I have a re-manufactured hermetically sealed 3 phase A/C compressor/motor with rather large current imbalance. The vendor insists that it was tested and in spec. My utility uses the grounded center tap of one winding to provide 240 volt 3 phase and also 120/240 single phase. My voltages are Vab=242. Vac=244 and Vbc =239 and voltages to ground are Va=212, Vb=120 and Vc=119. All of these voltages are within the utility's specs. My compressor motor currents are: Ia=11.5, Ib=8 and Ic=11. All these readings were taken when the area electric load should be low because of relatively low A/C demands. The voltage imbalance calculates to be 1.1% while the compressor current imbalance was 14.3%. The other day I had slightly larger voltage imbalance (1.7%) and greater current imbalance (22%).
I think this is very inefficient and maybe even damaging to the compressor. Fortunately, the load is only about 2/3 of the motor rating. I will try checking the windings using a single phase 120 volt source and compare amps. What do you all think? If you would like to reply to me directly, my email address can be deciphered from: ralft(at)hal-pc(dot)org.

 

I checked the windings using a single phase 12 volt source connected to each winding and all were 2.1 a. This seems to confirm that the problem is with the utility BUT what could that be???? If you would like to reply to me directly, my email address can be deciphered from:

ralft(at)hal-pc(dot)org.

 

Ralf...

1) While I disagree with the Ampere-Unbalance-Factor you calculated (I got about 21% with the Ampere values listed) I do agree with your conclusions.

2) Hermetically-sealed motors are often wired for dual-voltages. Perhaps, one of the phase-groups could have a reversed coil.

3) Presumably the 12-Vac tests were carried out at the starter by measuring the voltage between each of the motor lead-pairs. Did you happen to test from phase-to-ground (earth)?

4) When you applied the test voltage between, for example phases A & B, did you measure the resultant voltages between B & C, and C & A?

5) While it is true that the mid-phase grounded supply (see Control List Archives on related threads) should not materially affect the 3 phase-phase voltages, were the six voltage measurements made with a "true-RMS" voltmeter?

6) Did you request "Shop Test Data" from the re-manufacturer!

7) Finally, did you observe any unusual sounds emanating from the operating unit at any time during runup?

Phil Corso (cepsicon [1t] ao; [dot] com)

 

Ralf... further to my earlier reply:

8) With motor in operation, measure current in the Ground'ING' conductor that connects the supply transformer’s mid-point connection to ground (earth).

Phil Corso (cepsicon [at] aol [dot] com)

 

If the numbers you quoted are the transformer secondary currents, then I would expect an imbalance with a higher current on the leg with a centre tap. It has to supply the single phase loads from one winding obviously.

If the numbers are for the compressor alone, then the problem is likely with the motor. The motor draws the current and the transformer supplies it. There might be one bad winding inside the motor that is causing it to draw less current on that phase. If that is the case, then you may need to replace the motor or have it re-wound. For a small motor it is often cheaper to just replace it.

 

Thanks Phil and I need to explain that the transformer is utility owned. That being said, I will look at the transformer pole and see if I can hook onto the grounding conductor. But, most likely, there will be multiple grounds on the transformer. I am gathering up some information before replying to your earlier post.

 

Thanks "M",

The currents are for the compressor itself. It is not practical to change out the motor alone. The motor is enclosed by the same housing as the compressor and the refrigerant cools the winding.
I agree that the motor may be suspect since I tried shifting the motor T leads changing the motor leads at the contactor. By doing this I got the current imbalance down to less than 10% and this was with the higher voltage imbalance of about 1.7%. This tells me that the motor may have some inherent imbalance after all. Is 10% imbalance worth "Going after"?

 

Ralf...

9) Unless you exchanged the "T" leads an even number of times the motor could now be oerating in reverse.

10) Have you noticed a change in cooling ability?

11) Can you provide me with the amperage values that resulted in an AUF of 10%.

12) What are the Namplate Hp, Amps, Volts, and Rpm!

13) Re your "Going after" the 10% unbalance. Using typical parameters for a 3ph, 240V, 10Hp (USA) motor, a 1.7% VUF could increase effective rotor current by 15%.

14) I haven't yet determined the decrease in efficiency, but rotor-loss is increased by 22.5%. Besides increasing energy costs, I'm sure you don't want to add heat to the refrigerant!

Phil

 

As Phil Corso has said, if you're not careful when you are switching the leads around you can end up with the motor running backwards. The way you reverse a 3 phase motor is to switch any 2 leads.

There are 2 electrical factors that matter here: voltage, and current. The transformer supplies a voltage, and the motor draws a current. If the voltages coming from the transformer looks fairly similar, then that throws suspicion on the motor. If the transformer couldn't deliver the current on a particular phase, then I would expect a large voltage differential on that phase (the load would pull the voltage down). I would also expect to see smoke starting to come out of the transformer before too long.

It is pretty hard to test a motor properly without special equipment, except by hooking it up and running it. I would take any compressor "certification" with a grain of salt unless I knew what sort of tests they ran. Their test may have consisted of hooking it up and seeing if the compressor ran. If they are a compressor re-builder rather than a motor re-builder they may assume the motor is fine as long as it runs. On the other hand, I certainly can't prove from where I am sitting that there is anything wrong with the motor.

What would concern me about a large current imbalance is that it is a symptom of a problem waiting to happen. I would not be surprised if something failed completely after running under load for a while. That failure might be just the thing refusing to start, or it might fail in a more exciting manner.

I am assuming in all this that there are just 3 or 4 wires coming out of the motor and into the motor terminal box. If you have much more than that then you might have a motor where the individual coil leads are brought out so they can be hooked up in series or parallel, or with different tap arrangements so the motor can operate on different voltages. If that is the case then I would suspect that the leads for one phase may be hooked up in the wrong combination or with the wrong tap. Have a look at the motor name plate and see if it is rated for multiple voltages.

I see that Phil Corso has already mentioned this point. If you haven't already checked this and double checked it, then check it now before you do anything else.

 

>1) While I disagree with the Ampere-Unbalance-Factor you calculated (I got about 21% with the Ampere values listed) I do agree with your conclusions.<

I agree and apologize. I simply entered the wrong volts for that set of amp readings

>2) Hermetically-sealed motors are often wired for dual-voltages. Perhaps, one of the phase-groups could have a reversed coil.<

This one is wound for 208/230 v. only

>3) Presumably the 12-Vac tests were carried out at the starter by measuring the voltage between each of the motor lead-pairs. Did you happen to test from phase-to-ground (earth)?<

No but the motor is not grounded so there would be no current. I have verified that it is not grounded.

>4) When you applied the test voltage between, for example phases A & B, did you measure the resultant voltages between B & C, and C & A?<

I did this yesterday and it is difficult because of the fluctuations in the utility voltage. I will tabulate the results separately nut they seem to show that the voltage across the 2 windings in series is not split 50/50. It is more like 49%/51% on 2 of the 3 possibilities.

>5) While it is true that the mid-phase grounded supply (see Control List Archives on related threads) should not materially affect the 3 phase-phase voltages, were the six voltage measurements made with a "true-RMS" voltmeter?<

All readings were done with a Trpplett true RMS meter.

>6) Did you request "Shop Test Data" from the re-manufacturer!<

I did yesterday and test data is available. I am expecting a phone call today.

>7) Finally, did you observe any unusual sounds emanating from the operating unit at any time during runup?<

No sounds and the motor seems very smooth.

 

>9) Unless you exchanged the "T" leads an even number of times the motor could now be oerating in reverse.<

The compressor is bi-directional so it doesn't matter. That being said, all I did was to shift the left 2 T leads to the right one space and brought the right T lead to the left hand space. I cannot imagine why reversing the rotation could help. Do we all agree on that?

>10) Have you noticed a change in cooling ability?<

Probably better than with the old compressor in part because it seems to run cooler at slightly lower amps

>11) Can you provide me with the amperage values that resulted in an AUF of 10%.<

Sorry but no!! (Dummy me)

>12) What are the Nameplate Hp, Amps, >Volts, and Rpm!<

The compressor N.Pl. info is incomplete and all it says is: M# 06DW3166FC1200, S# CH1015-107, Volts 208/230, 3 Ph., 60 cy., L.R. Amps 100 The incomplete N.Pl. may be because I bought it on an emergency basis and the motor had to be changed out from 480 v. just before I got it. The old compressor N.Pl. It replaced a re-man. compressor with the following N.Pl. info: M# D6DM3166FC1200, S# 00E40745, 3 Ph., 5 HP, 60 cy., Volts 208/230, F.L. Amps 21.5/18.8, L.R. Amps 95.0/86.0

 

>...I would take any compressor "certification" with a grain of salt unless I knew what sort of tests they ran.<

I doubt that their testing is very sophisticated but I am expecting to get that data. The business owner has been doing this all his life and he has a good reputation with the Carrier service people.

>...I am assuming in all this that there are just 3 or 4 wires coming out of the motor and into the motor terminal box. If you have much more than that then you might have a motor where the individual coil leads are brought out so they can be hooked up in series or parallel, or with different tap arrangements so the motor can operate on different voltages....<

There are only 3 T leads brought out to a terminal block just as with the previous compressor.

 

>8) With motor in operation, measure current in the Ground'ING' conductor that connects the supply transformer's mid-point connection to ground (earth).<

I looked at the utility transformer bank and there is no ground at the transformer itself that I can see. The center tap of the one unit is connected to a messenger which carries the 3 hot conductors to the various residences including mine. All of them presumably have driven grounds just as I do.

 

Ralf...

15) That messenger (is it bare?) is the ground'ED' conductor. It is carrying the unbalanced neutral-currents associated with 120V loads.

16) It probably is connected to the neutral-bar in your panel and then to the driven ground-rod.

17) My point was to determine if the A/C unit ground'ING' conductor is carrying a current when the compressor was operating! If so, it confirms that one of the windings is inadvertently grounded.

18) Had you done a continuity check from the 'T' leads-to-ground?.

19) BTW, an inadvertent reversal of a coil in one phase-group will result in two of the 'T' leads showing higher currents than the third!

Awaiting answers to other questions.

Regards, Phil (cepsicon [ at ] aol [dot ] com)

 

Thanks Phil!!!

>15) That messenger (is it bare?) is the ground'ED' conductor. It is carrying the unbalanced neutral-currents associated with 120V loads.<

The messenger is bare and the other 3 conductors are insulated.

>16) It probably is connected to the neutral-bar in your panel and then to the driven ground-rod.<

It is although it is connected to the ground before it enters my panel.

>17) My point was to determine if the A/C unit ground'ING' conductor is carrying a current when the compressor was operating! If so, it confirms that one of the windings is inadvertently grounded.<

Will check.

>18) Had you done a continuity check from the 'T' leads-to-ground?<
Yes; several megohms

>19) BTW, an inadvertent reversal of a coil in one phase-group will result in two of the 'T' leads showing higher currents than the third!<

I think "M" mentioned that. How to verify?

>Awaiting answers to other questions.<

I think I have answered all except to get an update on all measurements. If not, please refresh me. I will do testing 1st thing tomorrow AM and also late today. These is some significant variation.

 

Ralf... if further testing indicates amperes are less unbalanced, then problem could have come from improper termination of 'T' Leads!

Search Control.com Archives for the following thread:

http://control.com/thread/938749565

It covered Faulty Motor Terminations!

Phil

 

Ralf... further to my 28-May (22:13) reply:

20) I interpreted your 27-May (13:01) comment to mean the present motor was originally operated at 480V operation, but was reworked for 208/230V?

21) Only a reciprocating-piston type compressor can be operated in either operation! The fact you didn't hear any unusual sounds confirms it is probably a recip!

22) Can you confirm the single-phase 12V test-voltage mentioned in your 24-may (18:35) was 12V, and not 120V?

Regards,
Phil

 

Here are 2 sets of readings:
Ambient temp. 94 F: Va-b=241.6, Va-c=245.6, Vb-c=235.3, Va-Gnd=213.2, Vb-Gnd=118.2, Vc-Gnd=117.3, Ia=11.5, Ib=8.8, Ic=10.6.

For these readings I calculated the voltage unbalance to be 2.3% and the current unbalance at 14.6%. I think these voltages are outside the utility's spec and I will complain again next week.

Ambient temp. 76 F: Va-b=242.2, Va-c=244.1, Vb-c=240.5, Va-Gnd=211.6, Vb-Gnd=120.3, Vc-Gnd=120.1, Ia=10.5, Ib=9.5, Ic=9.6.

For these readings I calculated the voltage unbalance to be 0.73% and the current unbalance at 6.4%.

Does anyone believe that there is enough motor imbalance to fix. Please remember that I achieved the better readings by swapping the T leads at the contactor

 

Thanks PHIL,

>20) I interpreted your 27-May (13:01) comment to mean the present motor was originally operated at 480V operation, but was reworked for 208/230V?<

I doubt that it was operated except for shop testing. When I called, he said that he had the compressor but would have to swap the motor which he did for me on an overnight basis.

>21) Only a reciprocating-piston type compressor can be operated in either operation! The fact you didn't hear any unusual sounds confirms it is probably a recip!<\

It is a recip. I think 4 cylinder with 2 heads. But please understand that I rotated the leads so there should have been no change in rotation. I asked the question whether anyone could see any possible benefit by reversing it.

>22) Can you confirm the single-phase 12V test-voltage mentioned in your 24-may (18:35) was 12V, and not 120V?<

It was 12 volts (nominal).

 

Phil Corso spent lots of time communicating back and forth to help bring this problem to a close.

As it turned out, it appears that there was some sort of motor winding defect which I essentially took advantage of to compensate for the utility’s 3 phase voltage imbalance. Before starting this thread on 5/24, the phase to phase voltages were Vab=241.9, Vac=245, Vbc=237.6 and these readings are better than what I now expect to see in the late afternoon when the neighborhood’s SINGLE PHASE A/C load is maximum. (Each week the temperature is significantly hotter in the Houston TX area.) This voltage imbalance is calculated as 1.6%. With those readings, the currents were Ia=12.5, Ib=8.5, Ic=11.5 and this imbalance calculates to be 21.5%. I made various measurements to try to pinpoint a motor winding problem without much success. I applied a single phase 12 vac source to each T lead combination and measured the resultant current; all were the same. Then, I repeated the test and measured the voltage drop across each the 2 windings that were in series and found that they were all within a percent of being balanced. This slight imbalance motivated me to try rotating the T leads on the load side of the contactor one phase at a time. In other words, the conductor on the right was removed and put in place of the conductor on the left. The 2 conductors on the left were moved one space to the right. This significantly reduced the current imbalance form 15% to 9.5%!

Phil suggested some other checks such as voltage drops across the power panel circuit breaker, power lines and the contactor. I did this by using an extender wire from the power panel to the compressor location. All were very low, 0.3 vac and almost identical.

Phil took my recent WORST CASE current imbalance of 14.6% and calculated efficiency loss and his findings were a pleasant surprise to me. I quote: “Line-current, rotor-current, and losses have increased. But interestingly, because applied voltage is higher than the motor's rating, efficiency has only decreased by 0.5%. Had the problem been under-voltage, the efficiency decrease would have been about 2%!”. My motor is nameplated for 208/230 v. but my voltage runs just over 240 v.

We both find the HVAC industry practice of still using hermetically sealed compressors surprising. Why force the motor’s heat into the refrigerant and then use more fan energy to cool it? Likewise, why is the air handling fan motor always put in the conditioned air space? I will post these questions to an HVAC users group/forum.