Reply to Thread

Originally Posted by coolerik

I agree its tough to spot if yer only geting the drop on startup, hence why I like the idea. Should see it whil running though, unless its that bad that it wont even start, in which case should still be pretty easy to find with any ol true rms meter..

True RMS is irrelevant for this. Digital display is pretty much useless in seeing rapidly changing values. Don't underestimate what your senses can notice when presented in the right form. We're incredibly sensitive to flicker and can read a lot from a pattern. If its at the just right beat, we can notice flicker that you'd be hard pressed to see with a power quality analyzer. The slight static we can hear in music is also something we notice very well, but hard up with test equipment.

Rigging up three light sockets isn't hard. If you're using just one meter, by the time you check the third voltage, you don't know if the 5v difference is from line variation from other loads or bad connection somewhere and they tend to be temperature sensitive. The starting current gets applied cold. But loose connection gets warmer and stays warm when there is current.

Compared to FLA, LRA can be 5-6x, and inrush can be 10x. The line providing acceptable voltage at LRA is the key to successful start. Many commercial machines use single phase from three phase source to power controls, so a 7v drop on 230v might get over looked by some techs as insignificant but this means 70v drop at inrush or about 40v during motor starting.

I can rig up the truck with a few rulers, duck tape and arrows cut out of bright pink paper and mark the ground every 6", and top and bottom of the sidewall with bright tape and video it from the side on a bright day using a consumer grade high speed camcorder. Install high performance leaf springs and repeat. When I go back and watch it frame by frame, and eyeball the different markers I setup, I can gather actual events that happened in sync with each other as the truck goes over a speed bump. These values are pretty much useless if they were not captured in real time.

Some dude's commercially made sensor that can capture one channel and spits out 125,252.2 lbs with 5lb accuracy would be pretty much useless here even though it would cost many fold over my entire setup, because it just spits one value.

Getting a commercial setup that can gather the data I need even if I don't need a great accuracy would cost an enormous amount.

I agree its tough to spot if yer only geting the drop on startup, hence why I like the idea. Should see it whil running though, unless its that bad that it wont even start, in which case should still be pretty easy to find with any ol true rms meter..

I like the idea, and I dont have a 3 phase meter either, defantily not poopooing the idea, but ive always been able to find a sharty line with my meter.
Might build one anyway, like u say, nice to see all 3 at once..

Originally Posted by steve wright

Wouldn't the bulbs actually be connected parallel to the windings?

Yep. You're just visualizing voltage as seen by each winding on the compressor and you use three bulbs so you can see all three at once. The big oval and switches are a dramatized representation of a loose connection of a 3 pole contacter with one bad contact, so its not part of the tool. I put it together as a delta since I had spare 230v indicator bulbs, but if I was to build another one, I'd do it as wye so I can use 120v bulbs. In wye layout, the one bulb that corresponds to the weakest phase dims. So, to build the wye version, you wire up three bulbs, but literally hook them up like the letter "Y". The center node can be useful if neutral is available, but if not, you need to cover it up for safety reasons. I just built it, because you can't observe all three phases at once with any single phase multimeter and I can't justify buying a three phase test tool. It's not a precision instrument. You just use it to get a sense of if and where weak phase is.

Frosted bulbs in 15 to 40W are what I use. They need to be the same bulbs on all three or it won't quite work right.

It's a primitive archaic tool without UL ratings or stupidity proof features that exposes line voltage if the glass is broken, so I'd only making what you actually personally use or else its way too much liability.

Originally Posted by coolerik

yah, looks like that too me. Series wouldnt work... neat idea, but nothing my meter wont tell me

Which meter(s) do you have? I would love to get one just like it if its not too costly. I'm not aware of any reasonably priced meter which can show you a phase specific single digit percentage dip simultaneously.

Originally Posted by steve wright

Wouldn't the bulbs actually be connected parallel to the windings?

yah, looks like that too me. Series wouldnt work... neat idea, but nothing my meter wont tell me

Wouldn't the bulbs actually be connected parallel to the windings?

Originally Posted by ICanHas

This is what you use to diagnose it. Those round things are light bulbs. Our eyes are incredibly good at picking up flickers. The bulbs need to be 240v rated for this setup. You can expect that they will flicker, but they all need to dim the same amount. If one remains brightly lit, but the other two dims, you can figure out which contactor is bad. You have to use some common sense to use this hack job.

If you hook it up right below disconnect then, AB dims on compressor start, you know phase 1 is weak.
After turning off the breaker feeding it, place this above the disconnect but below the breaker and you still experience the same pattern, its upstream of the disconnect.


Build it on a plywood but label it clearly so the user doesn't get confused.
1 bad = AB dims
2 bad = AC dims
3 bad = BC dims

I very much enjoyed this thread, I like your illustration. Placing the bulb in series with a leg to verify steady power.

Thanks for this diagnostic tool. Brings me back to building my first vacuum pump from an old GM Fridgidaire refrigerator compressor (they ran in a vacuum normally). And the old Simpson 260, then when the Beckett meters came out before the head designer left to start Fieldpiece.

In the days of Moncrief, Luxaire, borg and Stewart Warner, and on and on.


Enjoyed the blast from the past. After 34 years I still
like them.

Thanks

Originally Posted by N2fords

One thing that is overlooked is they see voltage at all 3 legs but when the system starts the voltage drop from a leg, bad wiring loose connections corrosion weak breaker etc.. Always pull the Breaker if its hot to the touch.
Many compressors are replaced when it was wiring all the time that was the problem.
Bruce

This is what you use to diagnose it. Those round things are light bulbs. Our eyes are incredibly good at picking up flickers. The bulbs need to be 240v rated for this setup. You can expect that they will flicker, but they all need to dim the same amount. If one remains brightly lit, but the other two dims, you can figure out which contactor is bad. You have to use some common sense to use this hack job.

If you hook it up right below disconnect then, AB dims on compressor start, you know phase 1 is weak.
After turning off the breaker feeding it, place this above the disconnect but below the breaker and you still experience the same pattern, its upstream of the disconnect.


Build it on a plywood but label it clearly so the user doesn't get confused.
1 bad = AB dims
2 bad = AC dims
3 bad = BC dims

Single phasing. Nothing unstuck.

You have lost credibility.

You appear to be a humble man and should bounce right back and smarter too.

How about the one that was growling and tripping breaker, is it still running?

Before you change that compressor with 0 ohm readings on the windings, make sure it's not the internal overloads.

I was about to comment to the thread until I read everyone else's comments. That's when I realized that all my ideas were covered. It really could have been a bad breaker single phasing. Perhaps a loose connection at the breaker, contactor or peckerhead. Who knows, perhaps a box-elder bug on the contacts which I've seen two or three times where I'm at. If it's running now and wasn't before I would 'tend to believe what the guy told you which was a bad breaker. Or talk to the service guy who checked it out and diagnosed it. I'm guessing it was probably the breaker like they told you.

Listen too the hot rod man....

Originally Posted by hotrodrob

you said it was 3phase and was drawing 65A. was that LRA, did you check each legs amp draw. i just went through this myself with a breaker back in a mcc was not making good contact on one of buss bars. it was an intermittent issue so of course when i checked the unit amp draws everything was fine till the breaker warmed up.

I'm thinking one leg on the breaker is opening under load as well. If your luck is like mine it will pick the leg thats not carrying the transformer primary or the condenser motor cuz that would just be too easy...


When your the guy giving the second opinion you always dig a little deeper to make sure, plus you have nothing to lose.

I diagnosed one just like that, up sold to a new condenser and when I powered it up I had a knockdown drag out fight with my conscience before I walked into my customers office and told him I just bought him a new unit because I mis-diagnosed in the first place. Luckily his mom was there, (True Story) and insisted he pay me, we negotiated a discount, still hurts...

Ive reversed em and unstuck them more than once. You can do it with a single phase if you know how. pull up a PDF on an old ANNIE. I always considered it a temporary fix until I could get in a compressor. I lived in a small town, compressors were overnight on a good day...

Originally Posted by Birmingham

i came across one today that was at 0 ohms all 3 legs on compressor, scary part was it didnt trip the breaker, the thermal overload in the compressor was catching it before it flipped the breaker, i have never seen a compressor shorted on all 3 legs not flip the breaker, before i replace the compressor im going to check the breaker.

How do you check to see if a breaker will flip? i really cant think of a safe way.

I had a unit with two 3 phase bristols that did that. One was about 5 ohms to ground on all 3 legs. The overload in the compressor tripped before the 60 amp breaker in the building did.

we have changed this compressor 3 times in 2 years, i would expect this compressor to read less than 1 ohm between legs and show nothing to ground, this compressor pulls massive amps, my meter jumps and does its im thinking thing then goes to 0. the internal overload trips almost instantly. i have never seen one do this. i am worried that the breaker is bad that feeds the unit, slow blow fuses in the disconnect so i see why they don't go but i wouldn't think the breaker would take this.

this also poses another question unit running great when i change compressor:
phase monitor says were all good
its a trane 10 ton can style reciprocating
not burned last time it was pulling high amps (6 more) on one leg normal on other 2 (Im thinking loose wire connection somewhere) so changed all the wiring we could get access to.
no voltage imbalance on the contactor load side between legs or to ground.
new refrigerant every time new driers never any acid so no suction drier expansion valves working like they should checked in heat and cool working well. Package unit weigh in the charge.

Originally Posted by Birmingham

i came across one today that was at 0 ohms all 3 legs on compressor, scary part was it didnt trip the breaker, the thermal overload in the compressor was catching it before it flipped the breaker, i have never seen a compressor shorted on all 3 legs not flip the breaker, before i replace the compressor im going to check the breaker.

How do you check to see if a breaker will flip? i really cant think of a safe way.

Did the motor run?

Typically, 3 phase motors have a very low line to line resistance.

Most that I've measured are under 1 ohm.

i came across one today that was at 0 ohms all 3 legs on compressor, scary part was it didnt trip the breaker, the thermal overload in the compressor was catching it before it flipped the breaker, i have never seen a compressor shorted on all 3 legs not flip the breaker, before i replace the compressor im going to check the breaker.

How do you check to see if a breaker will flip? i really cant think of a safe way.

I am going to guess the compressor was single phasing.

I had one about a month ago with bad breaker. the blower acted as if it were locked up. checked all phases and found a phase with no power.

it was the main breaker that fed the disconnect.

always check phase to phase and make sure contactors are good.

Also no mention of checking wires in peckerhead. I have come behind other techs that said compressor bad only to find a burnt wire causing single phase. .02

One thing that is overlooked is they see voltage at all 3 legs but when the system starts the voltage drop from a leg, bad wiring loose connections corrosion weak breaker etc.. Always pull the Breaker if its hot to the touch.
Many compressors are replaced when it was wiring all the time that was the problem.
Bruce