What happens when a star wired balanced device (without a neutral) becomes unbalanced

[Inteificio]

Title says it all.

Admitting my inexperience here, but yesterday I was working on some machinery wired in star without the neutral connected outside the device.
If all phases are balanced then N has no current flowing down it!
One of those things, it is obvious if you think about it, just have never thought about it before.

So what happens on one of these devices if it becomes unbalanced either:

i) slightly: One motor winding runs slightly warmer than the others so has a higher resistance

ii) a lot: one phase drops

Also is this type of wiring only on motors?
Is there a specific name for a star with the neutral not being connected back to the board?

 

[Knobhead]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

You need to draw out a vector diagram to show voltage and current imbalance, I have a drawing of it but due to a cock up on my part I cant upload drawings at the moment. Sorry L

 

[Thomas123]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

good question, i too would like to know the answer to this.

would the line to earth voltage increase on the phases pulling more load? i have no idea.....

 

[Knobhead]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

This is a post I wrote for an international site.One to add to the experience pile. Got a call to a plant, all the office equipment had failed. On arrival I tested the supply to the office and all seemed OK. Called the lads from office services to come and sort their junk out. All power supplies smoked! I'm puzzled now, how could 4 go wrong together. Got talking to the plant manager and he tells me that the lighting had "gone weird" for a while. Went back and checked the Ph -N voltages again and all OK.It wasn't until lunchtime when I got talking to some of the other electricians that things became clear. Two of them had installed a 150mm 3½C cable in to the plant main switch board that morning and had disconnected the neutral - earth cables to make it easier to get their cable in, when finished they reconnected them. To quote one of them "I thought there was a bit of a spark!" What galled me was I'd fitted an engraved warning label to the earth bar when I installed it!Once the neutral is floating and unbalanced current drawn on the 3 phases will change the voltage between line and neutral.I’ve got to get IE sorted on my machine so I can upload drawings.

 

[Inteificio]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

Hi Tony,

Can you please expand, although top of my class on the calculations I can't work out what to do on this one.

Being top of a 2330 doesn't mean much, it is like being the best at kindergarten ;-)

 

[johnboy6083]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

Neutral needs to be grounded at the supply transformer. This at 0v or near as dam it. The line is at 230v. The potential difference is 230v.Now if the neutral is not connected to earth, and the voltage on the neutral rises, then your potential difference is altered, and it will play havoc with lighting, and anything else that requires a steady supply. I guess that motors won't like it either, as the torquewill rise and fall, along with the associated current and voltsge

 

[trong]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

3 phase, single phase, or phase to phase?

 

[Knobhead]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

Phase to phase voltage will remain constant. Phase to neutral voltage will vary if it’s not tied to earth. If you have a look at this thread http://www.electriciansforums.net/commercial-industrial-electrical/42533-three-phase-voltage-query.html I’ve explained it before. (saves me a lot of explanation) LOLAs I’ve said I’ve made a cock up with my PC so can’t upload drawings at the moment.

 

[trong]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

the danger of flowing neutral is that it could became phase to phase by voltage flowing through device with neutral wires. surely will burn devices.

 

[Inteificio]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

oooh good point, makes me want to stick a neutral on these unmaintained devices in awful condition!

 

[Andy 7671]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

OK, well forgive the ambiguity of typo, I only ask to find out where my thought process differs.

1) Why no info on whether the phase angle is leading or lagging 2pye xl or 1 over 2pye fc, comes to mind as a calc.

2) Why no reference to impedence triangle.

3) Showing ignorance but...I thought it was just Delta that didnt need a neutral.

Sorry Pete, aint hijacking, just wondering if practical use of my science and principles could help, I was so proud of passing this module, I vowed it would stay in my head but 4 years on and ..its hmmmm what was that again.

 

[johnboy6083]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

Delta doesn't need a neutral, but I think your confused regarding distribution systems and final circuits. Star is used to supply both 1ph and 3ph customers as the star point becomes the system neutral. I believe that there are customers who have a delta secondary on their tx, and I believe that an artificial neutral can be made.

Tony is probably the man with experience in this field.

 

[Knobhead]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

An artificial neutral can be created for a delta secondary transformer using an auxiliary zigzag transformer as shown on page 12 of http://www.eon-uk.com/distribution/CiCdocs/01%20Technical%20Documents/CN%20Combined/Earthing%20Manual/E2%20Earthing%20Manual%20Earth%20Guidance%20Notes.pdf this show a 11KV secondary but it’s just as valid for 400V

 

[Chr!s]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

It all depends upon the loads/ impedance's of each phase. If each phase is balanced then the star point will be on the right position. if the load or loads are not balanced the the star point will move away from the correct location. Worst case scenario would lead to phase to neutral voltage of 400 v +-.

 

[Inteificio]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

Hi Andy,

"Showing ignorance but...I thought it was just Delta that didnt need a neutral."

This was kinda the point of the thread, I spotted some equipment with a floating neutral (think that is what it is called).

So it was wired in star, but the centre point of the star was not connected to anything else!

I realised that if the phases are perfectly balanced then no current flows down the N, so it would not be required.

The limit of my knowledge was what happens if there was a problem throwing the phase balance off.


You have me curious, what module are you referring too?