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I'm seeking to better understand the workings of a Square D medium voltage MCC by reviewing the schematics. It operates the attached motor on 3-phase 4160VAC.

Since the unit also needs 120VAC single-phase power to operate the control circuitry (relays, monitoring equipment, etc), there is a transformer, CP1, as shown below, to output 120V from 2 legs of the 3-phase supply.

In the snippet below, you can see the drawing of the 4160 to 120 VAC transformer. I don't understand why the 120VAC secondary side is drawn as two windings in parallel, with X2 and X3 cross-connected to the opposite 120VAC legs, instead of a single winding. What is the benefit of this configuration? Is it higher current output, less heat, or maybe just the only schematic the designer had handy in his CAD software :) ?

Disclaimer: I would never venture to work on any medium voltage equipment. We have EEs and qualified technicians for that. I work with low voltage control signals only. I was perusing the schematics to understand some control wiring (low voltage) associated with this MCC. I have no experience or qualifications in medium voltage work but was just curious when I ran across this in the schematics. It never hurts to understand more than you need to for your job.

enter image description here

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    \$\begingroup\$ Almost all transformers are like this. If you're looking at your first one ever and it's a 4160VAC unit - BE CAREFUL. Seriously. Things like this should not be surprising you when you're handling this much power. It should make you worry about what else might surprise you. How are you involved in this project? In almost every jurisdiction in the world you need to be qualified to either design or install such things and being qualified you would not have this question.... this feels really scary to me. \$\endgroup\$
    – J...
    Commented Feb 4, 2022 at 16:12
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    \$\begingroup\$ @J... don't worry I would never venture to work on any medium voltage equipment. We have EEs and qualified technicians for that. I work with low voltage control signals only. I was perusing the schematics to understand some control wiring (low voltage) associated with this MCC. I have no experience or qualifications in medium voltage work but was just curious when I ran across this in the schematics. It never hurts to understand more than you need to for your job. Thank you for the cautionary words. \$\endgroup\$ Commented Feb 4, 2022 at 20:38
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    \$\begingroup\$ Ok, good to know! And yes, nothing wrong with learning, absolutely. You'd be surprised what some people try to get away with! The facility SLD suggests that you have internal access to these drawings so it always makes me nervous thinking it might be some cowboy trying to save a few bucks or something. Makes sense now. \$\endgroup\$
    – J...
    Commented Feb 4, 2022 at 21:00

2 Answers 2

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The transformer allows for multiple different configurations. It has two separate 120VAC secondaries which are completely separate.

It allows the two 120VAC secondaries to be connected in parallel like in the picture to allow a 120VAC output with double current than using single secondary alone.

It also allows two 120VAC secondaries to be connected as two completely separate 120VAC outputs that are isolated from each other.

It also allows the two 120VAC secondaries to be connected in series to have a single 240VAC output.

And it allows to have the two 120VAC secondaries to be connected in series to have a typical North American split phase output with center tapped neutral and two 120VAC live phases.

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    \$\begingroup\$ @RyanGriggs: You will also find transformers with dual primary windings to allow use with 120 or 240 Volt inputs. \$\endgroup\$ Commented Feb 3, 2022 at 20:13
  • \$\begingroup\$ @PeterBennett - that would be rare as it does not make effective use of the winding space in the transformer so it would not be as efficient. The most common method I have seen is as seen in the diagram. For small changes in input voltage there may be taps to provide 200, 220, 240V input. \$\endgroup\$ Commented Feb 5, 2022 at 1:26
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It's so you can wire it up different ways to get different combinations of output voltage/current. Take a look at page 11 of this brochure to see some examples.

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  • \$\begingroup\$ Makes sense: it's for configurability - use the same transformer for multiple applications. \$\endgroup\$ Commented Feb 3, 2022 at 20:10

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