Need Help!- Bridgeport VMC 800-XP2

[Helmuth]

Hello:

I have a question regarding power for my Bridgeport VMC 800-XP2. It has the Fanuc 18i-MB control. I invested a fair amount of hard to come by cash on a PT-380 digital phase converter, and wiring it all up, but ran into somthing I didn't anticipate. My line voltage is 251+ volts. More over, the PT-380 provides a delta configuration 3 phase where the line to ground voltage is 125 on legs 1 and 2 but 208 on leg 3. I'm afraid to flip on the power because I have heard about voltage balance problems on Fanuc controls. Does anyone out there have experience with this? The machine is intended for 208 but has a transformer tap for 220. Can I just disconnect the L3 input to the surge protector or put a resistor in line to drop it to 125? I realize it would be best to simply put in a 240 delta to 208Y/120 isolation transformer but I'd rather spend the money on a new Kurt Vise and some tool holders.

Best regards: Helmuth

[gus]

First, no resistors, don't even think about it

a quick look at the spec sheet implies that the output is balanced. am I misreading?

[Helmuth]

Hello Gus:

This is a bit long winded but here goes. The three pase power from the Digital Phase Converter (dpc) is balanced very nicely. The single phase input at my shop is very high, 251.8 volts, line 1 to line 2. The three outputs from the dpc are all within 1% of that. The problem is that it is a 'delta' configuration and the voltage level between the output legs and ground are not equal. This configuration works very well for most motors that operate by the voltage level between the lines. I heard that an isolation transformer would be necessary to run a CNC with a modern control and am finding out this may be true. I guess that is why larger manufacturing facilities have so many of them. Isolation transformers provide outputs with the voltage to ground the same on all three output legs, commonly referred to as a 'Y' configuration. By installing a step down isolation transformer, my troubles would go away. A 240 'delta' to 208 'y' transformer with a power rating of 30 to 45 KVA would solve my problems nicely. However, It is another expense I wasn't prepared for. I opted for the dpc for the phase balance properties and the simple fact that I wouldn't have the huge start-up current rush common to rotary converters. An AC motor draws 2 to 3 times its power rating on start-up. To run a 25 hp spindle cnc , I would need at a minimum a 50hp rotary converter. A 50 hp rotary converter could draw nearly all of the current supply the rural household line transformer could output. In combination with the normal current draw from the house, I could easily overload the transformer or send a power rush down the main line that would make my neighbors really unhappy. I could have spent between $7,500 and $10,000 to have utility three phase installed but then all of our 240 volt appliances would have to run off of 208 or I'd have to install a step-up transformer to bring it back up to 240 in the house. Then I'd risk unbalacing my three phase.

Best regards: helmuth

[jmelson]

Originally Posted by Helmuth Hello Gus: The three phase power from the Digital Phase Converter (dpc) is balanced very nicely. The single phase input at my shop is very high, 251.8 volts, line 1 to line 2. The three outputs from the dpc are all within 1% of that. The problem is that it is a 'delta' configuration and the voltage level between the output legs and ground are not equal.

Most CNC controls that run off the machine's 3-phase power do not USE the neutral wire, and in many cases there is only a 4-wire cable - 3 phases and safety ground. They assume that with the various services available, it is just a LOT simpler to run any single-phase loads such as computers, fans, small power supplies, etc. from line-line. Of course, you MUST!!!!! check this out to be sure that is the case on THIS specific machine. If it has a big 3-phase transformer with taps for the various mains supply voltages, then this is much more likely.

So, if that is the way the machine is wired, you needn't worry about how the neutral of the phase converter is set up. It is set up as a center-grounded delta scheme, allowing you to run 120 V loads off the 220 supply, just as you normally do in 220 V single-phase buildings. Actually, my guess is the converter is just passing your 220 V single phase with neutral straight through, and generating the extra phase relative to neutral, which is actually about 207 V.

If the machine DOES use the neutral wire, then you will have to study the layout of any load that connects to neutral and make sure you supply the correct voltage to it.

Jon

[Helmuth]

Hello Jon:

I expect you to be exactly correct. However the schematics I have for the machine's wiring are not clear enough to verify if any of the Fanuc control elements use the ground plane for anything other than safety. The machine was assembled by Bridgeport in England but it uses a Seimens electrical enclosure populated with a Fanuc 18i MB control system to run the show. According to the schematic L1, L2, & L3 distribute to the main power supply, the surge protector, and some motors like the coolant pump and spindle motor fan. Every thing else taps off of L1 & L2 alone.

Best regards: Helmuth

[machintek]

I do have a old service bulletin that states that the Siemens requires balanced phase to phase. Having said this, I never had a issue with delta or Y. But your voltage is too high. The digital phase converter puts out a third leg that is very stable but is still high. When we install machines with Fanuc controls we stay within 208 to 238VAC. Above that a transformer is required to bring it down.
By the way, the voltage drop using a resistor varies with current. E=IR where volts = current X resistance. The current the machine uses varies with load and accel thus a resistor will do you no good.

George

[Helmuth]

Hello George:

Thanks for your reply. You confirmed what I had heard before, that Fanuc systems need a tight control on their input power. Putting in a transformer to drop the voltage is the right thing to do, plus it provides the delta to wye conversion to equalize all potential power distributions. I was just hesitant to shell out the additional cash when it is so hard to come by.

PS: I was considering a resistor only on the line to the surge protector to drop the voltage. The current through the surge protector should be constant.

Best regards: Helmuth

[jmelson]

Originally Posted by Helmuth Hello Jon: I expect you to be exactly correct. However the schematics I have for the machine's wiring are not clear enough to verify if any of the Fanuc control elements use the ground plane for anything other than safety. The machine was assembled by Bridgeport in England but it uses a Seimens electrical enclosure populated with a Fanuc 18i MB control system to run the show. According to the schematic L1, L2, & L3 distribute to the main power supply, the surge protector, and some motors like the coolant pump and spindle motor fan. Every thing else taps off of L1 & L2 alone. Best regards: Helmuth

In the US, it is illegal to use a single wire for both safety ground and neutral. You don't say where you are, but it seems logical that this would be the same everywhere. Your last sentence also seems to say the same thing.

Jon

[underthetire]

Well, most important is the 251. Too high, period. Fanuc should be 220 +/- 10%. I would never install a machine over 240, too close to max limit. Another common, potentially fatal thing I see a lot of is ground rods with bus grounds together. Don't ever do that.

[gus]

If your machine has no hookup for neutral, then it would not care relative to ground. I would check an industrial surplus for the transformer you need. surprising the machine has no 240 tap, nother reason I won't buy a fanuc

[Helmuth]

Hello Jon:

The input to the machine is only the three power legs L1, L2, and L3 and a ground wire. The ground lead is flagged on the schematic as Customer Earth entering the isolator and Earth Plate on the inside. I have this connected to the ground lug in the phase converter where it goes out to a brand new, 8' long, ground rod in the ground (boy was that a pain here on a Colorado hillside). There is no neutral connection in the phase converter. It is relative to ground that the phase converter ouputs voltages vary. I have a copy of the schematic in .pdf form and could email it directly to anyone interested. Just let me know. I thought the voltage was too high before and am more convinced now than ever. I won't even turn the machine on until I drop the voltage. I guess I wont get that new Kurt vise I've been hankerin fer.

Thanks a bunch for all of your inputs. An opportunity to discuss this is really what I needed.

Best regards: Helmuth