Delta Transformer| Center tap and Ground or Corner Ground

[zemingduan]

Thanks for your input and information. The VFD part is inspiring but may be not suitable for our conditions. The most time we need to deal with existing 2ph 5 wire service is that the owner wants to reuse the existing service, service entrance due to budget or time schedule. The waiting time from utility is usually long for new service.

For this project, we decided to specify Scott T transformer with 240V 2ph 4wire input and 120/240V 3ph 4wire high leg delta output and ground the center tap. We will let the electrician to check the availability of the transformer. And we will let the owner to change the service to 3ph if the it's hard to get the transformer.

As for the 208V - 480V step up transformer, I did some research online, using transformer with wye secondary for grounding purpose is overall easier than using delta to delta transformer.

We do have some projects several years ago that the electrician knows the vendor still selling the transformer and four-pole fused disconnect switch.

 

[tortuga]

One thing to double check is if a VFD is already part of the installation to run the elevator. You wouldn't want to use a VFD to create 3 phase power to supply an elevator system that contains a VFD; but if the elevator system already _includes_ the VFD it may pay to investigate its true input requirements.

yes indeed I should have been more clear, I was talking about the elevator VFD / Controller, not adding another one. I have seen some elevator VFDs with DC bus terminals for the purpose of connecting big Dynamic Braking Resistors (DBR) to them.
as I understood at the time you can't use these terminals to connect and external DC source to the drive, because those terminals are connected to the Dynamic Braking Transistor in the drive, so it would not be connecting to the correct part of the DC bus. But if one can get a elevator drive with a dc buss input terminal that would be intersting, then two 240V DC rectifiers could possibly be used in parallel off the two-phase system.

 

[garbo]

Its too bad DC input vfd's are not a thing.
Seems there are lots of use cases to decouple the rectification from the drive, or make it modular, and now add two-phase to the list.
I mean EV's must just be using DC VFD's? Ore are they using DC motors still?

years back I was interested in a idea @gar also had, that would use DC VFD's as part of a energy use reduction system using DC vfd's in CNC machines on a common battery bank / DC bus to a small groups of CNC machines in an area. This would also allow the machine to keep running long enough to shut down properly if primary power was lost or for an alternate source to come up to speed.

A vendor could then offer optimized rectifiers for whatever system a customer has;
For 240V two-phase that could make a decent 8-pulse rectified waveform (commutation every 45 electrical degrees).
Then offer another supply optimized for hi-leg delta, corner grounded etc.

Up to maybe 15 years Danfoss VLT drives from maybe 10 to at least 125 had two terminals that were attached to the DC Buss. I would often verify the Danfoss LCB ( Local control panel ) DC Buss read out with a meter on those terminals. Were always within a volt or two. I asked the Danfoss tech about them and he said he only came across one customer that used another Danfoss drive to power up another drive thru these terminals. I know on 100 HP & larger drives . Can only remember seeing pre charge on DC Buss on drives maybe over 60 HP. When I retired gave co workers several over 225 pages of Danfoss drawings for their numerous drives. Never saw remote DC Buss terminals on any 6 or 18 pulse drives made by ABB in the last ten years.