open neutral

wmhwdh00 · Sep 27, 2005

I have a 240v 3ph Delta system with a high leg that I transformed down to 208v 3ph Y. I have the center of my Y gnded. I am using the 208 to power up two power supplies for students to use. 1 ps has no inter load so it is ,seemingly, working great. The other ps has a small internal resistance so it is not showing 120v from L1 to N. The other phases are OK. All of my phase to phase reading are good (around 211v).
How do I correct my neutral problem? No matter how I hook my neutral I have the same problem. If you need more info. let me know and I'll try to get it for you.
Thanks, Mark H.

charlie b · Sep 27, 2005

Re: open neutral

Here are my questions:

What is the nature of these two power supplies? I mean do they take 120/208 volt, 3-phase input, or 120 volt single phase input, or 208 volt single phase input, and what is their output?

You say that one power supply doesn't give 120 volt single phase. What does it give?

You said, "no matter how I hook my neutral . . . ." I don't understand. How many ways can the neutral be hooked? Are you talking about the neutral that is connected to the grounded 208Y, or the neutral that is from the power supply?

Do the power supplies create a new "separately derived system"? What I mean is do the neutral wires on the output of the power supplies connect to the neutral of the 208Y/120 transformer secondary?

wmhwdh00 · Sep 27, 2005

Re: open neutral

Our school had to have 3ph 208 to power lab volt training equipt. so we had to install an x-fmr to go from 240 to 208 3 ph. (208v 3 ph. Y is the input)
1. One power supply varies 3 ph 208v to 0V (all 3 phases) or I should get from N to L1 = 120, N to L2 = 120v, and N to L3=120v (bec. it's wired as a Y config.), the other ps should give me a steady 208v ph to ph and 120 to N on all phases. (The second ps reads Ov from L1 to N.

2.PS #1= L1 to N = 120v (good reading)
ps #2= L1 to N = 0v (bad reading, there is an internal resistance on this ps, which leads me to say there is an open neutral?)

3. The center of a Y config has to be grounded, which is where I derive my neutral from correct?

4. Yes by the NEC you could say that they would be a sep der. syst., but the trainers are portable so we can roll them around. They are connectoed by a 20A 3 ph. plug connector and they are used to train for motor controls.

I tried dwnloading a pic to show you the trainer,but wouldn't let me.

charlie b · Sep 27, 2005

Re: open neutral

Originally posted by wmhwdh00: ps #2= L1 to N = 0v (bad reading, there is an internal resistance on this ps, which leads me to say there is an open neutral?)

It does sound like your power supply #2 has failed. Whether it be by way of an open neutral or by some other mechanism, it has failed. I don't know electronics, so I can't help you figure out what to do next.

The center of a Y config has to be grounded, which is where I derive my neutral from correct?

The neutral does get connected to the center of the WYE secondary.

I tried downloading a pic to show you the trainer, but (the site) wouldn't let me.

There is a way to post photos, but I don't know the process. I do know that you have to upload the photo to some host web site, and that you use the "IMAGE" button at the bottom of the "Post a Reply" page to point to that other web site.

rattus · Sep 27, 2005

Re: open neutral

photobucket.com is easy to use, and you can copy and paste the "IMG" address from photobucket.

wmhwdh00 · Sep 27, 2005

Re: open neutral

Charlie b the problem is I have 10 of these same ps's brand new (600 dollars ea.)and they all do the same thing. Lab volt techs are saying open neutral bec. ps #1 has no internal resist. and #2 does, so it's just enough to pull down L1 to N to 0v.

A way I used to check for open Neutral was on ps # 1 (the ps that read correctly) I plugged a jumper from L1 to N and turned on power and the ps sizzled a bit but nothing happened (breaker or fuse did not trip) therefor we believe it to be an open Neutral. As you say there is only 1 way to hook up sec. of Y xfmr (N or center to gnd) is there something I'm missing? xfmr is new!

ccjersey · Sep 28, 2005

Re: open neutral

Looks like you are saying the Labvolt tech think the neutral is floating and that is why you only get 120V L-N when there is no load applied L-N. I think this is called capacitive coupling.

BUT you are saying there is 120V L-N from the other legs of the wye, just not L1 which is somewhat of a load by itself. If you apply a small load like a light bulb between L2 and neutral and L3 and neutral, does their voltage hold up or disappear? If it disappears, then the neutral is floating.

Has anyone checked the PS plugs and receptacles and wire back to the transformer. Maybe someone at the manufacturer had a bad day wiring plugs.

EEPeder · Sep 28, 2005

Re: open neutral

I'm familiar with Labvolt equipment -- I built a source of power for one in a building lobby to use it in a demo for a college project. Where are you located? If you by coincidence happen to be in SoCal I'll come fix this for you for free ... I am happy somebody is doing hands-on instruction.
Alternatively, I am going to ask some silly questions. Your indication that the 1 power supply which showed voltage on all phases didn't have a load and didn't trip for a direct short of that phase seems to indicate that it also doesn't have a solid 120V source on that phase. Use a small load to swamp any capacitive voltage when measuring voltage in this situation. One of the resistor banks from the Labvolt equipment should be adequate for this purpose. Banana jack a single resistor across the phase to neutral output you wish to measure voltage on. Measuring voltage with a voltmeter alone (particularly digital) with its extremely low leakage current often creates this kind of confusion due to stray capacitive voltages.

Do you have 4wire + ground from your step down transformer to the receptacle?

open neutral

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