Industrial Control Panel Control Transformer question

[Bruno01]

I'm currently designing a control panel requiring a 240VAC 3 phase 4 wire supply (controlling motor loads). Though the majority of the loads will require the 240VAC 3 phase and the control 24VDC, I do need to install a 110VAC receptacle. Can I simply pull that power for the 110VAC receptacle from L1 and Neutral of the main supply (properly fusing it of course),or do I need to install a separate 240VAC to 110VAC transformer? Basically, do I need a separately derived neutral for my 110VAC loads? Appreciate the help.

 

[itsmoked]

Are you sure it's 240V with a neutral and not 208?

I'd use a line to line 240V to 120V control transformer with a fuse protecting the transformer primary. Then pick one of the two secondary leads and ground it with the system ground. That will define that side of the control transformer as the neutral for the outlet. Continue the ground also to the outlet ground as normal.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

If the panel is 240 Volts, four wire it may be a four wire delta. The voltages to ground from the three phases will be: 120V,120V, and 208V (Called the "High leg" or the "Wild leg".
If the supply is the more common 120:208 Volt system, you will have 120 Volts from any line to the neutral.
By the way, 110 Volts has not been a standard voltage in North America for over 60 years.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

If they are truly running the neutral wire out to the panel, you can use that, but the installation becomes more critical because you only have 120V from A or C phases to Neutral. The advantage to doing it with a CPT is that the incoming phase connections become irrelevant, not subject to the skill of the installing electrician or subsequent service techs. If the line power is a high amperage, the cost of running even a downsized neutral wire just for that control power is likely higher than the cost of a CPT.

That said, first check your 24VDC power supply, most of the good ones on the market will take anything from 90-270V or thereabouts, meaning you may not need a 120V source at all.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[LionelHutz]

Motors only need the 3 phase wires. Motors and motor controls don't need a neutral. No-one pulls (or wants to pull) 4 wires for motor loads. So, put a transformer phase to phase and only require the 3-phase wires for the panel.

 

[waross]

Don't forget this is not for control power,this is for a receptacle..
Rather than a 50 VA control power transformer we may be looking at a 2000 VA transformer.
I would compare the cost of running the fourth wire with the cost of the transformer before finalizing the design.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Good point.

The OP didn't indicate the size of system involved. Running a Neutral cable to a 300A panel would likely be cost prohibitive, running one for a 30A panel would likely not.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

Distance will be important also.
Once you determine the cost to purchase and install a suitable transformer, I suspect that the choice will be obvious for most installations to either install a neutral or use a transformer.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Is this a 120:208 Volt wye system or a 120:240 Volt four wire delta system?

Bill
--------------------
"Why not the best?"
Jimmy Carter