Delta Transformer| Center tap and Ground or Corner Ground

[zemingduan]

Here is the background. The building service is 120/240V 2ph 5wire from PECO in Philadelphia. I am an electrical engineer and plan to specify a 240V 4wire input and 240V 3ph 3wire output Scott T transformer to supply the 240V 3ph elevator (I am not using the single phase - 3ph converter since it will over load one phase of the two phase service.).

Do you make a center tap at one secondary phase and ground the center tap or do you corner ground one phase? I don't need to use the center tap for 120V loads. Chatgpt tell me the corner ground is the best solution And if I corner ground one phase, I need a fused disconnect listed for corner ground (i.e. no fuse for the grounded phase). Meanwhile, the elevator controller shall also be listed for corner ground. But the elevator/controller specification has limited information if corner ground is acceptable. And the vendor is not clear if the corner ground is acceptable for the elevator/ elevator controller. Any thoughts?

Another situation:

If the building service is 208Y/120V 3ph 4wire and the elevator is 480V 3ph. And you need a step up transformer. Do you use a 208V 3ph delta - 480Y/277V 3ph 4wire wye transformer and ground the neutral? Or do you use a 208V 3ph delta - 480v 3ph delta transformer and ground the center tap/ corner ground one phase?

 

[infinity]

Do you use a 208V 3ph delta - 480Y/277V 3ph 4wire wye transformer and ground the neutral?

That's what we would normally use. Even if you don't need the 277 volts it's just simpler to use a normal Wye system with grounded neutral than a corner grounded or ungrounded system. If there are VFD's involved the Wye system might be required.

 

[garbo]

Have not heard of a Scott T transformer beside these post in many years. Many years,ago I helped my dad install a Scott T transformer in an o!d Philly factory. Within the last year a poster asked if any one had any old two.phase equipment. I have not purchased a two phase 4 pole four fuse 240 safety switch since the 1970's Supply house only had them by one of the worst manufacturers back then. I recently did a Google search for two phase four fused safety switches and could not find anyone making them. The Philly area around Frankford & Cottman Avenue still has two phase. During one of the energy events they ( PGW ) refused to install gas to existing buildings so I had to install a 100 amp three phase service for a roof top heat pump on a store along with a single phase service. I questioned PECO on only supplying five wires in basement end box where they dud not increase the wire size that feed one leg on both services. I wasted time attempting to access the entire PECO/Exelon Blue book for Philly area.for a corner grounded three phase service. It is their blue book. I never came across a three phase corner grounded service in Philly but a few in surrounding suburbs. Hope that bussines has a spare two phase fused safety switch if you go with a two to three phase phase changer.

 

[winnie]

Some equipment doesn't care about L-G voltage, other equipment (in particular VFDs) care alot.

My design preference would be to use a Scott-T transformer with the appropriate tap on the teaser winding to create a 240/139V system, and to ground that tap.

 

[zbang]

to supply the 240V 3ph elevator

Is it really built for 240v? That seems kinda odd to me; I'd expect 208/120 and go that route if at all possible.

 

[Joethemechanic]

need a fused disconnect listed for corner ground (i.e. no fuse for the grounded phase

You should only need a 2 pole fused disco. Why do you need to open a grounded conductor?

 

[kwired]

Here is the background. The building service is 120/240V 2ph 5wire from PECO in Philadelphia. I am an electrical engineer and plan to specify a 240V 4wire input and 240V 3ph 3wire output Scott T transformer to supply the 240V 3ph elevator (I am not using the single phase - 3ph converter since it will over load one phase of the two phase service.).

Never been to the area, read things about two phase though, I thought there was no longer any two phase distribution and any Two phase services still supplied were derived from three phase through Scott-T transformers? If so wouldn't it be best to just have three phase service run to the facility? Some of them might be just an elevator is the only thing keeping them on two phase and if you updating one to three phase you maybe no longer need two phase or at very least have eliminated one more two phase item you do have?

Step up from 208/120 to 480/277 does make sense if you must have 480 for your elevator for whatever reason. Not that they won't work at all on corner ground or high leg delta but chances are they may have surge protection only designed for ~300 volts to ground.

 

[Joethemechanic]

two phase distribution

That has all been gone since before I was born.

I guarantee the primary is 3 phase

 

[tortuga]

Two-phase huh, someone from PA told me that 'two-phase' was short for 'go phillys!'

Here is the background. The building service is 120/240V 2ph 5wire from PECO in Philadelphia. I am an electrical engineer and plan to specify a 240V 4wire input and 240V 3ph 3wire output Scott T transformer to supply the 240V 3ph elevator

How many amps is the elevator 3 phase 240V VFD? And how large is the service?
I'd be tempted to get a larger VFD that accepts a 240V single phase input, then no transformer.
You'd need to re-balance some loads to keep the two-phase balanced if you go that route.
Since each line in two-phase has the same voltage to the neutral its better suited for a VFD than a delta transformer,
In 2026 you'd think someone could make a modular VFD that you could add a 2nd single phase 240V rectifier to, a 2nd input to the DC buss, then you could use both 'phases' all 4 lines as drive inputs to drive a 3-phase 240V motor, and your two-phase side stays balanced.