Hi Leg Transformer

[Tom DiRienzo]

This is my first post to this forum and hope to be involved often with what seems as an awesome group of problem solvers.
I am an Eng Tech at a Precious Metal Fabricator and leading the purchase of a CNC Lathe. We had a building expansion approx. four years ago and there were two Hi Leg 240v 3ph 150kVA transformers installed. The primary is 480 and the secondary is 240vac. Every other transformer throughtout the plant is a delta-wye, where these are delta-delta w-hi leg. This will be the first use of these "new" transformers and i would like to know the implications of their use. Several CNC manufactures of machines we do have, have said not to use them, but the manufacture of the CNC that I have purchased has not responded as of yet. The local transformer rebuilder has said to "just remove the tap that is grounded"...is this a reasonable thing to do? Is there really no issue at all?
I appreciate any and all help that I recieve...
Thanks...

 

[hmspe]

Just removing the bonding jumper is not a good idea -- it would remove the fault current path that would allow the fuses or breaker protecting the feeder to the lathe to open. For that matter, can you remove the bonding jumper? 150KVA seems like a lot for a single lathe. Will there be other loads on the transformer?

I can tell you that I've seen CNC equipment installed on 120/240V open delta with the bonding jumper installed, and everything worked properly. On the other hand, if you do not install per the manufacturer's requirements you may void your warranty, and violate NEC requirements.

My recommendation would be to put out a request for proposal which states the requirements that you have for the lathe and states that the power available is 120/240 V, 3 ph, 4 wire delta. Let the manufacturer(s) suggest a solution.

Martin

 

[don_resqcapt19]

If the new equipment is 3 phase 240 only, there will be no problem using the 3 phase 4 wire delta system.
Don

 

[Tom DiRienzo]

Sorry for the bit of confusion...
This transformer supplys a panelboard which would supply the CNC. This panelboard has not been tapped for use since it was installed.

I have been in contact with the CNC manufacture, but due to vacations they have not been able to give a timely response.

Thanks for the reassurance that all will be well...