Zigzag Transformer

[mshields]

I have a 3 phase, 4 wire load that happens to be fed from a 3 phase, 3 wire panel.

The feeder is already run. What I'm wondering is whether or not a zigzag type transformer is worth looking at as a solution.

The load is an MRI (for a lab not for people but the electrons don't know the difference).

I know I can put in a delta wye transformer but voltage drop being a significant concern, I'd have to put one in that is low impedance and even then, would probably want to to oversize it to boot.

Would love to hear your thoughts?

Thanks,

Mike

 

[jim dungar]

Most zig-zag transformers are non-standard products so price and availability should be considered.

 

[mshields]

Which is indeed a factor. But do you know whether or not one of these could even be a solution.

 

[jim dungar]

Yes you could use a zig-zag, but it there is a reason there are not as common as delta-wye configurations.

Voltage drop would not be my first area of concern as most standard transformers have taps to deal with it.

Remember, transformer impedance, regulation, and efficiency are not directly related nor are they mutually exclusive.

 

[electrofelon]

Did I make this up, or in such an application could one use a Delta wye transformer and just connect to the wye side to derive the neutral and leave the Delta winding floating? If this is possible, please discuss the implications of this versus just using it as a conventional isolation transformer.

 

[winnie]

The derived neutral would not be a grounded conductor. It would be an approximate 0V circuit conductor that wasn't grounded. Not sure if this has any implications for the load.

You would introduce transformer impedance only for the derived neutral, which would mean poorer regulation for the L-N loading.

Is there any chance that the L-N portion of the load can be supplied by a control transformer? Many large 4 wire 3 phase loads only have a small L-N component for controls and the like.

A wye:floating delta transformer will derive a neutral on the wye side, but you'd really like your voltage adjustment taps on the wye side for such an application.

You can build a zig-zag transformer from a bank of 3 single phase 1:1 transformers.

Jon

 

[Besoeker3]

Different phase shifts are one reason we have used them.

 

[synchro]

Does the equipment manufacturer require an isolated ground? If so, since the existing feeder doesn't have a neutral conductor I doubt it includes an isolated ground. A delta-wye isolation transformer would make an isolated ground easy to do.

In any case I think it would be wise to contact the manufacturer of the MRI to find whether there are any additional requirements beyond just 3-phase 4-wire.