View Full Version : Problem Delta to wye 3 phase

07-02-2008, 12:23 AM
I was told i needed a transformer to run my cnc machine. I have a 230v 3 phase delta service and they told me i needed to get a $3k+ transformer to convert the delta to wye. I am not electric savoy and would like to see if there is an electrical geek out there that can help me. The reason i ask is because everything runs fine and the manufacturer told me i needed it but could not give me a reason other than they have installed a machine befor on delta and had problems with it.

Any Takers?

07-02-2008, 08:47 AM
For ground referenced supply a Star or Wye supply is usually used and the star point grounded, rather than one leg of a Delta.
I assume yours is ground referenced and not isolated?

07-03-2008, 01:33 PM
I am not sure what you mean but it is refered to as delta, or grounded b phase by electricians.

07-03-2008, 09:58 PM
You have the three phase system that Al the Man referred to as the delta referenced to ground. Grounded B phase Delta is what I have always called poor mans 3 phase. The grounded B phase system only requires a 2 pole circuit breaker for each circuit rather than a 3 pole CB. The voltage to ground on the A phase and C phase is 240 volts each with the B phase wire being grounded. The voltage between the A, B and C phase wires is 240 volts each. A full 3 phase system also has 240 volts between each phase wire but all 3 phase wires show approximately 138 volts to ground. Your machines manufacturer may have had problems with a machine on a grounded B phase system before and the transformer "fixed" the problem. If it is running OK I don't see a problem however it may void your warranty if not wired the way the mfr specifies. Also the machine mfr will probably blame any problem you have in the future on the grounded B phase system. I'm not familiar with modern CNC machines or their control systems so I can't comment on any specifics of their wiring that could cause a problem with grounded B phase.

Kevin Taylor
07-04-2008, 11:03 AM
I have 230V three phase with a wild leg leg to leg is 230V and two of the three to ground are 125V and the one is 200V give or take I always make shure that the wild leg dosen't get into a control voltage that requires 120v. Haven't had a lot of proublems over the years but my old BPT CNC seem's to have voltage isues from time to time . I started working on a lathe that came with a transformer on it and I think the voltage seem's to get averaged out on the output side will have to check. Kevin

07-07-2008, 01:22 PM
From phaseconverterinfo.com,

"If the three-phase equipment has 120V phase-to-neutral circuits that cannot be isolated and connected to either leg 1 or leg 2, the output of the phase converter must be passed through a 240V delta to 120/208V wye isolation transformer. This transformer will establish ground and neutral at equal distance from all three legs and deliver the power in a wye configuration.
This is especially important when operating some newer CNC machines that have certain Fanuc or Siemens controls. These controls handle regenerative power from the spindle motor by putting the regenerative power back onto the line utilizing a phase-to-neutral connection on all three legs. If these machines are powered by a phase converter with delta configured power, the regen circuit on the leg manufactured by the phase converter will operate at a voltage that is too high. An isolation transformer must be used to convert the power from delta to wye configuration."

Basically, with regenerative power your wild leg would go REALLY wild.