240V Step Down Transformer to 208V for Brother Speedio - Do I need to wire X0?

I have the basics down and have a 15kVa Hammond step down transformer. Already running a 15hp RPC. Brought line voltage in and checked the outputs and have 208-210 on the output. The one thing I'm unclear on is the wiring needed for ground. There's a ground lug inside of the case. I'm assuming I need to run ground from the RPC side to that and ground from the CNC side to that. But I've also read about the X0 terminal being connected to that as well. I'm hoping someone can better explain that portion of it to me.

This is residential service. 200A main to a 100A sub in the garage. 70A breaker feeds the RPC. The transformer will only be in between the Brother and the RPC.

First, what's the transformer? 3p-3p delta-wye?

As a rule, step-down are delta-wye, but there might be exceptions. As a separately derived system to run a branch circuit, X0 would be jumped to ground on a wye secondary.

When running one machine, you don't have to, but I don;t know why you would not.

JST said:

First, what's the transformer? 3p-3p delta-wye?

As a rule, step-down are delta-wye, but there might be exceptions. As a separately derived system to run a branch circuit, X0 would be jumped to ground on a wye secondary.

When running one machine, you don't have to, but I don;t know why you would not.

Click to expand...

Yes. 3p-3p Delta-Wye. So I just need to run a jumper from the X0 terminal to the case ground lug?

If you have a delta to Y transformer, then just ground the neutral of the transformer (likely the X0 terminal.)

anyhow, friend of mine has a brother speedio and well, when the spindle stops from 16K rpm, it shoves 70 amps back up the ac line.

That 70 amps pushes the 208 volts generated by the RPC up to 250-260 volts, which causes the spindle drive to trip. He is running a 20hp American rotary and some buck boost transformers to drop the voltage to 200 volts at the machine.


So anyhow, what I did was take a 5.6KW furnace heating element and a 25Amp solid state relay wired from the rpc generated leg back to the neutral of the 120/240v service. the furnace heating element comes with its stock thermal circuit breaker for safety. (no fan to cool it off). Some lightbulbs are wired up so when the element turns on you see the bulbs light up.

The solid state relay is driven by a full wave diode block and a string of about 25, 12v half watt Zener's, which "remove" enough voltage such that the ssr trips if the voltage gets above, about 220 volts ac iirc. a 150 ohm 1 watt resistor is in series with the Zener's.

two 12v Zener's in series are across the SSR to protect it from over voltage spikes which could be presenset in the event of a power surge.

I went through a couple iterations of this, tried 24v 1/2 watt Zeners and a 50 ohm resistor. once one zener got damaged and its voltage drops out, the rest slowly failed and the heating element stayed on.

I have verified the ssr is turning on within the first line cycle and the voltage is clamped fast enough.

also for what its worth i added a 5hp motor to the 20 hp rpc and it didn't make a measurable difference. there is some talk of folks running his machine off of a 30hp idler and it mostly works. i don't like "mostly works" 30hp idler is only going to be 50% stiffer than a 20hp machine.

if you need a resistor to clamp the RPC voltage as a solution feel free to contact me i can build you one.

Well, run it to something grounded. And to the machine case.

The case I would ground to the incoming ground. The X0 can be to that, also, and to the incoming ground if you like. That's convenient, and grounded.