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  • Two VFD on one circuit... possible problems?

    My lathe and milling machine each have their own VFD. I feed the two VFD's on one 220 V circuit. 20 Amp circuit breaker in the breaker box. There is about 25' of 10 AWG to the lathe VFD, and then 25' of 10 AWG to the mill. Each VFD is a 1.5 HP TECO. They are in parallel. Lathe motor is a 1.5 HP 3Ph and mill is a 1 HP 3Ph. This has worked fine for a year, but I've never tried running both machines at once before last night.

    Last night I was doing a series of long cuts time-consuming on the mill using the automatic X feed (with limit switches). I had done 5 passes when I thought "It will be more efficient if I use the lathe to make the brass knobs while this thing is cutting". So I set up the lathe, and turned it on.

    As soon as I turned on the lathe there was a "thyunk" from the mill as the carbide cutter broke.

    Dang ... it was one of my better cutters.

    I have done a few small experiments since then, but I can't see any reason for what happened. I have turned on the mill (cutting air), and then had my wife turn on the lathe (also cutting air). I couldn't hear any difference in the sound of the mill nor could I see any momentary slow down (although it's possible there was a small variation that I didn't see). I haven't tried anything with the mill under load since I have a limited number of carbide cutters left (in fact, only 1 left).

    Was there some interaction, or was it just really unfortunate timing?




  • #2
    If it really happened simultaneously with your turning on the lathe something happened. You might have had a slightly slower startup on the lathe if the voltage sagged, but the mill vfd would (could) have faulted if materially affected. Have you looked at any error logs? How are you programmed to handle voltage sags - like Hitachi and other AVR function. etc. Were you really "optimally hogging" by pushing the mill to the close to the limits? What's the acceleration time of the lathe? Do you have quality vfd? What the manufacturer recommended "fusing" for each vfd - it's often double what you'd expect based on motor size)?


    At work we have many instances of multiple VFDS on "same circuit", and some at 15hp pairs/triples. No issues, but each is fused separately before the common breaker "just in case". This is more to protect the main circuit for an individual VFD failing "short" than interaction..
    Maybe it's a simple as you took your eyes of it and it behaved badly. Happens to me all the time on my bandsaw when sawing large blocks
    Last edited by lakeside53; 02-02-2020, 02:08 PM.

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    • #3
      Just an unfortunate coincidence probably.

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      • #4
        The only possibilities are complete chance (most likely), a faulty VFD unit which has become susceptible to RF noise on the supply (least likely) or a supply voltage drop sufficient to significantly reduce the mill motor speed (VFD will usually error)
        If you benefit from the Dunning-Kruger Effect you may not even know it ;-)

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        • #5
          Were both VFDs powered up, or did you flip a switch to energize the lathe VFD? I would also assume that you are starting the machines using the VFD on/off controls with a reasonable soft start parameter. Applying line power to a VFD can result in a pretty severe current surge and momentary voltage drop as the link capacitors charge up, especially if it is applied at a waveform peak. And switching on a load to a VFD which is already supplying voltage can cause a surge current fault. But that would not likely affect another VFD on the same circuit.
          http://pauleschoen.com/pix/PM08_P76_P54.png
          Paul , P S Technology, Inc. and MrTibbs
          USA Maryland 21030

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          • #6
            I'm gonna vote unfortunate timing. It's possible that kicking on the lathe caused enough of a load that the mill spindle slowed for a bit, but if you were in the middle of a cut you would've noticed the change in sound I'd imagine

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            • #7
              I'm with Paul. If the lathe has not been powered up for a while, the capacitors were likely discharged and would have caused a voltage sag at startup. That might have caused the other VFD to fault but the power feed would have kept moving.

              This would not repeat until the lathe VFD was left idle (and powered off) long enough for the caps to discharge.

              My VFD came with a warning to make it the only device on the circuit. Same warning was on my TIG welder.

              Dan
              At the end of the project, there is a profound difference between spare parts and extra parts.

              Location: SF East Bay.

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              • #8
                I was going top suggest unfortunate coincidence.... BUT

                You obviously had the feed going (you said so) on the mill. Dunno how aggressive, but carbide suggests "somewhat" aggressive. When you turned on the lathe, possibly the mill VFD was NOT affected, but the FEED was. It slowed the feed for just a moment of time, then resumed, and that "hiccup" of feed caused the breakage.

                It's hard for one VFD to affect another as to voltage, since most have bus capacitors, and can ride through most interruptions. And the interruption would not change the speed, just the power. Unless you were taking a high power cut, that should not do anything.

                The scenario the two Pauls suggest is possible, but unless it caused a fault, which would have been noticed since it would likely stop the VFD, it is unlikely to have made any difference, since the frequency sets the speed, and voltage does not affect it.

                The one issue here is that you have created a worst case situation. The lathe and mill are on the same wire, daisy-chained. It appears that the lathe is halfway down 50 foot of wire with the mill on the end. That does create nearly the most possible chance of problems.

                Did you find the mill running, or shut off? If running, do you have it set up to ride through overloads and faults, or to shut down on them?

                TECO should have a surge device on the input, to cut the inrush current. I do not know if it is a resistor & relay, or a PTC, but should not make much difference.
                Last edited by J Tiers; 02-02-2020, 05:00 PM.
                1601

                Keep eye on ball.
                Hashim Khan

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                • #9
                  If the feed was slowed momentarily, the depth of cut would be less and safer.

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                  • #10
                    Originally posted by old mart View Post
                    If the feed was slowed momentarily, the depth of cut would be less and safer.
                    And then it resumes potentially with a jerk as the feed motor speeds up then gets loaded down and settles again to the prior rate..... Many of the feed setups have a dial controlled feed, and there is no spec on what happens if the voltage drops and returns to normal.
                    1601

                    Keep eye on ball.
                    Hashim Khan

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                    • #11
                      Dan said, "As soon as I turned on the lathe there was a "thyunk" from the mill as the carbide cutter broke."
                      you didnt mention whether the mill continued running with broken bit, or, the power dropped out causing the bit to crash.

                      I assume you refer the power dropped out. I have seen many weird things happen with electricity. We once had a twin engine aircraft/a generator on each engine. if you replaced one generator, you had to adjust both voltage regulators. If both sides weren't happy, they would fight each other until one dropped out.

                      My first guess on your VFD's is the mill was already running loaded, then you turn on lathe and send a surge load through through primary power, and the mill VFD just wasn't happy and dropped out.
                      I have seen many avionics issues due to what I call 'dirty power', that is, power that is not smooth & steady, nor within tight voltage spec, nor within tight hertz spec.

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                      • #12
                        Originally posted by danlb View Post
                        I'm with Paul. If the lathe has not been powered up for a while, the capacitors were likely discharged and would have caused a voltage sag at startup. That might have caused the other VFD to fault but the power feed would have kept moving.

                        This would not repeat until the lathe VFD was left idle (and powered off) long enough for the caps to discharge.

                        My VFD came with a warning to make it the only device on the circuit. Same warning was on my TIG welder.

                        Dan
                        The capacitors have bleeds. If they are off for more than a few minutes minute they are discharged. They also don't have great surges (circuits to stop that..) unless you have a really cheap vfd, and Teco typically does thing correctly.

                        Check your VFD settings. What have you told it to do in an input voltage droop?
                        Last edited by lakeside53; 02-02-2020, 06:18 PM.

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                        • #13
                          Thanks everybody for brainstorming on this. Right now my current SOP has become "use EITHER the mill or the lathe but not both at once". I have to admit that since I'm retired and doing this for mostly fun and only tiny amount of sales/service, I can take my time.


                          Some clairifications and answers to questions:

                          The mill was still running after the cutter broke. I had to go and turn it off.

                          The automatic feed on the milling machine is on a completely different 110 V single phase circuit.

                          The VFDs seldom get powered all the way off. Only when I know I am not going to be using the shop for a few days.

                          I use the VFD panel button to turn on/off the lathe, and it has a gentle power up.

                          The lathe had not been used that day, but the lathe VFD had been on for hours before hand.


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                          • #14
                            I would suspect the auto-feed system. If it is a stepper motor, or computer controlled, it may have experienced a glitch, and momentarily advanced too fast and caused excess side pressure on the milling bit.
                            http://pauleschoen.com/pix/PM08_P76_P54.png
                            Paul , P S Technology, Inc. and MrTibbs
                            USA Maryland 21030

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                            • #15
                              OK, so regardless of undesirable circuit, the lathe did NOT cause a drop in voltage, because it was already charged up and waiting.

                              The idea of the feed glitch is out because the drop on the other circuit did not happen, PLUS the system is on another circuit, at least 25' of wire away from the VFD.

                              The mill VFD did NOT fault out, because it was still running, plus the things that might have caused that did not happen. Unless it was something that might cause a trip out, but the VFD was programmed not to do that, it is not one of the obvious things.

                              About the only thing left is that IF the wiring of controls involves long unshielded wires, and/or the control wires are in with the motor wires, THEN it MIGHT have had some interference cause a glitch that broke the cutter.

                              OR.... you had an aggressive feed set up, it was just gonna break, and happened to break right then.
                              1601

                              Keep eye on ball.
                              Hashim Khan

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