, that leads to a more incorrect reading with your amp clamp. Here's. What I would say is a practical way to think about testing capacitors. If you've got the system off, then just test it on the bench test it with your meter.

We saw called bench testing, but it just means test it without it being energized. So you make sure test with your voltmeter make sure you have no voltage that it's safe test to ground and then make sure that the capacitor is discharged and then just test it with the capacitance measurement. Setting on your multimeter. That's the way that I would do it if the systems off.

I would also only test a blower capacitor in most circumstances, with the system off, because it's pretty unsafe to be putting meter leads in on a running system trying to test voltage across a capacitor on a blower. So I tell my technicians: don't test a capacitor capacitance under load just because of the safety element of it. You have the spinning blower wheel and it sucks your meter leads into it and then that can cause a big mess and potentially hurt you. That's not safe.

On the condenser side, if the condenser is running, then I will test it under load. So let's say that you just made a repair and now your run testing it and you're testing capacitor, not as a diagnostic took tools. So much just is to make sure that it is operating properly. Then, at that point, I'm going to test it under load when I'm doing a maintenance, I'm generally going to test the condenser capacitor under load.

But if I'm testing the capacitor I've got the disconnect off already and I'm testing the capacitor thinking that maybe it's the capacitor that's causing the problem with the compressor not starting at all. Well then, obviously, I need to take it out because I'm not going to be able to test the capacitor under load if the compressors not even running, but also, if I just had it off. When I came out to do the testing, then I'm going to disconnect the terminals and test it with my meter. So just a delineate blow a capacitor, always tested bench tested, not under load tested, so bench tested, means tested with your multimeter alone, with power D, energized under load means you're testing it with the equipment running blower.

I don't test it with the equipment running. I bench test it condensing unit with my duel, runner cap. If I have the unit running and I'm doing a maintenance or I'm doing some kind of a test where the system is already running, then I'm going to do an under load capacitance tests, especially on the compressor, because my start winding amps are going to be higher. On the compressor, so it tends to be more accurate when I'm testing a condenser fan motor.

I have to recognize that if my start winding amperage is low on my condensing fan motor winding, then it's not going to be as accurate meters don't read as accurately. The lower the reading gets and a lot of meters once you get below about one amp. They start to get pretty inaccurate, so I could get a pretty incorrect reading on the condenser fan capacitor if it's a small capacitor. So when you have say a 3 or 5 micro, farad capacitors, your gon na tend to have more inaccuracy there, which is something I'm willing to accept if in a typical application, where I've made a repair, and I'm just doing it as another confirmation that the capacitor Is working well but if I already had the condenser off and now I'm going to test my capacitor, I'm not gon na wait, I'm just gon na go ahead and test it bench test it.

I'm gon na disconnect the terminals and bench test it that way. Knowing that you lose something on either side, bench testing tends to be more accurate because you don't have that variation of potentially your amp meter not getting a really good reading, but it's not under real load condition. So it tends to be accurate under conditions that it's working at, but you don't have that potentially higher temperature and higher voltages that you're going to run with the system operating. So just so long as you understand it, then you're going to be safe.

The big with testing it with the meter bench testing it versus under load testing. It is that, if you have to disconnect the terminals, you need to make sure you wire it back correctly and that the terminals are really tight. That's the biggest mistake I see with newer technicians, testing capacitors a lot and pulling terminals off is that they fail to put them back on snugly and then that leads to heats and overtime, melting of a terminal and then that's a big waste. So that's huge to those of you who listen to this and think capacitors don't fail, there's no reason to test them on a regular service call if you're in a place where air conditioners don't run very much or if it's not as hot or you don't get As many power surges, then that is probably true.

So if you live in a state where you don't have a very long cooling season, you don't get a lot of transients lightning strikes power surges, that sort of thing and it's not as hot you're, probably very rarely going to have capacitor failure when you're in a Place where it's hotter outside so they run longer and the ambient temperature is higher and you get more transients and voltage strikes and power surges. Then, in those areas, capacitors fail all the time and capacitors, don't always just fail, they're, not always just good or bad. I've heard a lot of guys say this well capacitors, either good or bad. That is false.

A capacitor can definitely read a lower reading, and that happens because sections of the winding inside sections of that metallic coded it's actually like a plastic inside that's metallic coating on both sides. Sections of it can burn out, and so you lose some of the capacitance and running a system on a weak. Capacitor is not good for it. It's not good for a compressor to run with a weak capacitor.

That compressor is going to run hotter and eventually it's gon na be more likely to fail. So there you go. That's my practical look at how to test capacitors, and hopefully you found that helpful. We will talk to you next time on the HVAC school podcast.