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#128852 - 02/22/04 10:28 AM Would this work?
C-H Offline


Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
You have an ordinary 120/240V, three wire system with a grounded neutral. Take the two hot wires and place an autotransformer between them. Make this an 120/240V autotransformer and ground the midpoint to a ground rod. No metallic connection with the original neutral. Would this work as a neutral? Or will this only let the smoke out of the transformer?

If you place a load on each leg, will the autotransformer only have to handle the unbalance current?

It seems too good to be true, and that usually means it is...

#128853 - 02/23/04 09:19 AM Re: Would this work?
winnie Offline

Registered: 09/15/03
Posts: 652
Loc: boston, ma
Sound a lot like the use of a 'zig-zag' tranformer to derive a neutral from a delta supply. I don't see that it _couldn't_ work, but I bet there are some subtle failure modes that make this a bad idea. However in the below I am just guessing.

For example, in the system you describe I bet loading that puts imbalance on one side could shift the potential of the neutral. How do you ground the neutral of the autotransformer? Since the system is not galvanically isolated, connecting the new neutral to ground would seem to create an ugly parallel paths problem.

I'd bet that using an autotransformer to derive a neutral on a single phase system is only viable when the neutral is not derived elsewhere, eg a 240V secondary without a center tap, combined with a 120/240V autotransformer to get the neutral. But at this point, why not simply use a secondary with a center tap.


#128854 - 02/23/04 09:42 AM Re: Would this work?
C-H Offline


Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Thanks! You input on both this topic and the other is appreciated.

Yes, the idea is that this should act just like a zig-zag-transformer.

The reason is too save on copper. If you can't balance the loads very well on a three wire system, you'd have a voltage drop in the neutral. This would be avoided by this setup, but of course you'd instead get a voltage drop across the autotransformer. But this should be less than it would have been in a real neutral conductor. That is, I think so. This of course depends on the transformer and the size of the wires.

Yes, the potential of the neutral will shift. But it does so on an ordinary three wire system as well because of the voltage drop in the conductors. But you could be right in that the effect will be more severe in this case and therefore cause problems.

If you leave the "fake" neutrals floating with respect to ground and treat them as live conductors, does that solve the problem? It leaves you with some problems when it comes to equipment ground, but I think it is manageable.

If this is such a great idea, why haven't one heard of it? There must be a catch somewhere. There isn't room for a 19th century innovation in the 21th century.

#128855 - 02/24/04 03:26 AM Re: Would this work?
pauluk Offline

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
I don't know about codes in other countries (I'm assuming this is part of your cheap- power-for-developing-places project ), but as far as the NEC is concerned, wouldn't such a set-up violate the rule about not using the earth as a return path?

#128856 - 02/24/04 08:48 PM Re: Would this work?
Scott35 Offline

Broom Pusher and

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Yes it will work - and emit no smoke (under normal conditions!).

Aside from the redundancy of establishing a "half potential" circuit conductor point + a system grounded conductor, it would be similar to an HID Ballast's input Autotransformer - only connected different.

Smoke would likely be emitted from the Autotransformer, if the system Power Transformer's Center-Tapped Noodle Conductor was also connected to the Center Tapped point of the Autotransformer.

In the example setup described by C-H, the Autotransformer would be a 240VAC winding, with a single tap made to the center of the winding (coil).
The tap point will be a 50% tap.
This will result in a Potential of 120VAC between the ends of the winding and the tapped point, with an overall Potential of 240VAC measured across the complete winding (coil).

Move the tap point from 50% to 75%, and the results will be:

* 240VAC across the complete winding - AKA L-L,
* 180VAC from one end of the winding to the tap point - AKA L-N (L-N ex.1),
* 60VAC from the other end of the winding to the tap point - AKA L-N (L-N ex.2).

If you had equipment designed to operate at 240VAC, connect it to L-L.
If you had equipment designed to operate at 180VAC, connect it to L-N ex.1.
If you had equipment designed to operate at 60VAC, connect it to L-N ex.2.

I could post a Schematic of this if needed.

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#128857 - 02/25/04 03:40 AM Re: Would this work?
C-H Offline


Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Thanks Scott!

Then we take the next step:

If an isolating transformer with a 3kVA rating is connected as an autotransformer according to the above, you could take out 6kVA from one of the 120V legs, right?

If you put a 50A three pole circuit breaker after the transformer, would this serve as sufficient protection for the transformer? I figure you could take out a maximum of 6kVA on either leg. (I.e. a perfectly unbalanced load) If the load is perfectly balanced you could take out 12kVA, the only limit being the circuit breaker?


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