24vac transformer size for Allinone DC upgrade

[Joe]

Hello,

Just a quick question on what size (in VA) would be a good choice for the 24vac transformer in our upgrade? I know it would depend on how may relays that I am to install, but I am just going to be using it just for the EStop circuit (as recommended). It appears on the drawings that there is a 5amp fuse in the circuit that uses the Centroid T1 transformer (which I know that the drawings are somewhat generic), but that value still seems large for the application. My upgrade is on a Bridgeport V2XT machine and I was planning on using the main transformers already on the machine, for which I do not have 24vac available. The installation will be the basic V2XT conversion without any abnormal bells and whistles.

On another note, we did just receive our new AllinOne DC controller, have not connected to it or bench tested yet, but am still gathering as much information as I can on a few more components and wiring schemes, before moving forward.

Thanks in advance

Joe

[cncsnw]

If you only power the one contactor with it, then you need to look up the specs for the contactor you are using.

If you use the Telemecanique LC1DT40-B7 contactor, then I think the inrush is about 100VA, and the sealed current is about 9VA.
If you use a Fuji SC-E2S-24VAC, inrush is about 120VA, and sealed current is about 13VA.

You probably don't need to size your transformer for the inrush, since that only lasts a few milliseconds. Just use time-delay fuses. Allow at least a couple times the total sealed draw.

On the other hand, the price and size differences between 50VA, 75VA and 100VA transformers are negligible. There is little reason to go smaller than 100VA.

[Joe]

Thanks, that sets me straight.

Joe

[martyscncgarage]

Follow up, kind of related.
On a knee mill conversion, you generally only need 24vac for the Estop contactor CNT1.
Requiring the 24vac transformer for it, yet the AIO DC power supply has 24vdc. Why not just use an Estop contactor with a 24vdc coil and use the power supply? The could generally does not draw that much current.

I'm trying to understand what CNT1 is designed on the schematics as an AC driven coil?

Marty

[diycncscott]

Many contactors can be driven off of either 24VAC or 24VDC, check your coil specs.

A few reasons NOT to use the 24DC from the ALLIN1DC PS:

1. Noise: Inductive loads produce a huge spike. You now connected that spike directly to the 24VDC feeding the ALLIN1DC.

2. Current draw. How much does you contactor use?, how much does your other 24VDC I/O use (with everything on)?

3. Do you really want to take down your ALLIN1DC if something happens to one of the peripherals that is connected to the ALLIN1DC PS?

[martyscncgarage]

Thanks Scott
Marty

[cncsnw]

You could, of course, put in a separate 24VDC switching power supply -- maybe an S-25-24 or RS-50-24 -- and run a DC-coil contactor that way.

Those power supplies are modestly cheaper than a transformer (but then, DC-coil contactors tend to be modestly more expensive).

You might be tempted to use a contactor with a 120VAC coil instead. However, if you are using an M39-style control pendant, the Belden cable that it comes with, which carries the E-stop contactor coil circuit to and from the E-stop button, is only rated to 30V. If you are supplying your own wiring to and from the E-stop button, and you use appropriately rated wire or cable, then a 120VAC contactor will work just fine.

[martyscncgarage]

You could, of course, put in a separate 24VDC switching power supply -- maybe an S-25-24 or RS-50-24 -- and run a DC-coil contactor that way.

Those power supplies are modestly cheaper than a transformer (but then, DC-coil contactors tend to be modestly more expensive).

You might be tempted to use a contactor with a 120VAC coil instead. However, if you are using an M39-style control pendant, the Belden cable that it comes with, which carries the E-stop contactor coil circuit to and from the E-stop button, is only rated to 30V. If you are supplying your own wiring to and from the E-stop button, and you use appropriately rated wire or cable, then a 120VAC contactor will work just fine.

Thanks Marc.
Even with proper snubbers DC small contactors are not recommended I suppose.
I would use 120. If needed then I'll stick with 24vac.

Marty