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#63430 - 03/15/06 01:03 AM newbie question  
Clydesdale  Offline
Joined: Nov 2004
Posts: 138
I was reading about Buck/Boost transformers in another post and was wondering what it was and does....seems to me it has something to do with zeroing in on a specific voltage with quality control as a priority.

Work Gear for Electricians and the Trades

#63431 - 03/15/06 01:16 AM Re: newbie question  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,057
It is just a step down transformer you wire in series with the load.

If you wire one end of the secondary to the line and the other to the load it will either add to (boost) the voltage impressed on the load or subtract from (buck) the voltage, depending on the polarity of the connection.
Say I have 208 and I need 240 I have a transformer giving me 32 volts to add to the 208. If I am going the other way (240>208) I use the same transformer wired the other way to "buck" it down to 208.

[This message has been edited by gfretwell (edited 03-15-2006).]

Greg Fretwell

#63432 - 03/15/06 09:51 AM Re: newbie question  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
In a Nutshell, a Voltage Bucking / Voltage Boosting Transformer is a small sized option to increase / decrease system Voltage for a given piece of Equipment.

What is being done is taking a relatively small size Isolated Transformer - a Transformer with Primary windings and Secondary windings which are not physically connected together, and making an Autotransformer out of it.

The Transformers used typically have Secondary Voltages in the area of 12/24V or 16/32V, and have VA (Volt-Amp) ratings in the 250VA through 2.0 KVA ranges.

The advantage comes in the small size - in both physical size and Apparent Power (Volt-Amps), vs. the connected load.


If a certain piece of Equipment required 5.0 KVA @ 132 VAC (Volts Alternating Current), a 120v x 12v Transformer with a 500 Volt-Amp rating could be used to achieve the increase in Voltage, yet have ample capacity to pass the 5.0 KVA load - even though the Transformer's rating is only 10% of that figure - or 500 VA.

The connection scheme results in an Autotransformer being created from an Isolated Transformer.

The Primary winding is connected as normal - across line Voltage. The "Common" side is tapped to the Transformer on one side, then continues to the load, where it is terminated.
The "Other Side" of the 2 wire circuit is terminated to the "Beginning" of the Secondary winding (X1 for example) & the remaining Primary lead is tapped to the same line.
The "Output" from the Transformer comes from the "End" of the Secondary winding (X2 for example), and is the "New" lead to the load equipment.
This "New Lead" and the "Common Lead" are the 2 wire Circuit to the load equipment, which has an increased Voltage.

To lower the Voltage, switch connections so the Line Current flows through the Secondary winding - from X2 to X1, then taps to the Primary lead, then goes to the load device.

To create a Reactor (AKA "Choke") - which limits current flow via "Ballasting Action", simply connect the Transformer as Voltage Boosting, but place the terminations so they are not "In Phase Polarity".
This will boost voltage slightly, but limit current flow to a certain value - like a Ballast for Fluorescent and HID Lighting does.

Take a look at the Schematics for Voltage Boosting setups, located in the Technical Reference section.

Here are some links:

Voltage Boost Transformer setups - 1 Phase

Voltage Boost Setups - Open Delta

Voltage Boost setups - Wye connected

Buck, Boost, Choke and Polarity Tests

Here's a simple Schematic for fun!

[Linked Image]


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#63433 - 03/22/06 07:37 PM Re: newbie question  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA


Any remarks or other Q's?


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#63434 - 03/22/06 09:46 PM Re: newbie question  
HotLine1  Offline

Joined: Apr 2002
Posts: 6,802
Brick, NJ USA
That's the most thorough to-the-point explanation I've seen!! I'm printing it to show to my class.



#63435 - 03/23/06 07:54 PM Re: newbie question  
Clydesdale  Offline
Joined: Nov 2004
Posts: 138
thank you for the responses!

how does the current travel through the transformer? from H1 to X1, H2 to X2 ect.?

#63436 - 03/23/06 09:32 PM Re: newbie question  
n1ist  Offline
Joined: Feb 2002
Posts: 183
Malden MA
For boost, you are feeding the primary (H1, H2) as normal and putting the secondary (X1, X2) in series between the line and load. It must be connected in-phase to add its voltage to the line.

For buck, you do the same configuration, but feed it backwards. The line feeds the series combination of primary and secondary, and the comes from the tap point.

Note that this is an autotransformer, and does not offer the isolation that a conventional transformer hookup would have.

[Linked Image]

[Edited to fix problem in the buck description and add schematic]

[This message has been edited by n1ist (edited 03-24-2006).]

#63437 - 03/26/06 11:05 PM Re: newbie question  
Clydesdale  Offline
Joined: Nov 2004
Posts: 138
Thanks everyone very much for the information. I should perhaps obtain one of these transformers to see exactly how I would wire it up in the field. Thanks again!

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