Buck Boost Trans

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I'VE GOT A QUESTION THAT EVERYONE IVE TALKED TO CANT ANSWER, AND IT TO ME THIS WOULD BE A VERY COMMON SCENARIO, I GOT A MACHINE THAT IS 30 AMP THREE PHASE NO NEUTRAL WITH 230 VOLT MOTORS ON IT,, THE SUPPLY VOLTS ARE 3 PHASE 208 VOLT, THE COMPUTER IS SHUTING IT OFF BECAUSE OF LOW VOLTAGE, ... SO IVE ORDERED BUCK BOOST TRANSFORMER AT 1 KVA.. BUT DO I NEED THREE OR TWO TRANSFORMERS?..THE GUY AT THE SUPPLY HOUSE DOESNT KNOW I ASKED IF ITS THREE PHASE OR SINGLE .. HE SAYS ITS BOTH... I THINK HE'S WRONG . BUCK BOOST ARE USAULLY SINGLE PHASE RIGHT ?... WILL I NEED 2 OR 3 AND IF TWO DOES IT MATTER WHICH TWO PHASE'S YOU USE( OPEN DELTA)......... IM GONNA BE DOWN ANOTHER DAY IF I ONLY HAVE ONE TRANSFORMER.......THANKS MUCH APPRECIATED:confused:
 
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The picture I have is you are considering putting a 19V buck-boost on two of the three 120V legs. If you put them on A and B, then your voltages will be:
A - N 139V
B - N 139V
C - N 120V
A - B 240V
B - C 225V
C - A 225V

That doesn't look good to me. I would think you would need three b-b xfm to use this method.

Have you considered using a 208D/240D(or Y) - 12.5kVA, and either be ungrounded D or grounded Y to the equipment?

Mild disclaimer: I have not ever had to do this, so hopefully others have some hands-on experience.

carl
 
No neutral connection to the load, means no neutral required by the transformer.

You can use an open-delta (2 unit) buck-boost transformer arrangement. This is the most common setup I have seen in manufacturers catalogs.

Typically the wye (3 unit) arrangements are used only when the kVA requirements are too great. Personally I have never been an advocatefor using a wye arrangement to change the entire 208Y/120 system, I prefer to buck/boost single phase loads independently.
 
I've seen two BB's used for 3-phase, but it always strikes me as weird. I'd prefer three.

Creative? How about three 120-to-240 transformers connected Y-D?
 
LarryFine said:
I've seen two BB's used for 3-phase, but it always strikes me as weird. I'd prefer three.

Creative? How about three 120-to-240 transformers connected Y-D?
Click to expand...

What would you do with the neutral on the primary side? With three 120V transformers, I would think that you would be required to connect the neutral, but when you use a Y-D transformer, I thought that it was a bad idea to connect the neutral on the primary side. (I don't remember the details, something about imbalanced load on the delta side showing up as neutral current on the wye side.)

If you are in a situation where buck/boost transformers are suitable, you end up with much smaller (cheaper) transformers and less loss in the transformer windings (cheaper), with simpler grounding and bonding (cheaper), then if you use an isolation transformer.

-Jon
 
winnie said:
What would you do with the neutral on the primary side? With three 120V transformers, I would think that you would be required to connect the neutral, but when you use a Y-D transformer, I thought that it was a bad idea to connect the neutral on the primary side. (I don't remember the details, something about imbalanced load on the delta side showing up as neutral current on the wye side.)...

-Jon
Click to expand...

Actually the delta secondary configuration will allow currents to circulate inside the delta thereby balancing the wye system primary side if the wye is grounded. If you don't ground it, everything will be OK.

Jim T
 
With this application (2) BB transformers will be fine. While supporting BB transformers as an applicatons engineer I have never come across any issues with an applicaion such as you have described. But remember your intent is to "tweek" the voltage but you will not get any isolation such as a common transorfmer would provide.
 
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