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I've seen a control circuit that uses a transformer in the plant. The supply voltage is 380V. What is the reason for this? Would it not be easier just to tap one phase and use neutral as your return path in order to get the same 220V? Infact, what is the general purpose of using a transformer for the control circuit? Maybe that's the point I'm missing.

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  • \$\begingroup\$ Balancing the load across the phases is one possibility. \$\endgroup\$
    – Solar Mike
    Aug 16, 2019 at 4:32

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In the industrial network the neutral is not always present. Most machines have three phase and PE connection. PE is used only for safety purposes, so no device may use it as power supply connection.

Another fact when using neutral and phase is that if neutral gets missing, your control voltage will become high, blowing up the PLC and other control circuit.

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I have an oven supplied with 80A 208V 3Ø. While the building (and panel) has a neutral, there is not a neutral supplied to the oven. Therefore, to get 115VAC 1Ø voltage as used by the control circuits, there is a 208V -> 115V transformer.

I converted this rotational molding machine from 480V to 208V and realize the main purpose for not supplying neutral to the machine was to not have to run a 5th 4 AWG conductor. Another portion of the machine is powered separately from its own subpanel and I chose to run a neutral conductor so I could use 115VAC 1Ø without using a transformer. This complicates the design somewhat in that the neutral/return now should have its own circuit breaker. Additionally, I made it so that this other part of the machine is protected with a 3Ø 30A breaker and also supplied via a contactor. This contactor is powered by the neutral from that aforementioned 115VAC circuit.

This highlights further design consideration: with 3 phases to supply voltage to the neutral/return, how should this path be protected. After completing the oven part of the machine, I observed less than 3A on the 115VAC control circuit and in total, the 208V 3Ø is under 35A per phase. For that other part of the machine's equipment, I chose 10A of protection on the neutral/return. Now, should any of the phases exceed 30A OR the neutral exceed 10A, all voltage paths to this piece of equipment are opened.

Rewiring this oven (and other part of the machine) has been eye opening to me. I have a BSEE and the choices in the design, while safe, were not what I personally would consider ideal. The entirety of my electrical career has been a study of where practicality and ivory towers meet, continuously fueling my sarcasm and irony. Before attending university, I was an Electronics Technician in the US Navy. In "the field" or "the lab" is reality. Reality doesn't care what "should be" or what the books said.

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  • \$\begingroup\$ Why do you think the neutral (second paragraph) required a circuit breaker? If that trips your star / wye point will float and P-N voltage may exceed that of the equipment. \$\endgroup\$
    – Transistor
    Aug 16, 2019 at 6:31
  • \$\begingroup\$ I intend for the 115V to only have low amperage requirements. Without the separate 10A breaker on the return, it would take the full 30A protecting the 3 phase motors to trip before protection kicked in. The 115V breaker could go anywhere but the safest and most conservative location was at the subpanel - to include supplying the contactor supplying the 208V 3Ø to begin with. \$\endgroup\$
    – Krista K
    Aug 16, 2019 at 6:37

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