How to diagnose a locked rotor compressor 

What's the meaning FLA

Great info.

Thanks a lot. I didn’t even know what the LRA was. So if I see that it’s possible to put a hard start kit on give it some more time before they replace? I’m now very concerned about what you said with the system being contaminated from burnt motor. I’ve replaced a few compressors(I just went from install to tech in March). I did replace the refrigerant and filter dryer when doing that, but could I’ve messed up the system for those customers if the motor was burnt?

@@jacobbell1934 Hard starts are a short term fix that could be useful in some cases. If you get a burned smell from the refrigerant in the system, I would recommend installing both a liquid line and suction line filter with replacement of the driers after a few days operation. GFM

1290457 In this application, its used as an outdoor thermostat to shut off the heat pump at lower outdoor temperatures. GFM

Ok, I was kind of leading that way. Thanks for the reply.

Agreed.This had happen to me once.But very rare.For 3-Ph compressors,we can release the rotor lock by reverse rotation.

Anand Gbk how can on reverse the rotor rotation?

gabriel peter by swaping any to phases going to the compressor.

I might need to learn what the hell you mean.

By switching the start and run leads going to the compressor you can reverse the direction of the compressor thus freeing the locked rotor. Then switch the leads back to normal and try it again. Only for three phase motors!!

Harbor freight CM1000a clamp meter measures in rush for 100 bucks. Not the kind of meter you can brag about to your friends but still better than nothing.

Thanks for the thoughts. GFM

you must of been new when you wrote this comment. dirty indoor coils not going to lock your rotor up, crank case heater failure could cause valve issues but i highly doubt rotor locking, system overcharge not going to lock your rotor and restricted metering device is not locking the rotor, usualy the causes for your rotor locking is an electrical issue whether bad capacitor or potential relay, low voltage, phase loss or simply motor seizing

Hi boss thanx for information

lmfao me too!!

It does but it is for a very short time and usually cannot be measured even with most peak hold meters. GFM

Thanks for the support. GFM

Yes, you have a dead compressor. GFM

The spark is from the contactor contacts opening. The start kit has no contacts inside. The tape is factory from Trane. The A419 locks out the A/C when ambient is below 60F. GFM

@@grayfurnaceman OK.. Thanks.

The pressures should correspond to the ambient temperature for a saturated vessel. Hope this helps. GFM

The compressor was reading high suction pressure and low head pressure. The compressor sounded like it wanted to start, but then it would just turn back off. I checked the capacitor it was good, I checked the compressor windings also and they were good. Do you think I should install a Hard Start?

SombraLocs if a hard start does not work take a piece of wood and put it on the compressor and hit it with a hammer, Might sound stupid but i have unlocked a bunch of them that way. Then the hard start will give it enough boost to start without locking up.

+Jerry Mullins thank you! It's 1am and this compressor has been stuck all day on the hottest day in history in San Diego. Couldn't sleep it's so hot tonight so I was hunting for answers. Good ol crack in the ass on that compressor with my rubber Mallet and it started right up. It's been locking up on and off for a couple years now. Usually it will start up if I shut it down for a day or so. I skipped all the trouble shooting and went straight for the slap. I did pay an AC tech to come check it out but by the time he got here it started fine. He topped off the gas, checked the capacity and couldn't find anything wrong. I guess this thing is on the way out. Glad it's cooling my room down tonight.

+grayfurnaceman Pressures on a "Locked Rotor" are static... 150 on both gauges... Are you paying attn?

It has been my experience that if a compressor stops running, it will stop again, regardless of capacitor replacement. You may get some extended time out of it, but the compressor is failing. GFM

It should not cause any damage, in fact it may reduce the head pressure a little. GFM

@@grayfurnaceman that is a good thing

If you want to play with your megger, go for it. The point of this video is to determine if the compressor has a locked rotor. If the windings are good, and the rotor is locked, it still has failed. GFM

@@grayfurnaceman LOL if you know that already why bother with beating around the Bush then? Just tell the customer the news and go from there. My reasoning for megging out the windings is to have a root possible cause for the customer information. I don’t function like many HVACR folks. I like to have all the information before I give someone bad news. Just saying.

@@richardmiller3808 Not a bad way to be. GFM

***** Thanks for the good info. GFM

If the breaker is not kicking off, there is no problem with the breaker. You could have a compressor failing. GFM

You are correct. Hard starts are not supplied from the manufacturer, except in certain circumstances, so they are not needed. GFM

If the capacitor is open, yes. If the capacitor is shorted, the amp draw will be higher. GFM

Are you sure the compressor was starting and changing the pressures? I am thinking you had a locked rotor. Also, the amp draw of run should be higher than you read. GFM

First, what was the original problem? Does the compressor not run? Does it run and not pump? GFM

The HPS (if it has one) is a low voltage control. The contactor should be deenergized if it is open. GFM

The circuit breaker is a slow opening circuit protection because it is normal for the compressor to draw 3 times RLA on startup. The internal overload is mounted in the windings of the motor, so it senses the overload (on the compressor only) very quickly. Remember the breaker is sized for the total ampacity of the appliance +25%. The overload is sized only for the compressor. GFM

Yes. Sometimes the overload opens, sometimes a fuse or breaker kicks off. GFM

Are you sure it is not going off on low pressure? That low side is pretty low. If the pressure switch opens, the contactor will open and close rapidly if there is no short cycle control and the compressor will probably not start due to unequal pressures. The amp draw is close enough to LRA to be a locked rotor and not shorted windings. Hope this helps. GFM

Do you mean the contactor is short cycling or the compressor overload? GFM

Yes

Thanks for the thoughts. GFM

Depends on the manufacturer. C is often not disconnected by the contactor. There is no right or wrong way. GFM

Why? The replacement compressor is 1/2 the size. GFM

blue moon Probably, there is a defect in the compressor windings. GFM

He didn't get shocked because he didn't touch the metal contact 🤔

I would certainly agree that the use of an aftermarket start kit has limited value. It is extending the life of a unit close to replacement. As for the loss of efficiency, I see very little change from the equipment efficiency when new. Usually, the failure to start is from bearing wear that has some, but very little relation to system efficiency. GFM

@@grayfurnaceman the efficiency i speak of would be related to secondary issues that may have caused bearings wearing out faster, i.e. refrigerant charge, dirty/contaminated refrigerant, high head pressure, etc. what i mean by lack of efficiency is more of the unit as a whole. if the compressor bearings wore out because it was pretty old then yeah not generally gonna see efficiency issues, but if the unit is maybe less than 12 years old in an environment where it’s only used for maybe 3 months of the year and if capacitor and electrical are ok, then chances are secondary issues caused the compressor to prematurely wear. For instance a unit that was grossly overcharged one year by some other company and for the next 4 years the tenant mentioned the unit was running non stop but never bothered telling the landlord. Head pressure was very high, compressor running non stop and high compression ratio was putting more stress on the compressor as well. the unit would give just enough cooling to be ok but it would run almost all day vs. before where it would only run for 3 to 5 hours. I do agree with your statement about efficiency, but I think it would be more of a case by case basis. more often than not i get caught up with failed compressors that aren’t that old, i haven’t come across many service calls for older ones so that’s also affecting my answer.

I do at least one a week. GFM

I can. The internal overload allows the compressor to start every time it cools down. This puts a load on the winding insulation. Eventually, it has to break down the insulation. I would have done what you did. GFM

Most of the time, when windings are not giving good ohm numbers, the compressor needs to be replaced. Yesterday i had a case like that where the compressor was giving bad winding lectures and the capacitor was bad too. I replaced the capacitor, switched on the A/A. One of the terminals overheated, maybe because locked compressor, gone loose and made contact with the piping, making short circuit with the grounded piping and leaking all the gas. The compressor didnt show continuity to ground or short circuit between terminals, just did not show known ohm values.

You have a dead short to ground. Baring a wire contacting ground, I am betting the compressor is shorted to ground. Unplug the compressor wires and check again. GFM

@@grayfurnaceman - That was the very first thing I did. Run, start and common pins on compressor to ground and all (OL). In fact I rechecked it yesterday. I have checked every single wire in the outdoor unit thoroughly.

@@jameswestii454 Remove and isolate all wires to the compressor. Then turn on the power. Ohmmeters do not always tell the tale. GFM

Breakers are delayed action devices. Especially the HACR ones for A/C units. Usually, the internal overload will cut off first. GFM

thanks again for the knowledge :)

If it is drawing 5 times the rated load, it is locked rotor. GFM

grayfurnaceman thank you.. how can I know ohming 3ph compressor and ground short???

3 phase will have the same ohm reading on all 3 windings. Check from winding to shell of compressor. Should be no continuity. GFM

Agreed GFM

Welcome GFM

+Tim Spriggs You could have a failed run cap. Also, the compressor is a very expensive part. Be sure you have checked everything. GFM

gotcha! I'm just studying HVACs on my own. it's fascinating. One of the best inventions too!

grayfurnaceman I hope the one drawing 119 amps the other day didn’t just have a bad run cap .. live and learn I guess I never knew that would case locked rotor amps they never mentioned that in school

+Andrew Riner Welcome GFM

Most compressors fail LRA. The 2 reasons for this type of failure are: Copper plating from air in the system (obviously poor install or service) and low charge that allows the oil to be pumped out of the compressor resulting in lubrication failure. I have worked on equipment that was installed from 1945 to present and I find that in most cases, the later equipment (of course with some low end exceptions) is better in quality. I would certainly not consider equipment made in the 1990s to be outdated. GFM

+grayfurnaceman I was speaking in terms of the install not so much the machine almost every system has an undersized return in volume and surface area what we knew about airflow was wrong back then when it comes to flex duct plus these new high efficiency systems are real sensitive and you dig a real deep hole real fast when you try to piece them back together using 410a 14/16 seer coils and compressor technology not arguing just saying soon we won't even be using capacitors and contactors it will be inverter drives and variable speed

Amperage will be locked rotor.because if the capacitor has failed, the compressor will not start. GFM

thanks GFM

There are 3 terminals to the compressor. One is common. One end of both the run and start winding is connected there. The other end of the run winding goes to R. The other winding is the start winding which has its end connected to S. Start is primarily there to start the compressor. The run winding has the majority of the electrical load as it powers the motor when running. Hope this makes sense to you. GFM

Actually it is quite clean. GFM

Does the unit have a low pressure switch? GFM

Go with the original on this one. GFM

If the capacitor is good, and power is available, the compressor is mechanically locked up. Most common failure of compressors. GFM

grayfurnaceman Thanks for your reply. I hit it with a hammer, but it didn't start. I tested it with my meter and I get continuity in all three pins. Is there any other why I can try to unlock it and make it work? I read about some people using a rubber hammer, but I'm not sure if you need to hit it when the unit's working or when everything's off. Thanks!

For 35 years I have tried using a hammer on a stuck compressor. Rubber, 16 oz, 8 pound. Never worked. The only possibility is to install a hard start kit. GFM

grayfurnaceman I just installed a new compressor and this is occurring. How could this be?

kiyonexus im facing same issue. Wondering if replacing booster pack will do the trick. Is there a way to test a booster pack?