xfmr voltage measurements 2

[BlaineVT]

We have a 230V-3P-3W system. We are working on a design build project for a renovation to an existing commercial building. The owner wants an elevator, so we had the electrician double check the voltages. The electrician measured 280V L-L and 160V L-G. After some discussions, we decided to call the power company and have them verify our system. They explained that they are using three separate 138.5/277V xfmrs connected like a wye, but used as a delta for the 240V-3P-3W system. The phasor calculation makes sense 138.5V*sqrt(3)=240V

Did my electrician measure the L-L voltage wrong or use the wrong meter (should use a 3phase meter)? 280V appears to be higher than normal, but the power company insists everything is normal.

As it turns out, they can use the same 138.5/277V stock xfmr for a 277/480V-3P-4W system (277V*sqrt(3)=480V).

We want to convert this system with a 3-phase 15kVA 240V-120/208V xfmr to connect our traction elevator. This makes sense on paper, but has anyone done this before?

Thanks, Blaine

 

[jraef]

If there was little to no load on the transformer when the L-L voltage was measured, it's possible for it to read high. Transformers are designed with a "turns ratio" that provides the proper output voltage under load, but that ends up high when there is little to no load.

L-G measurements are pointless on an un-grounded delta system (assuming that's what you have because you didn't report that he showed zero V on one phase when measuring L-G) because the voltage measurement has a capacitive element to it since there is no established reference to ground.

If your elevator needs 208V AND you want to power 120V lights and receptacles from this service, then yes, you need a 240-208Y120 transformer.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[stevenal]

ANSI transformers provide rated voltage at no load.

Tell the power company that ANSI C84.1 allows no more than 252-254 V at the service for 240 V nominal systems. No, a hand held single phase volt meter works fine.

 

[BlaineVT]

Thanks for the responses. I was starting to over think this one and needed some opinions.