Buck Boost transformer neutral

[dbaryl]

Hi,

I have a 3 phase, 480Y/277V panel and a 3 phase, 208Y/120V panel available to me.


I need 240V, 2 Hots with a Neutral.

What is my best option, with a buck/boost transformer. I'm thinking 208v to 236v, and pass thru with the neutral straight to my load. Is this correct?

 

[petersonra]

Hi,

I have a 3 phase, 480Y/277V panel and a 3 phase, 208Y/120V panel available to me.


I need 240V, 2 Hots with a Neutral.

What is my best option, with a buck/boost transformer. I'm thinking 208v to 236v, and pass thru with the neutral straight to my load. Is this correct?

You would end up with 236 V L-L and 136 V L-N.

 

[winnie]

As @petersonra describes, with a standard buck/boost arrangement on 208V, you won't get your desired balanced 120V to neutral.

With the standard arrangement one leg will be 120V to neutral, but the other leg will be much more than 120V to neutral and the phasing is wrong for a true 120/240V supply.

Best bet is a single phase transformer, eg a 480:120/240V transformer or a 208:120/240V transformer fed from the respective panel.

Jon

 

[hillbilly1]

You can with an open delta buck/boost, but you will have only one leg that is 120 volts, so it will be tricky to find which leg on the equipment needs 120, and if more than one leg needs 120, your out of luck.

 

[hillbilly1]

Just read it again, the op is needing single phase, so just one transformer, but same situation, only one leg will be 120. As others have posted, better off with single phase 120/240 transformer.

 

[LarryFine]

From a previous thread:

EV Charger Voltage Range

Hello, Thanks for all the great help with my daily questions. Do you think that #1 THWN is large enough to feed most electric car chargers that use 40-amp, 240-volts? The breaker will be 50-amps, the receptacle will be NEMA 14-50R, the voltage is 208-volts, the distance is 400', the conduit...

forums.mikeholt.com

EV Charger Voltage Range

Hello, Thanks for all the great help with my daily questions. Do you think that #1 THWN is large enough to feed most electric car chargers that use 40-amp, 240-volts? The breaker will be 50-amps, the receptacle will be NEMA 14-50R, the voltage is 208-volts, the distance is 400', the conduit...

forums.mikeholt.com

 

[James L]

With the standard arrangement one leg will be 120V to neutral, but the other leg will be much more than 120V to neutral and the phasing is wrong for a true 120/240V supply.

Exactly. It boosts one leg to 158v, and the other leg simply feeds 120v through the terminal

 

[LarryFine]

What if:

Some time ago, I suggested that, if you select a unit so the secondaries will be connected in series, they could be wired to opposite ends of the primary, maintaining equal voltages to the neutral.
For example, looking at the boost diagram below, separate the secondaries and wire X3-X4 to the H4 end, and X1-X2 to the H1 end.

In other words, wire it like you would an open-delta setup, but with only one transformer, and no neutral connection to the transformer.
It would basically look like this diagram, but with one winding in each place where this diagram shows two (and there would be no "neutral" or B phase connection to the unit):

A 240v primary with a 12/24v secondary on 208v will develop a 20.8v boost, for a 228.8v output.

A 16/32v secondary on 208v will develop a 27.7v boost, for a 235.7v output. (all numbers theoretical).

What makes my suggestion "radical" is splitting up the secondaries to maintain equal L-N voltages.

I don't know why B-B transformer makers don't suggest this. Maybe we should ask if it's a good idea.

 

[winnie]

A 16/32v secondary on 208v will develop a 27.7v boost, for a 235.7v output. (all numbers theoretical).

What makes my suggestion "radical" is splitting up the secondaries to maintain equal L-N voltages.

IMHO your proposal works but is generally of little utility. It maintains equal L-N voltages, but produces weird L-N voltages that would not be useful if L-N voltage matters.

In the example you give, where a 240:16/32V transformer is used to boost 208V to 235.7V, you get L-N voltages of 132.2V. This might be low enough for a device expecting 120V L-N, but it is excessive.

If you actually need 120/240V single phase, then all of the autotransformer arrangements get complex and give at best a factor of 2 improvement in transformer kVA. At which point it is probably simpler to use a normal isolation transformer.

A useful suggestion to keep in your pocket for when L-N balance matters but exact L-N voltage is not relevant.

-Jon

 

[LarryFine]

In the example you give, where a 240:16/32V transformer is used to boost 208V to 235.7V, you get L-N voltages of 132.2V. This might be low enough for a device expecting 120V L-N, but it is excessive.

Wouldn't it be half of 235.7, or 117.85? I'm using the transformer CT as my reference, not the source neutral.

I know there would be a voltage between the two, because the center of two lines is not the center of the wye.

What would the L-N voltage be if you used the 12/24v configuration for the 228.8v L-L output as I described?

 

[wwhitney]

Wouldn't it be half of 235.7, or 117.85? I'm using the transformer CT as my reference, not the source neutral.

Then you'd be supplying equipment with a "neutral" that has a positive voltage to ground (60V).

Cheers, Wayne

 

[LarryFine]

Then you'd be supplying equipment with a "neutral" that has a positive voltage to ground (60V).

Yep, can't bond that puppy.

 

[jim dungar]

Wouldn't it be half of 235.7, or 117.85? I'm using the transformer CT as my reference, not the source neutral.

I know there would be a voltage between the two, because the center of two lines is not the center of the wye.

What would the L-N voltage be if you used the 12/24v configuration for the 228.8v L-L output as I described?

Don't forget the 120/208V exists because your source would be an open Wye (remember a phase shift is involved) not because you are taking the center point of a winding.
If you could find the perfect 208:240 buck-boost arrangement, you would still have an open wye of 136/240.

 

[winnie]

Wouldn't it be half of 235.7, or 117.85? I'm using the transformer CT as my reference, not the source neutral.

If you use the center tap as your voltage reference, then yes, the voltage is half of the total.

I know there would be a voltage between the two, because the center of two lines is not the center of the wye.

Exactly. Equipment is designed with the expectation that if the neutral is used as a circuit conductor, it is also a grounded conductor. In your proposed setup, the center tap 'neutral' is very much not grounded. The importance of this elevated neutral voltage depends heavily on how the equipment is designed. I wouldn't want an elevated neutral voltage on a screw shell lamp, for example.

What would the L-N voltage be if you used the 12/24v configuration for the 228.8v L-L output as I described?

I get 129.1V

-Jon