Have you every seen a 120/240V 2PH 5Wire to 120/208V 3ph 4wire transformer?

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zemingduan

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Philadelphia,PA
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Electrical Designer
I am working on an electrical design of a building that is an old 120/240V 2ph 5 wire electrical system in Philadelphia. The new elevator is 208V 3ph. And this building is required to have stand by power for the elevator since it's over 6th floor but not high rise. I design a standby power panel which supplies the elevator motor, car lighting, hoist way GFCI outlets, machinery space outlets and pit sump pump.

I have seen a 120/240V 2PH 5Wire to 208V 3ph 3wire before. I want to see if there is a 120/240V 2PH 5Wire to 120Y/208V 3ph 4wire transformer. So that I can supply the normal power of the standby power panel from the transformer secondary. Thus I can avoid another 208V 3ph delta to 120/208V 3ph 4wire whey transformer.

By the way, are the car lighting and hoist way and machinery space outlets required to hooked to the generator? I search the IBC. It looks like they are required only for high rise building fire service or self evacuation elevators.
 
jim dungar said:
You need to look for a 'Scott -T' transformer.
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I think the Scott -T transformer is 208V 3ph 3wire delta instead of 208Y/120V 3ph 4wire. Are you able to get neutral from Scott -T transformer? I really don't' know. So I am asking is there a 2phase 240V to 3ph 4w 120/208V Whey transformer.
 
A 'Scott' is the special configuration that converts between 2-phase and 3-phase.

While I have not seen one, there doesn't appear to be any reason you could not get a 208Y/120 output like you can with a generic T configuration.

I found a reference to a Taylor connection, but that appears to be a 3wire delta output.
 
You would need to have a neutral tap on the secondary teaser winding of a Scott-T transformer to get the neutral. That might be harder to find. An advantage of having a delta-wye following a Scott-T is that you will have more flexibility to get balanced 208/120V outputs by adjusting the taps on the delta windings.
 
synchro said:
You would need to have a neutral tap on the secondary teaser winding of a Scott-T transformer to get the neutral. That might be harder to find. An advantage of having a delta-wye following a Scott-T is that you will have more flexibility to get balanced 208/120V outputs by adjusting the taps on the delta windings.
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Thank you synchro. You answer my questions perfectly!
jim dungar said:
While I have not seen one, there doesn't appear to be any reason you could not get a 208Y/120 output like you can with a generic T configuration.
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Thank you Jim too!
 
I have wondered about this before, but what about just using a 3 phase 240V elevator motor and VFD?
I wonder if a VFD manufacturer could support using 3 hots from a Philly two phase system.
In theory the inputs are just converted to DC
 
zbang said:
The AB phases aren't always connected with the CD phases.
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I think they call them (or used to call them) aa and bb, and if its grounded 5 wire they all share a common neutral.
But even if you had to use a single phase to 3 phase 240V VFD thats gota be cheaper than two transformers.
 
drcampbell said:
If not, are there any VFDs out there that will accept a DC input? Then you'd need only a rectifier, no transformer at all.
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Some VFDs have a provision for connecting DC link chokes between the rectifier and the DC bus capacitors. So presumably with such VFDs you could connect external rectifiers to the DC bus at this point. Unless there's sensing circuitry or other complications that prevent this. You would want a connection to both + and - sides of the DC bus (I believe some VFD's only have a provision for connecting a link choke on one side of the bus).
You could use two 4-diode full wave bridge rectifiers, one for each 240V pair of two-phase hot wires. I also don't see why you couldn't connect the VFD AC inputs to 3 of the 4 two-phase hot wires and use the intermal rectifiers for them. And then the 4th hot wire could be rectified with a pair of external diodes connected to the DC bus.
 
Yes, the hardware's all there but the limited experience I have trying to power up VFDs with the DC bus resulted in the software raising an "Input Phase Imbalance" or "Input Phase Lost" error. (likewise when they were supplied from a delta circuit) I just have high hopes that one or more VFD manufacturer have released something flexible enough to accomodate that.
 
However, all of this may be a moot point because the application mentioned is an elevator and so using a VFD to power the elevator motor may not be acceptable for a number of reasons. For example, it may feed back regenerative power. It may already be using its own VFD. And I think it's very possible the elevator manufacturer would not allow it. But good thought anyway.
 
Ah, yes.

This can't be the first elevator installed in Philadelphia. The elevator manufacturer probably encountered and accommodated two-phase power in the past, so it's likely only a matter of calling an application engineer and figuring out how to specify 2-Phase Input Power.
 
zbang said:
The AB phases aren't always connected with the CD phases.
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By "connected with" do you mean at a stable voltage relative to?

Looking at the Scott-T diagram below from wikipedia, it seems like the R3-R4 voltage is undefined relative to the R1-R2 voltage (although the phase relationship is fixed at 90 degrees.) So if you wanted 2-phase 5 wire, you'd need a center tap on R3-R4 and on R1-R2, which would be connected to become the common neutral point? Or you could get 2-phase 3 wire by connecting, say, R1 and R4 ?

Thanks,
Wayne


Scott-T.jpg
 
In my limited Philadelphia experience, and this was a long time ago and I was just starting out, the two phase came directly from PECO energy.
You could get various voltages like 12.5kV, but the phase angle was 90 degrees.
At the generation plants they would also have a turbine that generated at 25 hz for the rail system.
I suspect the rail system must have been converted to 60 hz by now.
Then industrial customers used customer owned scott T to derive 3 phase 120 deg phase angle from the incoming two phase.
If say b shorted to aa you would defiantly blow a fuse, I remember that.
My guess is a 5+ story building in center city is a primary customer, so they must still have two phase at the substation level?
It would be cool if someone who works a PECO was on here, I would be interested to know how far upstream the two phase still goes?
The voltage relationships were like this:two_phase_5_wire.png
 
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tortuga said:
At the generation plants they would also have a turbine that generated at 25 hz for the rail system.
I suspect the rail system must have been converted to 60 hz by now.
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Amtrak DC-NYC and Harrisburg, and the old Reading lines (now SEPTA), are still 25Hz and will be for a very long time. IIRC the Safe Harbor Dam still generates that, but the rest is a mix of converters.

Why still 25Hz? It's the cost of conversion- not only almost all the power distribution except the catenary wire, but also good parts of the signaling system. Any there wouldn't be much gained by changing.
 
Wow, very cool, I love rail history stuff and historic electric stuff thats still working, chugging along.
 
zbang said:
Well, I meant exactly what I said,and the diagram confirms that- the windings R1/2 and R3/4 have no (electrical) connection so unless one is added, they're 'not connected'.
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OK, that makes sense. So to spin a 2-phase motor, do R1/2 and R3/4 need to be connected? Will the motor spin with just a 90 degree phase difference, without a defined voltage magnitude between R1/2 and R3/4?

Thanks,
Wayne
 
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