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3 phase - Open Delta Circuit from electrical box

RML

Plastic
Joined
Sep 21, 2023
I have a 3 phase circuit, but it's an Open Delta configuration (see below). The readings indicate a 240v 3 phase system, but the open delta gives the middle leg a higher voltage. My question is when hooking up wires to a CNCs main power connection how do I avoid the machine from using just the middle leg. On a standard 3 phase, ALL legs would read 120v if measured across the legs to neutral. This would give a standard 3 phase 208v. It's calculated by multiplying 1.73 x 120 = 208v. I'll admit sometimes this is confusing when determining the proper connections at the CNC. I don't want to run the risk of frying something because the middle leg has too much voltage and the machine is expecting a lower voltage.
Maybe someone can make some sense of this. Thanks


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There is no totally correct answer for all CNC machines. If one were to label the inputs to the CNC as R, S, and T, then in many cases you would connect your "B" wire to "T". This is because many builders take any single phase loads from R and S. One really needs to study the wiring diagram of the machine to determine what, if any, issues there might be in connecting the wild leg to any particular input.
 
You are correct in saying that it depends on how the manuf configures their main connections and what they ultimately control within the system. I've heard that as long as you stay away from the controller, most of the motors can handle the extra current as if it were just a normal circuit. But again, I can't verify this as I haven't taken delivery on my machine yet. But hopefully thru careful communication with the manuf coupled with advice from members like yourself, I can navigate thru this.
 
Good question. I'm asking the manuf what the machine expects when a certain leg is connected to one of the main connectors within the control panel.
 
Even with single phase loads it won't matter if the machine doesn't use a neutral since phase-to-phase is always 240 VAC in a delta setup. Another question is does the CNC actually expect 208?
Yeah, this is kind of what I was implying by saying it varies from machine to machine, who built it and where. I have seen things like cabinet fans that are 120V connected one line to a hot leg and the other to neutral instead of using a 240-120 transformer. A setup like that is not gonna be happy if that hot leg is the high leg.
 
There are also cases where 120v loads get connected line to ground. Not neutral.. expecting 120v line to ground ie 208 phase to phase.

The machine gets shipped overseas... and fails.

Incompetence on the machine designer. If it needs a symetrical neutral they should tell you that and give you a 5 wire terminal block.
 
I have a 3 phase circuit, but it's an Open Delta configuration
Maybe. Or not, (did you verify there are only 2, single phase transformers feeding your system?)

but what you do have is a delta system with one phase center tapped
On a standard 3 phase, ALL legs would read 120v if measured across the legs to neutral

This is becoming the standard but was not for a long time. Over seas there seems to be 3 main standards: 120/208; 240/415; and 347/600. There is a lot of 277/480 in the usa as well.
 
I've heard that as long as you stay away from the controller, most of the motors can handle the extra current as if it were just a normal circuit.
I don't know for sure what you mean by that, but 3 phase loads such as vfd's and motors are connected only to the 3 phases and ground. The neutral being a different voltage from each phase has no bearing on the phase to phase currents through the motor.
 
The manuf has verified that the machine is setup to use 240v 3 phase open delta. They asked about 208v 3 phase and I said there are 2 transformers feeding into my building and the utility company has verified that it is indeed an open delta. The manuf said they have problems when the customer 'thinks' they have 208v 3 phase and the machine is shipped that way, then when they connect up the power issues arise and components aren't happy with the extra voltage a 240v 3 phase system delivers.
 
I would think that if the controller is fed by an in cabinet transformer instead of direct fused lines from the input power taps, you are protected. Cabinet fans or other 110 volt accessory's that are using the neutral or ground as the 2nd connection for 110 could easily be protected with an added transformer.
When I moved into my shop I had the power company install 208 Wye giving me peace of mind.
For reference, I paid $2300 in 2016 to have the transformers installed. They support a 600 (3x 200 amp) service/panel
My neighbors shops have Delta have told me about some of the Uh Oh's when they added a single phase breaker to their panel and weren't aware of the above concerns. Poooof.....smoke
 
The manuf has stated that while they draw power from the 240v 3 phase, they route the power to components that need 120v thru a transformer, into a separate control panel, then out to the device. They do that so customers don't hook up 3 phase power to components that would probably get fried.
 
I appreciate all the comments and assistance with this configuration. This was a good post.
 
I have a 3 phase circuit, but it's an Open Delta configuration (see below). The readings indicate a 240v 3 phase system, but the open delta gives the middle leg a higher voltage. My question is when hooking up wires to a CNCs main power connection how do I avoid the machine from using just the middle leg. On a standard 3 phase, ALL legs would read 120v if measured across the legs to neutral. This would give a standard 3 phase 208v. It's calculated by multiplying 1.73 x 120 = 208v. I'll admit sometimes this is confusing when determining the proper connections at the CNC. I don't want to run the risk of frying something because the middle leg has too much voltage and the machine is expecting a lower voltage.
Maybe someone can make some sense of this. Thanks


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This is exactly what we have. I just always put the Hi-Leg B In the center tap and have never had a problem. I have wired over 30 machine tools that way fron CNC's to manual Lathes to Surface grinders, saws and hydraulic presses for 30 years and have never had a problem.
 
There are also cases where 120v loads get connected line to ground. Not neutral.. expecting 120v line to ground ie 208 phase to phase.

The machine gets shipped overseas... and fails.

Incompetence on the machine designer. If it needs a symetrical neutral they should tell you that and give you a 5 wire terminal block.
Can someone tell me what international standard would allow any equipment having anything wired in such manner?
 
The manuf has stated that while they draw power from the 240v 3 phase, they route the power to components that need 120v thru a transformer, into a separate control panel, then out to the device. They do that so customers don't hook up 3 phase power to components that would probably get fried.
This the standard arrangement on most equipment.

The way it gets the neutral on the 120v circuit is by bonding one lug of the control transformer to ground only for fault detection. The ground will not and should not ever be carrying a load. This transformer specifically will most likely have two voltage input ranges 200 and 230. If you are on a 240 delta high leg, connect to the 230.

The comment about 120v being created from a hot phase to ground, absolutely never occurs on any piece of equipment from even slightly reputable manufacturers.

There are rare cases where some VFDs don’t like high leg delta or corner ground delta. Since cnc equipment has VFDs by design, this is the reason to check with the manufacturer. I was just in a shop the other day with some Mitsubishi screw machines that were not capable of accepting the high leg service so they had to install a 240v to 208v isolation transoformer to feed them.

There have been lots of posts on here about vfds and unbalanced 3 phase, the extent of my knowledge regarding that subject is that sh** could blow up if you don’t check.
 
I was just in a shop the other day with some Mitsubishi screw machines that were not capable of accepting the high leg service so they had to install a 240v to 208v isolation transformer to feed them.

This was probably because the majority of Japanese machine tools are designed internally to run on 200V 3-phase (the larger machines are 400V inside). And many of the drives (especially Mitsubishi) can be sensitive to overly-high voltages.

The U.S. standard of 240V works fine most of the time, but when the voltage tolerance spikes above 255V, you can blow a spindle or servo drive. Most power companies only promise what, +/- 10% on voltage?

In the past, CNC machines came standard with isolation main transformers with changeable taps, to guarantee the machine sees the right voltage internally.
But large transformers aren't cheap, and they are mostly an option on today's CNC's.

ToolCat
 
Ok, I've seen 200v machines installed on 240v 3phase without issues. I think there are things to be concerned with regarding some VFDs, I'm not sure if the specific model of VFDs in this case required lower 208v or required a balance line-ground connection. I didn't do the install, just assumed someone did their homework and the information I received was correct since it looked like a pretty thorough setup. I will correct my previous post for clarity. From my quick research to refresh my memory, some VFDs have surge protection of some sort to protect them and that's the part that can cause issues with unbalanced line-ground connections. The bigger the difference, the bigger the concern.. possibly?? High leg delta is 120v 120v 240v to ground, corner ground delta is 240v 240v 0v. Regardless, knowing that there are edge cases with some VFDs out there, It's worth mentioning.
 








 
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