Help me understand correct method to use a control transformer 2

[Langless28]

I have a machine that is single phase 240vac operation. Power in is L1,L2, and the grounding conductor. I would like to step down the voltage to 120vac so i can power a single convenience outlet meant to only power a small ionizing fan that is to be used on the unit.

The control transformer is 240/440 primary with 120vac secondary coil (single coil). From what i have read is you are supposed to obtain the grounded (neutral) conductor by bonding X2 of the secondary to ground. My questions are as follows:

1. Is this the correct method? From what i have always understood was that the grounded and grounding conductors are only ever tied together at the main panel.
2. If it is how you do it, can it be bonded directly at the place of installation? does it need a dedicated groundING conductor all the way to the main power entry of the machine?
3. Does it need a neutral? can the voltage be "floating". If so, what are the benefits and negatives of this?

 

[itsmoked]

1) Yes it is the correct method. The transformer creates a "New Source" unrelated to the facility's existing one, hence the need to assign a ground reference to it.

2) You should use the ground that came with the 240Vac you mentioned, that way there will not be any potential difference between the two grounds in your machine/system. You can ALSO then ground locally if you still desire.

3)No! And not allowed in the USA.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

This is the correct method. You will need to still have the grounding conductor for the control panel run back to your source, and bonded to the panel, do not create a new grounding conductor at the installation site (although you can bond it to your building ground if necessary, ie if you have VFDs or other sources of common mode noise). But the grounding of the secondary of the control transformer is like a new separate source for 120V power to use Inside of that panel, so the hounded secondary becomes the neutral for your 120V circuit from that point on. Floating 120V without reference to ground is possible, but not a good idea because people troubleshooting it later will be EXPECTING it to be referenced to ground. I used to have a designer working with me who came from the Navy, he never bonded the CPT neutral to ground, he claimed they never did that on ships. But in the industrial world it was a never ending source of field problems.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[jraef]

LOL, I took too long to pull the trigger on that one and Keith beat me to it, yet I still made a spelling error (hounded instead of grounded). Good to see that Keith and I are on the same page however.

But Keith, 250.21 allows for the choice to ground or not ground the secondary of a CPT circuit if it is used just as control power. Whether or not a light fixture inside of a control panel is "control power" or not might be debatable however. But still even so, just because you can doesn't mean you should...


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[itsmoked]

I knew I was under-the-gun so worked feverishly!! LOL I did force myself to proof read it though.

I did not recall 250.21! Seems ridiculous anyway to leave it all so ambiguous.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

They have to accommodate all those old Navy guys I guess...


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington