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Joined: Jul 2005
Posts: 2
djk101 Offline OP
Junior Member
I am building a glass melting furnace using low volt/high amp molybdenum elements.
I have a, single phase, dual 240/480 to 120/240 25kva transformer. I plan to run 240v in series through the primaries. I plan on jumpering x2 x3 to ground and using them as a center tap to get two 60v outputs at x1 and x4. How will that affect my transformer amp rating? Any problem with this setup?

Joined: Dec 2004
Posts: 329
The major limiting factors on a transformer are wire size and the core area. The wire size limits the current you can draw and the KVA is limited by the core area so:
25Kva/240 volt secondary=104.1666 amps
Theoretically if you keep the 60V load at that limit or less (104Amps) you should be fine as the lower voltage will keep you well within the power limits of the core. P=IE
120V*104A=12,480 KVA

[This message has been edited by IanR (edited 07-05-2005).]

Joined: Oct 2000
Posts: 2,722
Broom Pusher and
Hello djk101, and welcome to ECN!

Need to clarify something on your posted question.

Are you trying to get two 60 Volt 2 wire (or a 60/120 Volt 3 wire) outputs from this Transformer, or just a simple 120/240 VAC 1Ø 3 Wire output?

You can do the 120/240 VAC 1Ø 3 Wire setup by connecting X2 and X3 in Series, then "Tapping" this jumpered connection (at either X2 or X3), which will result in the Center Tapped Neutral conductor.
Voltage will be 120 VAC between X1 or X4 and the center tapped conductor, and 240 VAC from X1 to X4.

Unless this Transformer's Secondary has multiple taps on each Split Coil, you cannot get an output of 60 VAC from any two "X" points.
It sounds like a Split Coil Primary + Split Coil Secondary isolation Transformer - the common types used in "Buck-Boost" setups.
The Primary has 4 "H" terminals (H1, H2, H3, H4). For an input voltage of 240 VAC, H1 and H3 get connected together, H2 and H4 connect together (Parallel connection), and input goes on H1 and H4.

For an input of 480 VAC, only one jumper is used, which is placed between H2 and H3 (Series Additive connection) and input lands on H1 and H4.

Similar connections for the Secondary coils - 240 VAC output uses one jumper, connecting X2 and X3 in Series additive. 240 VAC output is between X1 and X4.
(this can also be the center tapped scheme I described before).
And, for an output of 120 VAC, X1 + X3 get connected, and X2 + X4 get connected (Parallel scheme).
120 VAC output is across X1 and X4.

As to the Apparent Power ratings of Split Coil Transformers, the combination of both Windings is where the ratings are applied.
For example, if the Transformer's Power rating is 10 KVA, each coil has a rating of only 5 KVA.

Let me know if I have mistakenly read your post, and if you have questions.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Feb 2005
Posts: 693
Scott, Doug's intention is to connect the primaries in series, i.e., for 480v, and then connect them to 240v, which will result in the secondaries to produce half voltage.

He then wants to connect the secondaries in series also, resulting in 120v with center tap from what would normally be 240v w/center tap, ending up with 2 x 60v. outputs.

He's concerned with the resulting available VA. Well, I'd say that Ian is correct: the same secondary current, at half the voltage, will result in half the KVA.

Doug, keep in mind that you can also use the secondaries individually or in parallel. If the two 60v loads are evenly divisible, you can series them for 120v use.

This would allow you to parallel both the primaries and the secondaries, again running the loads in series, and getting the full 25 KVA capacity.

Larry Fine
Fine Electric Co.
Joined: Jul 2005
Posts: 2
djk101 Offline OP
Junior Member
Ian, Scott, Larry,
Thank you all for your input.

Scott- Yes, two 60v, two-wire outputs, with a maximum 105 amps available from each. I have no intentions of using 120v.

Larry, your description of my set-up is precisely what I am doing.
I anticipated my output capacity to be cut in half to about 12.5 kva, which is very adequate for my application.
I will run a split, balanced load, at 60v.

Probably more than you want to know, but:
(I initially planned 120v in series through all my elements. My understanding of molybdenum heating elements, and real-world practice, is that they run best at lower voltages and high amperages. (there might be an arcing problem, or something. I don't know) Thus, the decision to split the series.

Their resistance is so low that they are almost at a dead short when cold, with resistance rising with the temperature, so I will run a phase-angle fired SCR with Current Limit to control them. The Watlow SCR is designed for transformer-coupled loads. The furnace is capable of 3000 Deg.F but will be idled at 1800F-2200F with 2400F maximum.)


Joined: Feb 2005
Posts: 693
Doug, keep in mind that SCR's only conduct in one direction, like a diode. This will limit your power to about half, like a half-wave rectifier.

Unless you're going to "69" SCR's for a full-wave current, look into using triacs (unless your controller already is a full-wave device.)

"The Watlow SCR is designed for transformer-coupled loads."

Sort of a solid-state variac?

Larry Fine
Fine Electric Co.

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