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#185936 - 04/06/09 11:34 AM Buck and Boost Xfrmer
Niko Offline
Member

Registered: 08/17/06
Posts: 358
Loc: Campbell, CA
Hello All,

I have a light industrial client that needs 120-240 3 phase for their machinery and the supply is 120-208 3 phase.I am planning to use (3) buck-boost transformer in order to get the voltage that I need.

My question is the grounded/neutral that i need on the secondary side (240-120) is the same grounded point that is available on the 120-208 primary side. Right?

Because this is a buck and boost set up and the primary and secondary windings are electrically connected. Am i correct with this explanation?

Thank you.
Edward
_________________________
Be kind to your neighbor, he knows where you live


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#185940 - 04/06/09 10:08 PM Re: Buck and Boost Xfrmer [Re: Niko]
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
Buck boost transformers are not usually the preferred method for creating a 4 wire secondary. When you raise the line-line voltage to approx 240V you will also raise the line-neutral voltage to about 136V.

Do you really need the 4th wire for your load?

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#185942 - 04/07/09 12:13 AM Re: Buck and Boost Xfrmer [Re: JBD]
Niko Offline
Member

Registered: 08/17/06
Posts: 358
Loc: Campbell, CA
I don't really need it. But it would be nice to have. Otherwise i can install new 120V circuits from the main panel.

Thanks.
_________________________
Be kind to your neighbor, he knows where you live


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#185943 - 04/07/09 12:16 AM Re: Buck and Boost Xfrmer [Re: JBD]
frenchelectrican Offline

Member

Registered: 02/06/03
Posts: 938
Loc: Wi/ Paris France { France for ...
The B/B requirement will be depending on the connection on the equiment if you do need the netural for the equiment then you must use 3 B/B in wye format and follow the manufacter connection diagram very carefully if not need to use the netrual at all then 2 will work.

However before you get the B/B make sure you double check the nameplate of that equiment if can run on 208 volts if not then use the B/B { keep in your mind you will have to size the B/B }

Merci,Marc
_________________________
Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)


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#185945 - 04/07/09 09:33 AM Re: Buck and Boost Xfrmer [Re: Niko]
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
Originally Posted By: Niko
I don't really need it. But it would be nice to have. Otherwise i can install new 120V circuits from the main panel.

Thanks.


Unless 136V L-N is nice to have, do not run the neutral to the load.

More than 90% of the B-B installations I have ever designed have been 2 transformers in open-delta. The only time I have gone to a wye connection is for the larger capacity it offers.

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#185947 - 04/07/09 11:48 AM Re: Buck and Boost Xfrmer [Re: JBD]
BrianP Offline
Member

Registered: 02/01/07
Posts: 65
Loc: Minnesota
120/240V 3-phase sounds to me like a center-grounded (high-leg) delta. You can't get 120/240 with a wye configuration. It is either 120/208 or 138/240. If you need the 120/240 delta, you can't do that with boost transformers.

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#185950 - 04/07/09 01:46 PM Re: Buck and Boost Xfrmer [Re: BrianP]
Niko Offline
Member

Registered: 08/17/06
Posts: 358
Loc: Campbell, CA
Thank you all for your input.
I will need to install 120V circuits from the main panel and only do line-line load off of the BB transformers.



Thanks again.
_________________________
Be kind to your neighbor, he knows where you live


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#186033 - 04/12/09 02:29 AM Re: Buck and Boost Xfrmer [Re: Niko]
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Niko:

I have a few comments per your post.

Quote:

I have a light industrial client that needs 120-240 3 phase for their machinery and the supply is 120-208 3 phase.
I am planning to use (3) buck-boost transformer in order to get the voltage that I need.


Please refer to the comments below - denoted "Customer's Machinery Requirements"


Quote:

My question is the grounded/neutral that I need on the secondary side (240-120) is the same grounded point that is available on the 120-208 primary side. Right?


Correct.
The Grounded Conductor of the System is _"COMMON"_ to all (3) Transformers in the Wye configuration, and as typical with any Auto Transformer configuration, the "Output" is NOT an SDS (Separately Derived System).
Therefore, the Common Grounded Conductor of the Auto Transformer's Output, is still "common" to the Transformer feeding the System, for which the Auto Transformer originated.

Here are some Drawings of the 4 Wire Wye Auto Transformer configuration:


SCHEMATIC: 1 LINE DIAGRAM


SCHEMATIC: PICTORIAL DIAGRAM

Refer to Voltage Boost Transformer Schematics - Wye Connection for more information.


Quote:

Because this is a buck and boost set up and the primary and secondary windings are electrically connected.
Am I correct with this explanation?


Yes, you are correct.
In "NEC Lingo", the Output IS NOT an SDS.

................................................
................................................

"Customer's Machinery Requirements"

If the Customer's Equipment Matrix requires a large new connected Load value, you might consider using a Delta / Delta Step-Up Transformer to feed one or more Panelboards for the Equipment.

You could use a Transformer configured as either:

*A: 208V 3 Phase 3 Wire Input x 240/120V 3 Phase 4 Wire Output;

or;

*B: 208V 3 Phase 3 Wire Input x 240V 3 Phase 3 Wire Output.

Feed a Panelboard from the Transformer. Connect the Equipment requiring 240V to the new Panelboard.

*** Reason for using "Option "*A" above (the 4 Wire Delta):

1: If the Equipment contains Auxiliary Circuitry derived directly from the Branch Circuit, which is rated at 120V, use the 4 Wire Delta configuration.
Make sure to verify the auxiliary "120V" load connections are only tapped from Line A or Line C (the Lines with 120 Volts to ground).

2: Use of "Standard" 120/240V rated OCPDs (Over Current Protection Devices) in the new Panelboard, instead of "Fully Rated 240V" devices.

3: Disconnect Switches for the Equipment may be 3 Pole type, without the need of any Auxiliary Contacts (to disconnect a separate 120V control circuit), or an additional stand-alone Disconnect for the aux. circuit (if allowed per AHJ).

If the Equipment DOES NOT REQUIRE a 120V Circuit derived from the Branch Circuit directly (tapped from Branch Circuit, with a Grounded Conductor brought in with the Branch Circuit), then use the 3 Phase 3 Wire Delta Output Transformer "Option *B", feeding a Panelboard.

The OCPDs in this Panelboard shall be "240V Rated". Do not use "Slash Rated" devices - i.e.: 120/240V Rated devices.

If you propose to use the 240/120V 3 Phase 4 Wire Delta Transformer - with an SDS Output of 240/120V 3 Phase 4 Wire, perform the following:

1: Connect the Transformer Terminal "X0" to the GES (Grounding Electrode System), along with the Equipment Grounding Conductors (EGC) - if used.
Bond the Transformer enclosure to "X0" also.
The System's Grounded Conductor (AKA "Neutral") will derive from "X0".

2: Color Code the Conductor deriving from Transformer terminal "X0" with White Coloring (Tape, etc.).
Terminate this Conductor to the "Neutral Bus Kit" in the Panelboard.
*** NOTE: remove any Ground bonds between the Neutral Bus and the Enclosure, & / or auxiliary Ground Bus Kits.

3: Color Code the Conductor deriving from Transformer Terminal "X2" with Orange Coloring (Tape, etc.).
Terminate this Conductor in Position "B" on the Panelboard's Bus Kit (via the Panelboard's Main Breaker).
This Bus will have +/- 208V to Ground.

4: Color Code for the remaining Two Ungrounded Conductors may be what ever choice you wish.

5: Bond the Transformer's Enclosure to the "Primary Circuit's EGC", in addition to the Secondary side bond.

5: Label the Panelboard's System Voltage accordingly (240/120V 3 Phase 4 Wire Delta).
Make note of the L-G Voltage for Phase "B" is +/- 208V.
Install a "System Color Code" guide for each System's Color Code.


If you propose to derive a new System with only one Voltage available (240V 3 Phase 3 Wire), then either use a 240V 3 Phase 3 Wire Delta Output Transformer ("D1"), or the before mentioned 3 Phase 4 Wire Delta Transformer, without bringing the Grounded Conductor to the Panelboard ("D2").

* D1: (3 Phase 3 Wire Delta Output):

1: Choose One "X" Terminal for Grounding, thereby creating a "Corner Grounded Delta".
For example, Terminal "X3" is chosen, which results in Phase "C" becoming the new System's Grounded Conductor.

Connect the GES, EGCs and Transformer's Enclosure bond to Terminal "X3".

Color Code this Conductor White.

2: Color Code the remaining Two Ungrounded Conductors as needed.

3: The Grounded Conductor may be Terminated in the Panelboard in one of the following methods:

a: Across the Bus Kit - via the Panelboard's main Circuit Breaker:

In this case, the Panelboard would be a 3 Phase Panelboard.
The setup would have the Grounded Conductor terminated in "Phase C" position.
The Conductor from Transformer terminal "X1" will be in the "Phase A" position.
The Conductor from Transformer terminal "X2" will be in the "Phase B" position.

The "Neutral Bus Kit" may be used for EGCs, and should be bonded to the Enclosure.

b: Terminated to the "Neutral Bus Kit":

In this case, the Panelboard would be a 1 Phase 3 Wire Panelboard - with a Neutral Bus Kit.
The setup would have the Grounded Conductor terminated to the Neutral Bus Kit, and would not feed through the Main OCPD for the Panelboard.
Lines "A" and "B" would feed through a 2 Pole main Breaker, installed in the Panelboard on the Line side of the Bus Kit.
(Alternate option would be a Fusible Disconnect external to the Panelboard, connected between the Transformer and the Panelboard.)

The Conductor from Transformer terminal "X1" will be in the "Phase A" position.
The Conductor from Transformer terminal "X2" will be in the "Phase B" position.

The "Neutral Bus Kit" MAY NOT be used for EGCs, and SHOULD NOT be bonded to the Enclosure.

4: OCPDs shall be 240V Rated.

* D2: (3 Phase 4 Wire Delta Output):

Perform the same as would be done with a "Normal" 3 Phase 4 Wire Delta output, altering the following:

a: Ground the System via the "X0" Terminal.

b: DO NOT bring a Grounded Conductor from X0 Terminal to the Panelboard.

c: Bring an EGC to the Panelboard from terminal X0.

d: Terminate this EGC to the Panelboard's Neutral Bus Kit.

e: Bond the Neutral Bus Kit to the Panelboard's Enclosure.

f: Run the Three Ungrounded Conductors, deriving from terminals X1, X2 and X3 to the Panelboard's Bus Kit via the Main OCPD.

Option "D2" reduces the Voltage to Ground to a maximum of 208 Volts, with a "66% Chance' of 120 Volts to Ground.

Slash rated Breakers _MIGHT_ be usable. Verify with the AHJ.


----------------------------------------------------------
----------------------------------------------------------

If the use of Voltage Boost Auto Transformers is preferred over an SDS as described above, use the Open Delta configuration shown below:


SCHEMATIC: 1 LINE DIAGRAM


SCHEMATIC: PICTORIAL DIAGRAM

This configuration results in an output Voltage of 252 Volts 3 Phase 3 Wire.

Refer to "Voltage Boost Transformer Schematics - Open Delta Connections" page in the Technical Reference Section, at this URL:
http://www.electrical-contractor.net/for...Sche#Post148713

Contact me if you have questions or comments.

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

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#186042 - 04/12/09 07:32 PM Re: Buck and Boost Xfrmer [Re: Niko]
Niko Offline
Member

Registered: 08/17/06
Posts: 358
Loc: Campbell, CA

Thanks again to all of your replies, i learn something every day by just visiting the forums.


Scott,

Thank you very much for the detailed explanation.
_________________________
Be kind to your neighbor, he knows where you live


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#187595 - 07/02/09 10:59 PM Re: Buck and Boost Xfrmer [Re: Niko]
BobV3CORP Offline
Member

Registered: 02/28/06
Posts: 22
Loc: Watertown, Ma USA
Scott,

That was an interesting lesson. Thanks.
_________________________
BobV

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