Buck Boost / Step-up Transformer European vrs. USA standard question

[LarryFine]

In my opinion, the right thing to do here is run a new circuit, or better yet, a new feeder and sub-panel.

 

[winnie]

As I stated I would not use the trick of bodged together cords to combine circuits of different phases.

Not only are there safety issues (parallel neutral paths, male terminals that might be energized, risk of short circuit if a receptacle is miswired), you are in a school teaching future technicians.

You don't want to teach the lesson that 'any ole hack is ok if it works'. That is an attitude gets people killed.

The only exception in my mind is a listed tool designed to safely combine circuits.

Jon

 

[mark.richardson]

In my opinion, the right thing to do here is run a new circuit, or better yet, a new feeder and sub-panel.

In my opinion, the right thing to do here is run a new circuit, or better yet, a new feeder and sub-panel.

The whole idea is to have this be a portable teaching tool that can be exported not only from classroom to classroom but also to other sights.
In the past when we've done this with other equipment we've found two 120 v plugs from two separate breakers off different legs of panel.
I was dreaming as it turns out when I thought I could find a buck boost transformer that would work. I have duly been educated. Thanks

 

[Speedskater]

thanks, can you tell me who sells these surplus transformers?

I'm late, but they have several locations. The catalog pages don't give much info thou.

https://hgrinc.com/

 

[LarryFine]

This question was never addressed:

What about paralleling the neutrals and/or 'return' current not contained in the same cable or conduit?

Even ignoring the neutral issues for the moment, the two line currents would be on separate pathways, and possibly lead to conduit heating.

 

[synchro]

Of the problems pointed out by winnie, one of the most serious is having hot pins on a disconnected plug due to voltage that's backfed from the other cord through the load. I think protection from this issue is a must, whether it be with the product that winnie mentioned or another way.

One approach for protection is to have two single-pole contactors, with one of them in series with the line conductor from each cord. The coil of each contactor would be powered by the 120V L-N voltage from its corresponding plug. That way if a cord is not plugged in, then its contactor will be open, and therefore there will be no backfed voltage on the pin of that plug even if the other cord is plugged in.

 

[synchro]

This question was never addressed:

Even ignoring the neutral issues for the moment, the two line currents would be on separate pathways, and possibly lead to conduit heating.

Yes, not having the current return back through the neutral conductor of its own circuit has a number of issues. They include the possible heating you mentioned, higher magnetic fields from the large circuit loops created (possibly leading to interference issues), and violating a key principle within the NEC, even though it may not be within the domain where that code applies. Some people are apparently sensitive to 60 Hz magnetic fields, but that's something I know little about.

A way to have current return through the neutral conductor of each 120V circuit is to insert a winding from a relatively low voltage transformer that has a 1:1 ratio in series with the 240V load as in the circuit pictured below. This circuit has the same effect as putting the neutral conductors in series with the load, as is the case in normal 120V circuits. The voltage drop across the neutral conductors will be added to that of the line conductors, as it would be with L-N loads on 120V single-pole circuits. The total voltage drop on a percentage basis should be about equal to that in 120V L-N circuit when the same load current is drawn. There will be some drop through the transfomer itself but it should be relatively minor, as discussed below.



The transformer only needs to support the amount of voltage developed across the neutral conductors at terminals N1 and N2 without substantial distortion. It's likely that a 1:1 transformer for 24V would be more than sufficient. And so the kVA rating needed could be less than 1/10th that of isolation transformers necessary to support the 240V load. Even if the voltage drop across the neutrals gets high enough during motor starting, etc. to saturate the transformer, it just means not all of that voltage drop will get put in series with the load. Which is actually a good thing in this situation. The transformer would need to be rated such that its insulation supports 120V to ground.

If the impedance of a 24V transformer is say 5% and the rated current is 20A, then at 20A it would have a voltage drop of 0.05 x 24V = 1.2V, or 0.5 % of 240V. This is the drop you'd get if the neutral conductors had zero resistance. And so the contribution from the transformer itself to the voltage drop should be relatively low.

 

[winnie]

@synchro
I like the two solutions you've suggested to the problems of combining two 120V 20A circuits to create a single 240V 20A supply.

I am certain that the cheap 'Y' cables sold on Amazon to do this won't include the necessary protection.

I don't know about the more expensive apparatus which I linked would include the necessary protection.

-Jon