Disconnect Melting

JoelHunter1102 said:
After the 1st time we ran a bunch of calculations and made sure wire sizing/distance/etc was all good.
Click to expand...
Can you share your calculations?
What is the fan motor namplate rated voltage / HP.
Branch circuit size / breaker size?
You mentioned the wire is copper what is the wire size?

You mentioned the fan is 15 feet away from the disconnect.
How long is the run to the disconnect?

You mentioned you monitored the voltage was that on each phase L-L?
 
tortuga said:
Can you share your calculations?
What is the fan motor namplate rated voltage / HP.
Branch circuit size / breaker size?
You mentioned the wire is copper what is the wire size?

You mentioned the fan is 15 feet away from the disconnect.
How long is the run to the disconnect?

You mentioned you monitored the voltage was that on each phase L-L?
Click to expand...

OPs followup has been very good but, Yes, we want more!
 
ptonsparky said:
Do away with the reducers and put in 110 amp fuses.
rk
What is/are the actual loads? Types.
Single, three phase? Mixed loads?

I have no idea what an indoor shooting range requires. My outdoor is a 1/2 mile and requires daylight.
Click to expand...
Light up the target a little bit with one of those little rechargeable work lights. If it's too bright smear something on the lens.

I like shooting at night
 
herding_cats said:
C phase is bottlenecked somewhere. If it killed a 200A DC something else is wrong.
Click to expand...
I think I'd want to record current on all 3 phases for a few days and then look for something that deviates from the norm. Probably want to do voltage too.

I wonder if there is anything in the circuit like line reactors or PF caps? Or if it's supplying something something with lots of rectification
 
Joethemechanic said:
I think I'd want to record current on all 3 phases for a few days and then look for something that deviates from the norm. Probably want to do voltage too.

I wonder if there is anything in the circuit like line reactors or PF caps? Or if it's supplying something something with lots of rectification
Click to expand...
Like tons of reactive power? Yeah put a scope on it.
 
Out of Curiosity I would swap the phases and see if another phase burns up the same way. I believe the VFD can be programmed to correct rotation. If not just swap both line and load phases to correct rotation.

I know this is a simple approach but I would want to see if it happens to another phase.
 
Vinny B said:
Out of Curiosity I would swap the phases and see if another phase burns up the same way. I believe the VFD can be programmed to correct rotation. If not just swap both line and load phases to correct rotation.

I know this is a simple approach but I would want to see if it happens to another phase.
Click to expand...
The output of the VFD won't change.
Yes, if the problem follows, it's the load. Might be better to take all voltage, scope and thermal measurements before and soon after.
Waiting another three years will keep us waiting with abated breath.
 
I've flown a over 1000 miles to go put eyes and hands on mysterious problems like this, and most times it's somebody doing something stupid and whatever I'm being told on the phone seems to be something from an alternate reality
 
Joethemechanic said:
I've flown a over 1000 miles to go put eyes and hands on mysterious problems like this, and most times it's somebody doing something stupid and whatever I'm being told on the phone seems to be something from an alternate reality
Click to expand...
Read HVAC tech story recently, guy determined gas valve was bad. Unit was rather old and he talked client into replacing the furnace. New furnace wouldn't fire after it was ready for run. Found out the gas company had disconnected service. You would think they would have figured that out before determining old unit had a bad gas valve?
 
Back when I was a kid, we moved into a new house. First thing my dad did was put a window AC in. Thing made noise but didn't work.

Replaced the AC. Thing made noise but didn't work.

The 6-15 receptacle had been wired 120V. Whoops.
 
winnie said:
Back when I was a kid, we moved into a new house. First thing my dad did was put a window AC in. Thing made noise but didn't work.

Replaced the AC. Thing made noise but didn't work.

The 6-15 receptacle had been wired 120V. Whoops.
Click to expand...
Kind of worse yet, I had a builder change one my temporary 5-15 GFCI receptacles to be supplied by 240 volts.

Kind of hard on equipment that gets plugged in that is expecting 120 volts. I was a little surprised the GFCI circuitry survived that and still worked.

He had an air compressor that was dual voltage that had a external switch to easily change over winding taps. He had a 5-15 plug on the cord so he could plug in most anywhere but also knew that it often would trip the breaker on 120 volts and apparently would just change the configuration when possible to run it on 240 volts and do whatever he needed to get 240 to the receptacle and leave his cord as is.:censored:
 
I usually figure the place of the heat is the place of the problem. But this is kinda weird
In that you have replaced the disconnect several times. I might want to switch legs to find if the problem travels.
Not sure why it would but a guy I once worked for had said "Do something even if it's wrong but do something."
 
JoelHunter1102 said:
Good afternoon all,
I have a situation that is stumping me.

My coworker installed a disconnect (100A 3ph 208v) for a client and the disconnect had the C leg melt, without blowing the fuse. So we replaced it, it happened again. Replaced it for a 3rd disconnect, and this time upsized the disconnect to a 200A heavy duty disconnect, and used fuse reducers to put new 100A fuses in.
Lo and behold, it lasted longer than the others, but ALSO melted the C leg without tripping the fuse... it took a couple of years to do so, but still ended up with the same result.

After the 1st time we ran a bunch of calculations and made sure wire sizing/distance/etc was all good. After the 2nd time we hired an outside engineering team to come in and do some studies, they deemed everything was fine. We have monitored it ourselves and keep coming up with the equipment pulling about 68A per leg...so not enough to trip the fuses, but somehow still melting the disconnect.

We assumed weak internal connections in the disconnect itself (faulty from the factory), but at this point after three disconnects met the same end, my coworker and I are a bit dumbfounded.

Anyone have any thoughts?

Thanks in advance!
Click to expand...
swap the phases twice so the fan doesn't loose its proper rotation, it'll happen again but at least you'll know if is the wire or the fan itself.
 
JoelHunter1102 said:
As for startup loads, there is a VFD on the load side of the disconnect, between the disco and the fan motor. Which should be controlling the ramp up (at least as it pertains to load going to the motor).
Click to expand...
That is why the harmonics issue was brought up earlier. You may have a situation where there is a lot of single phase HID or LED lighting that is all powered from A-B or B-C phases instead of being more evenly distributed among all 3, to where the harmonic heating effects are then concentrating on B phase Then when that combines with the harmonic heating effects of the VFD, it’s too much on that one pole. A basic meter will not properly pick up on the higher frequency harmonic currents, you would need a harmonic analyzer. This might also explain the error codes being thrown by the soft starter.

I’m going to bet that there is no line reactor on that VFD since it was added to replace a soft starter. That would be my first go-to. Total Harmonic Distortion of current on a low cost Asian VFD** can be 85% of the FLC! If there is already high harmonic distortion from single phase lighting ballasts, it can do stuff like this.

** Most Asian based VFDs use only capacitors on the DC bus because it’s cheaper for them. This is referred to as being “unbuffered”, meaning that they do not use an internal DC bus choke to help dampen the harmonic distortion down to about 35% instead of 85%. Buffered VFDs, usually used in North American and European based VFD designs, tend to have lower THD values out of the box. Adding line reactors is still a good idea either way though.
 
Jraef said:
That is why the harmonics issue was brought up earlier. You may have a situation where there is a lot of single phase HID or LED lighting that is all powered from A-B or B-C phases instead of being more evenly distributed among all 3, to where the harmonic heating effects are then concentrating on B phase Then when that combines with the harmonic heating effects of the VFD, it’s too much on that one pole. A basic meter will not properly pick up on the higher frequency harmonic currents, you would need a harmonic analyzer.
Click to expand...
Good old harmonics, the OP mentioned he hired an EE to do some testing with a scope:
JoelHunter1102 said:
The outside company we hired to do the testing did use an oscilloscope as well as a handful of other tests, all of which apparently led them to tell us the disconnect was the problem. They then suggested we upsize it to the 200A HD disconnect...and here we are several years later.
Click to expand...
Would that miss harmonics? I suppose it could, I know its widly believed to be a non-issue anymore, but with so much low cost AC-DC rectification like you said its probably the likely culprit.
If he does not want to mess with the VFD and since he is using a 200A disconnect, presumably he has #3 CU (did we get the wire size?) , would you think pulling one size larger like #2 would displace the excess heat from the harmonics?
Then he could for sure use 110 A fuses like others mentioned, I imagine a large fuse will also generate slightly less heat?
 
You must log in or register to reply here.
Top