480 Wye to 240/120 Delta or Wye?

[Jody Boehs]

Hello,

I work in a plant that has 3 phase 480 Wye as the primary service. All the people I've talked to about 240 3 phase, say that 240 3phase wye does NOT exist. I have a transformer in this plant that does just that. It transforms 480 3ph wye to 240 3ph wye. I have measured each phase-to-ground as 136V (there are no heavy loads on this transformer, hence the higher voltage) and each phase-to-phase as 233V. And yet, all of my electrical suppliers tell me I cannot get a transformer that does that. Do I just order a transformer that is 480 to 240 Dleta and reterminate the windings to a Wye config?

 

[jim dungar]

Every major manufacturer of transformers builds a transformer with a 240Y/136V output. Most of them will have a delta winding on the primary, which can be used on 480Y/277V systems.

However, very few supply houses will stock these devices because they are not very common. You need to push your suppliers to look beyonf their 'stock' offerings.

It is not physically possible to have a 240Y/120V output.

 

[charlie b]

I would say it is a bit tricky to assert that something does not exist, when you can put your hand on one of them. But your suggested retermination would not work. You would get 240V phase-to-ground and 415V phase-to-phase.

 

[iceworm]

Jim is correct. 480D - 240/136Y is available, probably just not stocked at your local supplier.
I have only seen these used in two applications.

240V VFD, where the manufacturer does not want ungrounded, corner grounded, or CT grounded, Delta.​

Feeding a dedicated 240V 3phase piece of equipment. And the company does not want to use ungrounded, corner grounded, or CT grounded, Delta​

They work great. No ground detectors needed. No screwy phase voltages.

One could use the transformer to feed a 240V, 3 phase panel board where the loads were 240V 3phase, or 240V single phase. As Jim said, no 1phase, 120V. It would work fine. I just have not seen one.

 

[Jody Boehs]

Ok. Thanks. Now for another question. I am setting up a power distribution system in my plant here that needs to provide 480V 3ph, 240V both 1ph and 3ph, and 120V 1ph. I have, in the past, purchased a 480V to 240V delta transformer and then used the 2 phases that provide 240/120V for single-phase power. I did this due to the fact that my single-phase loads were very low. Is this a good plan or does anyone have a better idea?

 

[iceworm]

240V/120, center-tapped Delta works fine. 120/240 loads are often limited to 5% of the xfm capacity. Jim is the expert on this.

However, depending on the various loadings, you may wish to consider.
480/277 for all the loads that fit
208/120 for all the hotel loads. Also use for any of the nominal 240V 3ph or 240V, 1ph that will run on 208V
240D or 240/136 for loads demanding 240 3phase, or 240, I phase

Welcome to being the engineer-of-record

 

[Jraef]

240Y136 secondaries on transformers are often called "Drive Isolation Transformers" and can be found in the "specialty" sections of many transformer manufacturer catalogs. This is because VFDs and DC drives do not like being connected to delta power systems, 240V or otherwise. The same was also true for old large computer mainframes and UPS systems, so there were a lot of these made and installed for data centers.

 

[kwired]

Hello,

I work in a plant that has 3 phase 480 Wye as the primary service. All the people I've talked to about 240 3 phase, say that 240 3phase wye does NOT exist. I have a transformer in this plant that does just that. It transforms 480 3ph wye to 240 3ph wye. I have measured each phase-to-ground as 136V (there are no heavy loads on this transformer, hence the higher voltage) and each phase-to-phase as 233V. And yet, all of my electrical suppliers tell me I cannot get a transformer that does that. Do I just order a transformer that is 480 to 240 Dleta and reterminate the windings to a Wye config?

all wye systems will have line to line volts that is 1.732 times line to neutral volts. 208/120, 480/277 and 600/347 are most common below 1000 volts in North America, but any combination with 1.732 ratio is possible to build.

As mentioned 240 volts system might be desirable for drives where a corner ground delta or high leg delta is undesirable - but line to neutral volts is going to be too high to use for most typical 120 volt loads if connected to same system.

 

[LarryFine]

One could always use two transformers, one for 3ph and one for 1ph.

The 120v loads would probably be happier isolated from the motors.

 

[drcampbell]

I would take a step back and ask "How can we make everything work?" instead of "How can we provide 240-volt 3-phase power?"

You say there are no heavy loads on the 240v 3ø transformer. Are there any loads? Can they be reconfigured for 480v?

If at all feasible, don't install any 240-volt circuits at all. Put small consumers on a 120v single-phase circuit and everything larger on 480v.

 

[Jody Boehs]

240V/120, center-tapped Delta works fine. 120/240 loads are often limited to 5% of the xfm capacity. Jim is the expert on this.

However, depending on the various loadings, you may wish to consider.
480/277 for all the loads that fit
208/120 for all the hotel loads. Also use for any of the nominal 240V 3ph or 240V, 1ph that will run on 208V
240D or 240/136 for loads demanding 240 3phase, or 240, I phase

Welcome to being the engineer-of-record

So does anyone know why 240/120 loads are limited to 5% of the xfm capacity? Or how far over that 5% I can go? Or is it best to just get a single phase xfm to handle the 240/120 loads?

 

[Jody Boehs]

I would take a step back and ask "How can we make everything work?" instead of "How can we provide 240-volt 3-phase power?"

You say there are no heavy loads on the 240v 3ø transformer. Are there any loads? Can they be reconfigured for 480v?

If at all feasible, don't install any 240-volt circuits at all. Put small consumers on a 120v single-phase circuit and everything larger on 480v.

The only 3 ph load on the 240 side is a heat shrink tunnel that pulls 36 amps. I dont think this machine can be reconfigured for 480 as it has heating elements that are 240 rated, however, the fan motor may be reconfigurable. Other than that, everything else is 120V.

 

[jim dungar]

So does anyone know why 240/120 loads are limited to 5% of the xfm capacity? Or how far over that 5% I can go? Or is it best to just get a single phase xfm to handle the 240/120 loads?

The 5% limit, is based on the possibility that 100% of the single phase load will be between a single line and the neutral. This extreme unbalance can quickly overload a transformer that has a significant 3-phase load. You would actually have to do the math, but there is not a lot of leeway at full load. The limit comes from a 3-phase transformer being built on a common 3-legged core. This limit is one reason a lot of these circuits are fed from an open-delta configured bank of single phase transformers (which does not have this 5% limit)

In the last few years, or so, some manufactures have changed their 240/120V 3-phase transformers and not longer list a loading limit (my guess is they changed their core construction).

 

[Jody Boehs]

The 120 loads consist of a Hand Glue Machine that pulls 13 amps, a High-Res Printer that pulls 5-8 amps, a couple Markem Imaje 9450 Ink Jet Printers (these are used to print expiration dates) that pull 0.5 amps each, 6 Desktop Computers (only 4-5 get used at a time), and 2 regular laser Jet Printers for printing documents and tags.

These added up are around 42-50 amps assuming the Inkjets pull up to 10 amps each.

 

[infinity]

all wye systems will have line to line volts that is 1.732 times line to neutral volts. 208/120, 480/277 and 600/347 are most common below 1000 volts in North America, but any combination with 1.732 ratio is possible to build.

That's why I thought that the actual 240 volt wye, 4 wire system was 240/139.