120V 15A circuit in control panel with no neutral

[lesmar96]

I have an OEM requesting a 15A 120V circuit in their control panels. This control panel is going on a moveable piece of equipment so it will not be plugged in at the same place all the time and sometimes will be run off of a generator. Thus we cannot guarantee that a neutral will always be available.

Models will be in 230V 3PH or 460V 3PH. Is the best way to stick a 2KVA transformer on? Am I missing anything? Is there a better way?

What is the best way to get 15A 120V?

 

[Christoff84]

I would say the best way would be to put on a transformer, that way you are making your own neutral. Just remember, you need to account for both primary voltages.

 

[g.mccormick]

To understand better, you will have a control panel that is fed by 230 or 460V 3ph power and you need to have 120V 1ph control power inside. You will need to use a transformer. You can ground the neutral on the secondary side of the transformer to create a neutral connection.

 

[Aabeck]

If the 120V is for a programming PC make sure you fuse/circuit breaker it for only 4 amps (no vacuum cleaners or barrel pumps)

Also, I would put a line filter on it JIC.

 

[James Mcquade]

you also need to connect the x2 terminal of the 120 side of the transformer to panel ground to prevent a floating voltage.

also be sure to have a voltage sticker on the panels designating the incoming power. you don't need 460 volts on a transformer designed for 230.

james

 

[lesmar96]

you also need to connect the x2 terminal of the 120 side of the transformer to panel ground to prevent a floating voltage.

also be sure to have a voltage sticker on the panels designating the incoming power. you don't need 460 volts on a transformer designed for 230.

james

Good Advice. Thanks!

 

[AustralIan]

Normally, you would use a centre tapped 120V transformer, so you have only 60V AC difference between each conductor and the chassis. Here is a diagram.

https://duckduckgo.com/?q=centre+ta...uploads/2013/06/Centre-Tapped-Transformer.jpg

Vp=460, Va=-Vb=60, Vtotal=120

 

[jraef]

Technically, the definition of a "neutral" circuit does not include that it be grounded. Plenty of people use a "floating" neutral system. Everything on a boat is that way. All that matters to your control system equipment is that there is 120V potential between the two terminals of the device. Bonding the neutral to ground is just an added safety feature (for land lubbers).

 

[Aabeck]

Not boats, but I work on old model stamping presses that require double redundancy in the controls.

Two ways of doing this are twice the normal amount of relays, or put the relay coil in the center of an ungrounded circuit with duplicate contacts and timers on each side of the coils from the 2 sides of the 120V.

 

[James Mcquade]

jaref,

you are correct, it is an added safety.
i have always designed control panels this way ever since i did an install at a plant that had a 75 volt floating ground. we had to modify the machine that required extra mechanisms and had to get under the machine. when you pulled the trigger of the 120 volt drill, you got lit up. i didn't believe the guys working with me, so i tried to drill the hole myself.
we told the plant manager and he contacted others. turns out that the plant xformer was wired incorrectly, a big mess.

james