[eloid]

have a few questions regarding ground for transformers.

On the seconday side of xformer I have 2 50 v coil winding in series to give 100vc I also one 18vac coil and one 12vac coil.

the input primary side has 2 115ac coils in parallel.

The question is... the in coming ac ground on primary should it be common? to fram of cnc unit? I understand the secoundary side should be floating ground. So I need to create a common secondary ac ground? and dc ground ?these should should the be separted ? on the secondary side of transformer.

Thanks in advance

[Al_The_Man]

The Service ground should be taken to a common point in whatever electrical enclosure you have, the frame of the transformer is usually bonded to ground if it is mounted on a back plane that the above common point is mounted to, but still it is good practice to carry a ground conductor from a mounting screw to the central Earth common.
For the secondaries, it often depends on the secondary usage and what method you subscribe to as to isolated or non-isolated systems.
For example, if you have a transformer who's secondary is to supply 120vac to feed other various systems, PC or power supplies etc. then the custom is to usually to ground one of the secondaries at the transformer and carry this ground conductor to the common Earth plate.
This secondary point is then used as a Local neutral using a white conductor.
In the case of other secondaries used for DC power supplies, it gets a bit murkier.
If you want to ground the power supply DC common(s) then these should also be taken to the central Earth point, do NOT ground the respective AC secondary for these supplies.
If you chose to float the DC supplies, then neither the AC secondary or rectified DC should be connected to ground.
The important thing to remember is that although a neutral or a DC common is connected to ground, the ground itself should not be used as a conductor!
Spindle motor frame and any metallic part of the machine should have a bonding conductor back to the central common Earth point.
Carrying out these practises avoid alot of spurious noise etc.
Al.

[eloid]

In my case i have this transformer
http://www.antekinc.com/AN-15450.pdf
Its a 1500VA toroidal transformer so it has no frame for the ground?
I need 100vac for fans and driving my smaller dc supplys
that will make up branch 5v dc, 12v dc and 24 vdc
70 vdc for gecko
pc will come from it own 115 ac circuit feed.
attached is a drawing of the main power transformer, no sure if i should use
a isolated ground?

The Service ground should be taken to a common point in whatever electrical enclosure you have, the frame of the transformer is usually bonded to ground if it is mounted on a back plane that the above common point is mounted to, but still it is good practice to carry a ground conductor from a mounting screw to the central Earth common.
For the secondaries, it often depends on the secondary usage and what method you subscribe to as to isolated or non-isolated systems.
For example, if you have a transformer who's secondary is to supply 120vac to feed other various systems, PC or power supplies etc. then the custom is to usually to ground one of the secondaries at the transformer and carry this ground conductor to the common Earth plate.
This secondary point is then used as a Local neutral using a white conductor.
In the case of other secondaries used for DC power supplies, it gets a bit murkier.
If you want to ground the power supply DC common(s) then these should also be taken to the central Earth point, do NOT ground the respective AC secondary for these supplies.
If you chose to float the DC supplies, then neither the AC secondary or rectified DC should be connected to ground.
The important thing to remember is that although a neutral or a DC common is connected to ground, the ground itself should not be used as a conductor!
Spindle motor frame and any metallic part of the machine should have a bonding conductor back to the central common Earth point.
Carrying out these practises avoid alot of spurious noise etc.
Al.

[Al_The_Man]

First off you will not get 70v ac on the connections you show as I think you intended the 18v to be in series with the 50v which it is not if you tap off where you show it going to the Gecko supply.
You would need to drop the brown to the green down to the blue.
What puzzles me is if you are bringing in 120v for the primary of the transformer, why not use that instead of the 100vac sec. for fans etc, do they really require 100vac?
If you need a 5v supply you could wind on about 10 turns of small gauge enamel and run it into a bridge and a LM7805 regulator.
Although what is wrong with the PC 5v supply?
With a toroid there is really no frame available as the core is generally covered so in this case it does not matter.
Nice size transformer though.
Al.

[groov]

Although 1.5KVA is plenty for your application...I would make sure if the 18V secondary can handle the amount of current required by the gecko system if you want to use it in series with a 50V section. Most likely the 50V section(s) will have better current capabilities than the 18V section and this last section maybe your bottle neck.

groov