[OT-CNC]

Hi,

I need some guidance/education on control transformers. Here is what I'm looking to do:
I have 3phase lathe I'm converting to CNC. 3phase in is high leg delta 240v. I'm using this for the spindle and drives, which works great so far. In addition I need single phase 120v to power computer, 24v power supply, 5v power supply and misc relays.

The 3 phase in does not include a neutral wire, just a ground. I do not plan on pulling in a separate neutral. I would like to buy a control transformer that provides 120v with a neutral or use the existing transformer in the machine if possible.
I assume I can wire a control transformer to my 2 legs that are not the high leg, which gets me 240v single phase in on the input. The output will be stepped down to 120 if wired for 2:1.
How is the neutral generated?
The reason I ask is that the current old transformer in the machine gives me 120 when I check the output taps, but each output to ground gives me 80v and 40v. Isn't neutral and ground supposed to be at the same potential?
Unfortunately I have no info on this transformer, its off a Hardinge AHC lathe.
Any suggestions welcome.

Thanks
-OT

[greggv]

A 240 to 120 transformer will do ya. Ground one of the 120 legs and call it common also. You can use any of the three legs. Also, most power supplys will take 120-240 on the input without jumpers or switches. You might check that out, if all you need is 24 vdc, I don't think you need a transformer at all.

[Al_The_Man]

If you want a grounded 120vac control supply, as mentioned, you can earth ground one side of the secondary of the transformer right at the terminal and this is then the neutral.
You must either use the service ground, if provided, or set up a separate recognized ground by using ground rod and/or earthing any metallic water supply.
Al.

[OT-CNC]

Thank you for the replies. Seems simple enough. Any benefits of tying the leg with the lower to ground reading to ground? In my case the 40v one?
Also, I found the name plate of the transformer and it's rated at .750kva. this should give me around 6.25A at 120v correct? If so, what size breaker is recommended on the input side?
Last, will a 6A load create enough of an imbalance to be of concern on a 50A 3phase line?

[Al_The_Man]

6.25a would be correct, a 15amp breaker and a local 10amp fuse would be the best scenario.
40v is not really necessary, if you rectify it and make a DC supply the Common or -ve could be grounded, but not both the AC and DC side.
The 6a load will not even be seen. In any case it does not draw a constant 6a, only at full load.
Al.