3phase RMS voltage

Hey Guys,
Got an off-topic question, if I need to do this somewhere else or in PM, please advise...so here goes.

Having trouble with a CNC lathe dropping out as if E-stopped. It is doing this sporadically. The servo system is Fanuc. Have hooked up the Fluke and tried to record Mins and Maxs but not telling me anything, they are all from 465vac to 472vac. We happen to have an Amprobe DM II Datalogger that I hooked up to log data. I understand that it is recording in RMS, but I'm confused (after all these years) on relating that to the peak-to-peak values. The power in, as far as I know, is 3phase 3 wire delta...typical buss stuff. But even though the readings are RMS they seem way out of balance. Not being that solid in power quality and distribution...please help. The following are the RMS voltages phase to phase:

Vab Min 113v Max 117v
Vbc Min 112v Max 116v
Vca Min 195v Max 200v
All currents were relatively close. Any ideas would be greatly appreciated.
Bob

Huh?...

waggs said:

Vab Min 113v Max 117v
Vbc Min 112v Max 116v
Vca Min 195v Max 200v

Click to expand...

These readings are from a 3-phase line?...

I'd expect readings like this from single phase, with your a, b, c replaced with L1, N, L2:

V L1-N Min 113v Max 117v
V L2-N Min 112v Max 116v
V L1-L2 Min 195v Max 200v

Are you sure your b probe is connected to the third phase and not neutral (or ground). Or, is one leg of the 3-Phase grounded?... utoh

Does this machine have a phase monitor? The 'dropping out like an E-stop' might be caused by a phase monitor that's not happy with the incoming power.

beerchug

-Eric

Bob

What make and model of lathe?

3phase power

What make and model of lathe?
bob1371, machine is a Tornos Deco 26/6

Eric,
There is a phase monitor on the machine and I set the threshold higher though there isn't any readout or indicator...just...higher. I set the trip delay to 5secs. I double checked all connections to be sure I had them in the proper place...everything seems OK. Not sure on the grounding of the system, we have had to individually earth ground other machines to try and solve 'spooks'. The phase monitors were installed originally to try and prevent premature death of the servo amplifiers from undervoltage, so they claim. I installed them, they said do it...I asked why...they said do it...I said, OK. Anyway, now this problem crops up and the 'unbalanced' reading really threw me.

Bob

3-wire delta... No neutral wire, right? Voltage across two points higher than the other two...

Could it be that you are having a ground short? What are the voltages across each of A,B,C and the ground (i.e. machine base)?

I would assume that a decent size CNC machine would have to have a true 3-phase power supply, not the Edison "split"...

My guess is that you have one of a couple of possibilities.

One being an open delta. This is where 2 phases come in and they "float" the third.

Another possibbility is if you have true 3 phase coming in, there is an open winding on the transformer. The transformer would have to be a delta output for this to happen.

My personal experience has been with 240 open delta in an obsolete food plant. Phase A + B were 120 volt to ground, Phase C (the wild leg) ran 138 to 240 to ground, and mathematically was supposed to be 208. The ground was mid-point between phases A + B. I see this a lot in the country, and may be what was refered to as the edison split.

Incoming phase "1" is fed to a regular 240 center tap transformer, for normal house power, the phase "2" line is fed to a 240 volt transformer, tied to phase A on one output line, the other becoming phase C. since phase B and C are not tied together by a transformer, "C" floats, dependind on load, etc.

Works well out in the country on grain dryers, where a motor starter is often just a 60 amp disconnect switch (fuses optional).

anyway, hope this helps.....casey

It is possible there is no fault on the motor your looking at but with an unrelated electricial item. Are there any motors running on inverters on same suppy line? are these running when fault comes up? What does a frez meter show? I thory 60hz if US but faults on inverter then 120hz etc may be shown even diffrent hz on each phase. Just a thought, it caught me out in past. If on generator even more of problem. Any cranes using Ward Lenord systems also can give odd supply reading.

All best Eric North Wales

What is the primary desired 3 phase voltage coming in? Your readings look (as others have pointed out) decidedly wrong for any standard 3 phase supply.

You say the system is 'dropping out sporadically, as if ESTOPPED'. Does the behaviour actually seem as if someone pressed an ESTOP, and then instantly reset it? For all or even better, just PART of the line?

If the second case is true, I'd take a real hard look at the actual interlock loop, and power sourced from your master ESTOP circuit. If you have daisy chained wiring, it is very possible that one or more connections in the chain is oxidized, or a bit loose, and at some times due to temperature or vibration, it is opening. Then as soon as the load is removed, it 'heals' itself.

I've seen that happen often when things like power to contactors are sourced from one contact of a relay, and daisy chained. When all contactors are picked up, some connection eventually heats, and opens, then re-seals as soon as the contactors drop out.

An even worse case is daisy-chaining any control-level voltage through traces on circuit boards. One quick test I'd do is get hold of a decent Infrared temperature gun (or better, camera) and look for any odd hotspots on your control panels when the system is running.

Good luck.

3phase power problems

First off, thanks everyone for your input. The ideas were all great and gave me lots to store away for future use.

rdrast,
The primary input voltage is 480 and the readings were taken off the secondary of the xfmr. Not sure if this would have made that much of a difference in the reading or not. Btw, the wiring diagram shows a straight forward auto transformer and calls for 200v. The problem with the voltage seems to be gone today, but we have had many phantoms here in this shop, so I'm not surprised. I didn't look into the E-stop circuit chain as when watching the phase monitor and safety relay, they never dropped out or tripped...it had to be somewhere else. The real reason that the 'E-stop' type condition was happening was found when the operator 'remembered' that a spindle clutch had been replaced the day before and that everything up to that point had been fine. Yes, I had asked him what may have have been done or may have been changed and he gave me that tid-bit rather late into the night. Seems that the wiring had not been tucked properly out of the way and rubbed on the clutch as it was spinning...I'll let everyone guess the outcome. Anyway the 24vdc supply crowbarred enough to drop cycle as if E-stopped, except there wasn't any 'E-stop' msg on the HMI. Pretty much end of story. Just to add a short note, I went into this detail a little to maybe help some of the newbies out there with troubleshooting. Use your tools properly...the operators. Don't abuse your tools...the operators. And buy coffee for your tools...the operators. They will pay for themselves in the long run. Ask the right questions, listen and thank them afterwards. I've been at this for a long time and still learn something everyday. Thanks again guys, the time I spend searching the forum has been worth its weitht in gold.
Bob

Re: 3phase power problems

waggs said:

Seems that the wiring had not been tucked properly out of the way and rubbed on the clutch as it was spinning...I'll let everyone guess the outcome. Anyway the 24vdc supply crowbarred enough to drop cycle as if E-stopped, except there wasn't any 'E-stop' msg on the HMI. Pretty much end of story.

Click to expand...

Yup, that'll do it everytime... :nodi:

waggs said:

Just to add a short note, I went into this detail a little to maybe help some of the newbies out there with troubleshooting. Use your tools properly...the operators. Don't abuse your tools...the operators. And buy coffee for your tools...the operators. They will pay for themselves in the long run. Ask the right questions, listen and thank them afterwards.

Click to expand...

Well said, Bob! And thanks for updating us on the outcome...

beerchug

-Eric

Aw, the old Delta - Wye problem. Scared me the first time.

With 14+ years doimg CNC repairs I can help (sometimes).

When measuring 480 input to a CNC machine, measure at the output of the main transformer AND the input to all servo drives. Phase A to B, B to C and A to C -> should be about 465 Vac. Then measure A to ground, B to GRND and C to ground. You'll find the latter measurements can have a 'high' leg.

IMPORTANT!! When measuring main fuses with replaceable elements, ALWAYS ALWAYS unscrew and eyeball the inside!!
I had an 80 amp fuse cartridge (replaceable aluminum element) that had blown and flash coated the inside of the fuse body. My Fluke read 2 ohms - but thay aint no current!!!

You probably have a Fanuc 10, or so.

YES!!!! always ask the operator! The shop owner doesn't know squat, the floor manager a little more, and the maintenance dude can't find the prints. If you can't query the operator, it can take awhile.

When checking E-stop chains, they are usually 24VDC. Set your Fluke to AC and measure for AC ripple. ripple can bite ya when testing.

I always test for AC ripple on ALL DC supplies - at the board and at chip level.

After a lightning strike or equivalent look for shorted MOVs. Snip a MOV leg and run the thing. Had a good MOV, but when I pulled the PCB traces were blown off the back.

I've only had one PLC problem related to bad input power, usually a battery was flat and the power went down - who ya gonna call?

Blown I/O bricks (SSRs) are the culprit, that and bad forklift drivers stabbing a cable.

Another gotcha is hydraulic fluid dripping into the power distribution panel - eats contacts like crazy!

Rod - the CNC dude