Phase loss error on AC compressor

P

pipe_bender

Senior Member
Location
Boston
Occupation
Electrician
A HVAC tech is dealing with a 'phase loss' issue on a 208V compressor but when I go out to check all phases are present, its a older rooftop unit and it looks as if the disconnect was replaced recently but not by me.
There is a heatwave going on and the incoming utility voltage is about 198 per phase today, L-N 114 at the service and at the load terminals of the breaker (3-P 60) its a 208/120 building.
However up on the roof I am getting L1-L2 187, L2-L3 180 L3-L1 180 and 14- 60V Line to ground screw ( no ground wire in the conduit on the roof ). When the HVAC tech manually started a motor L2 - L3 start dropping fast, L1-L2 fluctuate but not much.
Any advise is appreciated
 
That doesn't sound good. When you checked voltage on the roof was the unit running under load or off? Either way you getting a lot of unacceptable voltage drop. Need to check all connections. And check voltage at the panel and roof when running. I know you said disconnect is new but I would check fuse clips if it is fused and knife blades etc.

Also seems like you EG is not a good connection although that may not be your issue. Maybe a hack splice under the roof when someone changed the unit???
 
Eddie702 said:
Also seems like you EG is not a good connection although that may not be your issue. Maybe a hack splice under the roof when someone changed the unit???
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Thank you yeah not much visible conduit, and no EGC pulled in to the disco. I asked for access to the top floor so I can look for a Jbox.
hillbilly1 said:
Sounds like a blown fuse somewhere due to the low incoming voltage. Everything else in the building working? Is this the only RTU?
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Yeah thanks I only checked one other disco on the roof measured better but still on the low side 189 - 190V.
It also had no EGC, they have not reported any other issues,
3 story office building probably built mid 1990's but has been renovated to have separate tenants around 2009.
 
Out in the boonies, I have seen 1 primary side utility fuse blow causing all of the running motors to single phase. The running motors acted like rotary convertors generating the missing leg. Standard voltmeters would not see the problem, generators would not come online but the drives got upset. We eventually went with a negative sequence sensing relay which opened the incoming service breaker.

For you technical people, this has been a summary description with many details omitted.
 
jim dungar said:
Out in the boonies, I have seen 1 primary side utility fuse blow causing all of the running motors to single phase. The running motors acted like rotary convertors generating the missing leg. Standard voltmeters would not see the problem, generators would not come online but the drives got upset. We eventually went with a negative sequence sensing relay which opened the incoming service breaker.

For you technical people, this has been a summary description with many details omitted.
Click to expand...
You clearly know your stuff, and not many people explain real-world issues that clearly.
I’ve been using a thermal cam lately that’s actually helped me spot weird electrical faults I would’ve otherwise missed.
Not trying to sell anything — just thought it could be genuinely useful in cases like the one you described.
If you're ever curious to try it out, I’d be happy to send one over — no strings. If you could share your email with me, I’d be happy to send over some honest thoughts.
 
pipe_bender said:
A HVAC tech is dealing with a 'phase loss' issue on a 208V compressor but when I go out to check all phases are present, its a older rooftop unit and it looks as if the disconnect was replaced recently but not by me.
There is a heatwave going on and the incoming utility voltage is about 198 per phase today, L-N 114 at the service and at the load terminals of the breaker (3-P 60) its a 208/120 building.
However up on the roof I am getting L1-L2 187, L2-L3 180 L3-L1 180 and 14- 60V Line to ground screw ( no ground wire in the conduit on the roof ). When the HVAC tech manually started a motor L2 - L3 start dropping fast, L1-L2 fluctuate but not much.
Any advise is appreciated
Click to expand...
Is this phase loss a fault code on a VFD or is it a more general purpose phase monitor that locks out controls if it doesn't like incoming power characteristics?

I could see your rather low voltage readings causing either to happen I guess.

Are the even lower readings at the roof taken with load or with no load? No load should be the same as at the beginning of the circuit, can't have voltage drop with no current.

If you are seeing other units with about same voltage readings I'd say they are possibly on borderline of having similar issues if they have any sort of phase monitor or VFD's This one either has worse VD on branch circuit or maybe even higher settings before fault code or lockout occurs.

VFD's don't exactly measure incoming voltage to detect phase loss. The first thing they have after incoming leads is a rectifier to convert to DC.

If there is high enough load on output to drain the DC bus to the point where ripples in the voltage occur, many drives are programmed to identify that as possible phase loss that is causing the ripple and that is what they may give for a fault code. Such drive would run with total loss of one phase as long as output demand is low enough to not draw down the DC bus voltage too far to cause such ripple in voltage.
 
Thanks everyone for the suggestions turns out it was indeed the jbox below it under the roof deck, the connections were very oxidized the original installer did not use noalox and they did a tape and brass split bolt splice that was probably not even rated for AL wire and there was a 2rd splice in a gutter in a electrical closet that was also extremely short. Due to the really short wires and extremely difficult to access ceiling jbox and having a spool of #6 cu in the van I just re-pulled the circuit. I even pulled in a 10 EGC. The AC is now back on and the Saturday cleaning crew is happy.
 
Sometimes with a RTU replacement the electrical feed ends up on the wrong side of the unit or a curb adapter makes the unit so high the wires will not reach so it forces a splice on or below the roof although repulling it is better unless it is a long run. Sounds like a job by an AC company.
 
Eddie702 said:
Sometimes with a RTU replacement the electrical feed ends up on the wrong side of the unit or a curb adapter makes the unit so high the wires will not reach so it forces a splice on or below the roof although repulling it is better unless it is a long run. Sounds like a job by an AC company.
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Nothing wrong with splicing, unless maybe AC guys did it. Experienced electrician will generally make a good splice. AC guys are the ones that maybe use improper connector if they were extending aluminum conductor, if it works when they leave it must be good to go.
 
I thought it would be better to repull but I did have to use some non OSHA approved methods to get the old wire unstuck pulling it out solo (my helper was late as usual). The splice in the gutter electrical room was in my opinion done correctly with rubber tape and looked original.
The one under the roof deck used a split bolt like this one with regular vinyl tape, which I doubt is marked AL/CU as they make a split bolt that is tin plated for AL. I did not unwrap the splice to see what was marked on the connector as it was full of that white al oxide fuzz and the tape ball clearly had gotten hot enough to make the tape brittle. Could be it was a proper connector and just condensation and a lack of noalox.
1751211723092.png
 
jim dungar said:
Out in the boonies, I have seen 1 primary side utility fuse blow causing all of the running motors to single phase. The running motors acted like rotary convertors generating the missing leg. Standard voltmeters would not see the problem, generators would not come online but the drives got upset. We eventually went with a negative sequence sensing relay which opened the incoming service breaker.

For you technical people, this has been a summary description with many details omitted.
Click to expand...
I prefer phase monitors that look at currents for this very reason. You can trick a voltage meter, but you cannot read current through a blown fuse.
 
jim dungar said:
Out in the boonies, I have seen 1 primary side utility fuse blow causing all of the running motors to single phase. The running motors acted like rotary convertors generating the missing leg. Standard voltmeters would not see the problem, generators would not come online but the drives got upset. We eventually went with a negative sequence sensing relay which opened the incoming service breaker.

For you technical people, this has been a summary description with many details omitted.
Click to expand...
I would think you need fair amount of low torque and/or high inertia load to sustain this rotary converter effect for very long.

I've seen grain storage with multiple aeration fans pushing air into same containment. One fan or even a group of fans is found running backwards, the cause - a blown fuse and enough air pushing in reverse direction to allow it to run in reverse on single phase supply without blowing other fuses and with overloads with no phase loss detection methods.
 
ptonsparky said:
Could it be as simple as 3 of these along with some logic and a timer
Click to expand...
Current monitoring only is great for individual loads. It is almost never used for starting generators especially if you are trying to see issues on the primary side of a delta-wye transformer, unless you employ negative sequence current sensing.
 
ptonsparky said:
Was this a generator issue? I missed it somehow.
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No.
The OP mentioned blown fuses single phasing and voltage measurements. I mentioned how measuring voltages only may not be the best way to see phase losses, especially on utility primaries, based on a generator system I was involved with.
 
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