Compressor Won’t Run Diagnosis
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- Опубликовано: 20 ноя 2019
- Bryan explains and demonstrates how to reach an accurate diagnosis when a compressor won’t run. In this case, the compressor doesn't trip a breaker or blow a fuse; it simply doesn't run at all.
The compressor is the "heart" of the HVAC or refrigeration system; it increases the pressure of the refrigerant and moves it through the circuit. It is often a loud component, so you might be able to determine that a compressor isn't running if you can't hear it. However, checking the amperage is the only surefire way to tell if a compressor isn't running; you can put the amp clamp around the common wire or one of the compressor leads.
Compressors may come in the three-phase and single-phase varieties; the former is more common in commercial applications, and the latter is common in residential and light commercial HVAC. You will need to take different diagnostic steps depending on if the unit uses a single-phase or three-phase compressor.
Compressor Won’t Run Diagnosis? Before starting your diagnosis, allow the compressor protector (or thermal overload) to reset. In three-phase compressors, the thermal overload breaks all three legs of power. In single-phase compressors, the thermal overload will break common or directly behind common. In any case, a solid visual inspection will be your first main step; make sure that there is no evident damage to the compressor or capacitor(s).
Then, you check the voltage. Start checking voltage at the contactor (in single-phase applications), and then measure it at the compressor terminals. If the voltage is good in both cases, then you can check the amp draw. If you detect excessive amperage, then the next step would be to check the winding resistances and resistances to ground. An ohmmeter can tell you if a compressor is tripping on thermal overload. When measuring resistance, make sure you have an accurate meter and that your leads make adequate contact with the terminals; otherwise, you might get an inaccurate measurement. If you find a resistance well outside the manufacturer's parameters, you have enough evidence to condemn and replace the compressor. Before you condemn a compressor, you need to know exactly which measurement was out of the specifications.
If the resistances are in range, re-check the wiring and the start gear. If all of the readings are normal, make sure that you have the right compressor for the application; you can verify it by checking the manufacturer data. Ensure that the pressures are equalized, and then you can restart the system. If the compressor doesn't start, replace it.
In some cases, a scroll compressor will measure improper resistance from the terminals to ground. Copeland states that anything less than 0.5 megohms is a failure, but anything between 0.5 and 20 megohms is NOT a failure; those values are more likely to indicate oil contamination or mild winding breakdown than a total failure.
If you read no amps, you need to allow the overload to reset. If it doesn't reset, then you need to replace the compressor. In the case of a single-phase compressor, an open thermal overload will result in no continuity between common and either run or start. However, you WILL read an ohm value between run and start, so be aware of which terminals you're measuring.
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Very nice 👍🏼
What an excellent free resource for HVAC people, very good information and presentation
I've been in HVAC for 15 years now and once I started listening to Brian Orr and Jim Bergman I realized how much I still had to learn. Thanks SO much for all your free education. I'm a better tech than ever now.
I'm thinking the same thing. These guys got my wheels turning again.
WE all appreciate you ,and Mike Niper for the information.
I had watched a couple dozen videos and read articles on a dozen or more sites about diagnosing my compressor not running before I found this. They all showed the same things (capacitor and contactor). I've done a LOT of maintenance and repair work over the years (especially industrial motors including 3 phase), but this situation over the holiday weekend was exasperating. Non-contact current detector showed current at the input and output of the contactor when engaged (though I cleaned all the contacts just to be safe), and all along the leads to the capacitor and motor. Seemed everything was getting power. Measuring resistance across the three compressor leads showed it just a little out of spec (didn't quite add up but only off by half an ohm or so), and nothing shorted to ground. I was baffled. All local distributors were closed for the holiday but Amazon same day delivery got me a new capacitor since I couldn't think of anything else to try and I wasn't going to pay holiday rates for an HVAC tech to bail me out when we weren't going to die, just be uncomfortable, by waiting a day. But of course the capacitor didn't make any difference at all. :(
Then I got more determined and hit RUclips again and eventually found this video. At the 2:27 mark (which by remarkable coincidence is my birthday, so that felt like a good sign) you show the Emerson troubleshooting flowchart. I immediately recognized the gap in my process, as I had only checked for any current running through the wires, not proper voltage. Some more confusion by the readings I got from my multimeter (zero or a fraction of a volt depending on where I measured, despite the current detector showing current in everything. Called my brother, an electronics engineer so I could explain what I had checked and in what order and all that to see if he could spot another gap or explain the voltage readings. He said to turn off the thermostat and measure again. Still zero volts, but then I only got a reading from the current detector on one side of the input! The circuit was open somewhere else upstream, and with the contactor closed I was reading that same 120v across the whole system!
Turns out the type of disconnect on this house (a recent purchase) has a fuse inside. I had never encountered that type before, and as the power was on with the breaker on and off with the breaker off it never occurred to me to look in there. I'm annoyed that none of those dozens of other sources said anything about verifying voltage or tipped me to the existence of a fuse in some types of disconnect boxes, but I'm grateful that you showed that flowchart to point me in the right direction!
You guys are under rated and under viewed! Ty for making good content.
I was taught to add the sum of the ohm value of common to run and the sum of the ohm value of common to start and the sum the two should equal the sum of the ohm value of start and run which means that the windings are good. If they don’t add up the windings are bad
Thanks for really informative video. Great to hear an expert freely share his knowledge.
Thanks for the encouraging comment
Great video, very comprehensive
Excellent video, super-useful. Thanks!
One common problem can be no problem at all. With older Copeland (scroll compressors), they have internal heaters that keep refrigerant from saturating into the oil of the compressor. Many times, people will turn off the breaker for the A.C. during the winter. The first hot day in the summer, they will turn the breaker on, and then immediately turn on the A.C. only to have the compressor draw high amps which then thermally kicks off the compressor.
Doing this during the winter months is O.K. The proper way to start the system is to turn the breaker on about 5-7 days before you actually start using the A.C. This will warm the compressor to start normally.
What a great video 👏👍🏻
very good class. thumbs up good job
Legit, and well explained!
GREAT VIDEO......
O.k. ur the guy I've been getting/ignoring his emails for years. OK I've changed my mind. I will pay more attention ton. Ur good .
Very good information bro
14:25 BE VERY careful (especially on older compressors) when removing wiring connectors from compressor terminal pins, with pressure behind them, if they're weak you could get injured or at least a nasty surprise if a weak pin pulls out with the connector.
How do you avoid this?
@@jericosha2842 recover the refrigerant charge first
Nice job and video
On a 3 phase annex air I'm currently working on. It's danfoss scroll with a vfd and it's getting winding break down causing an a45 earth groundfault on the drive not allowing the compressor to run. So I'm replacing in next week. So I'm guessing I'm getting back feed noise on my load side on my drive. Reason for winding break down is they are running in low ambient conditions without low ambient kit causing oil migration.
Do the Amps vary much or do they saty close to the FLA on the name plate
Is it possible at site to get stator coil for replace scroll double stage -20 c range sealed compressor.
Ericsson Copeland.
Just came from a job where compressor would not turn over. initially though bad cap because fan was running. 60mfd cap reading 58. checked voltage line side of contractor and load side and it was good. Pulled compressor plug off(power was off). Turned power on and voltage to plug was good. compressor was a little warm. Ohm’d out legs and they seemed within range. Thermal overload was not open. LG compressor so don’t know the resistance value but S-R equaled R-C, S-C. Ohm’s to ground reading OL. Clamped my meter on Common and Run (separately) and did inrush amps and reading OL. Clamped Start and reading like 5.6A inrush. Just for testing purposes I added a hard start and still nothing. You could hear humming but would not turn over. I condemned compressor.
What i don’t understand is why I got OL when I checked inrush on common then check on run and got OL.
Image result for single phase compressor winding resistance
If the compressor passes the above tests, take a multimeter set to R x 10,000 (10K) and check each winding to ground. You should have an infinite OL resistance to ground. A shorted motor will read 0 ohms across windings, to ground, or both. If it is both, you will need to replace the compressor.
I would like to know why I had 4 compressors (new) that were electrically open between C and S. Fresh install, didn't even install freon yet, no start
I have a question about a single phase scroll
Working on a mini refrigerator and the compressor only kicks on after you tap the side of the refrigerator. What causes that Ive tested for continuity.
BE VERY CAREFUL WHEN REMOVING ANY TERMINALS AT THE COMPRESSOR WITH A FULL REFRIGERANT CHARGE, IF YOU HAVE A WEAK PIN IT COULD BLOW THE CHARGE IN YOUR FACE.
Very true
Good to know.
That's crazy lol but if you need to do it, you can't take regret it can you?
Where did you get your clips?
Dear Bryan: question: a Copeland compressor was left on new unit, unused for years, pipes were open not connected...we tried to check compression but not working...it reads 0.4 ohm on each terminal. please advise your thoughts..it is locked...shall I check all diagnosis you laid out or it is rusted inside and useless? thanks I appreciate it. Robert
Can a thermal overload fail open ?
I had an odd repair I had to do at the coffee roaster I work at that I think you'd find interesting. We have these old german grinders that run on 208/230v single phase power. The grinders are ancient, some with stickers that read "made in western germany" so pre 89, somewhere around 84 we estimate. Several motors needed to be replaced due to excessive wear, so we ordered them from the manufacture which still makes them after all these years.
The schematic is your basic single phase motor with a current start relay, a start capacitor with bleed down resistor built in, a contactor, a thermal breaker and a thermal protector built inside the motor.
Every time we tried running the new motors when installed in the housing with the original wiring per the schematic sent by the company ( with all new components ) the thermal protector would trip and the motors would make a terrible buzzing noise. I ohmed out the switch, the contactor, the current start relay, tried multiple caps, changed all the wiring incase it had a break etc. I was going crazy trying to figure out the issue so I watched a gazillion hvac videos and decided to get a cheapo $14 amp clamp to see if I could figure out what was going on.
I hooked up one of the old motors which had worn bearings, but was otherwise functional and ran it with all the components I had confirmed 100% to be functional and it fired up no problem. I put the amp clamp on the common and found around 20 amps LRA which would drop down to 5.5a running.
Hooking up the new motor I found the start windings were constantly engaged. I learned how a current start relay works and as an experiment I switched on the new motor then using plyers with a rubber handle and wearing gloves I pulled the wire which feeds the start capacitor / winding to emulate a properly functioning current start relay and found 30-35 amps LRA and 7.5 amps running! This is a nearly 31% difference between running amperages!
I decided to pull up the data sheet of the current start relay and found it has a pickup of 16.2 amps and a drop out of 13.2 amps. If the circuit was to function properly I estimated a current start relay would have to have a 17.2 amp drop out. I tried to find some which are rated for this but they are expensive and I didn't want to gamble on my theory.
Digging even FURTHER I found out about the SUPCO APR5, an adjustable potential start relay. They are about $25 bucks and I figured it was worth the gamble. I opened the manufactures schematic in photoshop ( which I had already colorized and cleaned up previously ) and changed it to accept the potential start relay instead of the current start relay. I really liked the fact that the potential start relay is normally closed, preventing arcing of the points on startup, compared to a current start relay which is normally open.
After testing the circuit on the old motor to make sure I didn't smoke the new one, I hooked it up and it worked! I assembled it and ground a few pounds of coffee to confirm operation and now its back in action and running like new.
Hope you find this interesting, I know I did and I'm thankful for you and others who posted videos explaining all this theory. Now that I have such a good grasp of single phase motors, I wanna tackle 3 phase or at least get my hands on some more single phase motors to fiddle with! At least I know if I ever wanna transition into HVAC I would have quite a good foundation to work off of.
also, passive resistance of windings were greater on the new motors in contrast to the old motors, indicating it was over wound ( I assume ).
@@aaronfidelisrecine Way to go. The way you described it. You're growing a lot Mr. Grumman.
how can I get a copy of that chart?
The headache of replacing a compressor is always a good incentive to check & double check everything in the electrical chain leading to the compressor.
What about a compressor that IS running but pressures are equalized and pulling 11 amps of the 25RLA? New construction Carrier straight cool no pressure sensors found leak in evap coil. Standing pressures on arrival were 102 psi on a 95° day. Mechanically failed?
Just double-checking here, didn't you read 17.43 Mohms to ground with the fieldpiece and 5.8 Mohms with the megger and then make the statement that the reading with the megger was higher than the reading with the fieldpiece meter? Am I missing something here?
Hello there , where can i get that copy of the chart ? Possible to email me ?
I want to get trobuleshooting chart
Great video, but the text "NOT LOOSING VOLTAGE" at 17:52 was distracting...
Will it walk?
Don't forget your filter drier! 🤓!
You cover the most of the vapour compression system.
Screw compressor and centrifugal ,give some details.
My opinion to many rely on Megaohmeters!
For professionals only? Well excuuuuuuse meeeeee!
How come I am not a Hvac pro but I can still understand your lesion.
Why those unbalanced voltages on three phase? 🤔
It’s just a diagram I use for multiple purposes
There is so much disinformation out there and guys who just luv to hear and see themselves on youtube
Can she run? Common start run
note...... Your illustration showes an external OVLD and not an internal OVLD. but you are talking about testing a compressor with an internal OVLD just to nit-pick LOL
For PROFESSIONAL ONLY.? 😲 Wow.
Downvoting a free educational podcast. Somebody must of had their feelings hurt about the disclaimer...lol.
Probably a homeowner lol
I am confused when you say that a compressor isn’t necessarily completely fail, but may have a partially failed reading. The way I see it is it’s either failed or it’s completely 100% fine. I am not about to tell the customer that this partially failed compressor is fine, that would just be crazy, because now you own it, your telling them it’s good, and then what, you leave and tomorrow it blows up, ya ok, and who do you think is going to be blamed for that! No, the shit is either 100% ok, or it’s bad, period. I am not about to get in a situation where I tell the customer that it’s ok and then next day bam it’s a piece of shit, then YOU will be liable for that. Don’t think so, try it, and when you end up in court guess what, YOU will be putting in a new system for them for free. If you don’t think so, then go ahead and try it, and let me know how that works out for you.
So just to add to this, when it comes to residential ac, when there is a problem, check simplist things first, capacitor, contactor, condition of the wires, are any frayed shorted or broken, if none of these things is wrong, then the unit is bad period, and needs to be replaced, no point in replacing the compressor, especially if it’s shorted to ground, more than likely it burned up, and all that crap is going to be through the whole system, trying to flush everything out is nothing but a waste of time, and you will never get everything out, especially in the Acoil.
Compressor not running?? Time for a new controller, some probes.. and... a, yes ..some topup gas.
Professionals should not need to watch a RUclips video. Duh
Kindly provide this chart @hvacschool