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.

 

[garbo]

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

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.

Reply to last paragraph . Just my thinking but installing a 2nd input to a three.phase VFD would still not produce the normal DC Buss of approximately 670 volts DC that is normally feed by 480 volts three phase. ( 480 × 1.41 ). Or 335 volts DC Buss on a three phase drive feed with 240 volts. PM'ed ABB 18 pulse drives from 40 to 125 HP that had two added circuits that believe produced the 12 added pulses. Heard maybe 25 years ago that Detroit also has some two phase services. Being the DC Buss on VFD'S are only two wires some smart person could bypass the existing rectifier assembly ( 3 AC inputs & 2 DC output ) with a power supply feeding the required 335 or 670 volts DC.

 

[kwired]

n 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.

Why can't one tie two DC supplies fed from each phase together and tie the DC bus of a VFD into that? It likely won't even know there is no AC input to the rectifier of the drive.

 

[synchro]

... 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.

I concur with having a tap 1/3 up the teaser winding for grounding the secondary to get symmetrical L-G voltages.

Also, since this is not a standard commonly available transformer and possibly a custom build, I would specify that there be taps on the two primary windings (e.g., +- 2.5, +- 5 %) just to make sure that they will be present. This would not only accommodate the specific voltages from the service, but also allow the possibility of balancing the output voltages of the Scott-T under load.

 

[winnie]

I suspect that most VFD DC bus terminals are downstream of the precharge circuits, so that might be a bit of an issue.

But I bet one could design a stand alone box that had a 4 leg rectifier bridge and precharge circuit to feed a VFD. It is just a question of how non-standard do you want an installation to be

In order, my preference would be:
1) Convert over to 3 phase service. I bet an examination of the property would show actual 2 phase loads, just single phase loads and 3 phase loads connected by converters of some sort.

2) Run this load with a 208/120V wye, supplied by a Scott-T transformer with appropriate taps, if the load can tolerate 208V, to make this load easy to convert to 3 phase in the future.

3) Run this load with a 240/139V wye system, supplied by a Scott-T transformer with appropriate taps.

4) Run this load with a bespoke VFD that operates directly from the 2 phase 5 wire supply. IMHO this latter is the most elegant, but fails on cost (bespoke VFD) and not being standard; think about the next person who has to deal with the system.

-Jonathan

 

[kwired]

I suspect that most VFD DC bus terminals are downstream of the precharge circuits, so that might be a bit of an issue.

But I bet one could design a stand alone box that had a 4 leg rectifier bridge and precharge circuit to feed a VFD. It is just a question of how non-standard do you want an installation to be

In order, my preference would be:
1) Convert over to 3 phase service. I bet an examination of the property would show actual 2 phase loads, just single phase loads and 3 phase loads connected by converters of some sort.

2) Run this load with a 208/120V wye, supplied by a Scott-T transformer with appropriate taps, if the load can tolerate 208V, to make this load easy to convert to 3 phase in the future.

3) Run this load with a 240/139V wye system, supplied by a Scott-T transformer with appropriate taps.

4) Run this load with a bespoke VFD that operates directly from the 2 phase 5 wire supply. IMHO this latter is the most elegant, but fails on cost (bespoke VFD) and not being standard; think about the next person who has to deal with the system.

-Jonathan

I figured there would be more details but the basic concept would certainly be something possible.

 

[tortuga]

I suspect that most VFD DC bus terminals are downstream of the precharge circuits, so that might be a bit of an issue.

But I bet one could design a stand alone box that had a 4 leg rectifier bridge and precharge circuit to feed a VFD. It is just a question of how non-standard do you want an installation to be

In order, my preference would be:
1) Convert over to 3 phase service. I bet an examination of the property would show actual 2 phase loads, just single phase loads and 3 phase loads connected by converters of some sort.

2) Run this load with a 208/120V wye, supplied by a Scott-T transformer with appropriate taps, if the load can tolerate 208V, to make this load easy to convert to 3 phase in the future.

3) Run this load with a 240/139V wye system, supplied by a Scott-T transformer with appropriate taps.

4) Run this load with a bespoke VFD that operates directly from the 2 phase 5 wire supply. IMHO this latter is the most elegant, but fails on cost (bespoke VFD) and not being standard; think about the next person who has to deal with the system.

-Jonathan

Add to the top of that list run the elevator off of a 240V single phase VFD that outputs 3 phase. Thats off the shelf part.

 

[winnie]

Add to the top of that list run the elevator off of a 240V single phase VFD that outputs 3 phase. Thats off the shelf part.

I agree in general. In this specific case, the original poster specified that they didn't want to do this because it would overload a single phase of the service in question.

 

[tortuga]

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.

 

[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.

 

[winnie]

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?

Many/most VFDs have DC input capability; even though they have the input rectifier, they have the DC bus brought out to terminals.

I've seen documents from Hitachi about using their drives on a common DC bus, for situations such as banks of elevators where some drives might be regenerating while other drives are consuming power.

The basic diode front end is cheap enough that it probably isn't worth building units that don't have the front end.

-Jonathan

 

[winnie]

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.

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.

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.

Sounds reasonable.

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.

Agreed. As I stated above my preferences is to have a symmetric grounded system eg. 480/277V or 240/139V.

-Jonathan