Generate Neutral

[Alwayslearningelec]

Is there a way to generate a neutral without changing voltage?
Scenario is - Using 3/C cable to feed a xfmr but we want to also use that 3/c to feed loads that don't need to be stepped down in voltage. We will be tapping the 3/C to feed a xfmr and then also feed the loads that don't need to be stepped down but need 4W. Thanks.

 

[ptonsparky]

Use a transformer. Typical Delta - Wye configuration, but it would be a 208 secondary.

Small detail

 

[jim dungar]

Technically yes, you can install a grounding transformer to create a neutral. There are several types but, the most well known is a zigzag style. You could also use a drive isolation style with a 480V delta input and a 480Y/277V secondary.

In general it is cheaper to install a 4th conductor as opposed to a special transformer.
Just about every one I have designed was to fix a major screw up.

 

[winnie]

1) You can get an ordinary delta:wye transformer where the output voltage is the same as the input voltage, to get a system with a neutral.

2) In theory you could use an autotransformer setup (such as the zig-zag mentioned by jim dungar) to derive a neutral, but in practice this won't work. The problem is that this derived neutral is not grounded, yet any system we install that uses a neutral operates on the assumption that the neutral is grounded. It would probably work just fine most of the time, but with unforeseen failure modes (what happens when you lose a phase and your zig-zag shifts the derived neutral potential from near 0V to the average of the other two phases??)

You can't ground this derived neutral, because that would cause large circulating currents with the supply system ground...unless the entire supply system is ungrounded, in which case you've just turned your ungrounded system into a grounded one.

(Note: I'm discussing this in the context of something like a grounded 480V system in a building where the neutral was not brought along with a feeder. Rules are different if you are talking MV distribution system outdoors.)

-Jon

 

[infinity]

A 1 to 1 ratio transformer with a 3-wire primary and 4-wire secondary should work.

 

[wwhitney]

2) In theory you could use an autotransformer setup (such as the zig-zag mentioned by jim dungar) to derive a neutral, but in practice this won't work.

And it would violate NEC 210.9 or 215.11.

Cheers, Wayne

 

[zbang]

If the new loads will take a delta feed, just connect them; be mindful of tap rules and necessary OCPD.
If new loads need a neutral (3p 4w, usually wye connected), then you do need a transformer (delta/wye).

I think you need to study the various 3-phase connections and their grounding; that would probably answer a lot of questions.

 

[topgone]

We installed a grounding transformer on the load side so load circuits can have a ground reference point. Basically, a wye-delta transformer with the primary wye point grounded.

 

[winnie]

We installed a grounding transformer on the load side so load circuits can have a ground reference point. Basically, a wye-delta transformer with the primary wye point grounded.

In an ungrounded system (turning into a grounded system)? Or was this grounding transformer on the load side of a 3 wire feed from a grounded system?

Jon