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  • VFDs unplugged

    Overall do you think its better to completely unplug (no power) a VFD when not in use or leave it powered? I wonder how long all those little lights will last always on. Also I only have one outlet for 240V in that area for two machines, so one gets unplugged. Nothing seems to change when unplugged for long periods so I am thinking unplug it whats your opinion please. Cheers!

  • #2
    No problem leaving it unplugged. If you are not going to use it for a extended period its probably a good idea. That said, there is such a thing as leaving it unenergized for too long, the capacitors in it can cause problems. As long as you power it up at least a couple times a year, you should have no problems at all.

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    • #3
      +2 for what Sparky said, also a good idea to unplug if your area sees frequent lightening storms.
      I just need one more tool,just one!

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      • #4
        I've had lots of gear that has sat for years, decades sometimes, and when plugged in again it works fine. I just recently went through some stereo and pa gear that hasn't been powered for 10 years or more, and all of it survived. I did have a problem with my computer a while ago which I'm pretty sure was caused by a nearby lightning strike. I'd say you have more of a chance of damage by leaving it plugged in.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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        • #5
          I power mine down when not in use. If you are prone to power surges (I am) or whatever, some models will dump excess voltage into the braking resistor, some will not. Safer to be off-line. On the other hand we have many at work that are on 24x7 for decades.

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          • #6
            I always power mine down at the end of playing with any of my three machines using it, never encountered a problem yet after 6 years.
            Chris....

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            • #7
              Yes we are prone to lighting strikes and I didn't really think about that. I was more concerned about the display lights burning out and now think its better to just unplug it entirely for all those reasons. I can't see any good reason to leave it powered up when not in use. Thanks much!

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              • #8
                I always unplug the supply to the mills vfd, and everything else in the workshop at the museum when we go home, including the pc. It saves a little electricity and reduces the fire risk.

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                • #9
                  I dimly recall seeing something in the installation instructions that said there should be a power switch feeding the VFD. So I turn it off. When powered on mine has a fan that runs. That's my reminder to shut it off when I leave the garage.
                  At the end of the project, there is a profound difference between spare parts and left over parts.

                  Location: SF East Bay.

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                  • #10
                    I used to use Mitsubishi VFD's when doing retro-fits and they had a nice (optional) feature.
                    There was a link that could be connected that allowed the main power off, as in E-stop or machine Stop etc. but the link would keep the LV control (PLC) power on when in this condition,
                    when re-powered, the response was much more rapid.
                    Of course, when the main disconnect was operated, everything was off.
                    Max.
                    ,

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                    • #11
                      Originally posted by Sparky_NY View Post
                      No problem leaving it unplugged. If you are not going to use it for a extended period its probably a good idea. That said, there is such a thing as leaving it unenergized for too long, the capacitors in it can cause problems. As long as you power it up at least a couple times a year, you should have no problems at all.
                      Agreed, after upwards of 6 months off-line good practise is to reform the capacitors gradually, a variac and a 60w light bulb (proper one, not a new-fangled LED or CFL) in series with the live (hot) to limit current works well - slowly ramp up the variac voltage in e.g. 5 volt steps, a half-hour apart and watch the light bulb - a second or two after each step it should dim back down to a glow-worm (a couple of watts worth of light), indicating it's pulling only a small current. After reaching 60 - 70% of the line voltage on the variac you should hear (if it's a quality VFD) the soft-start relay close as the reservoir capacitors will have sufficient voltage on 'em, and the panel should light up The fan should kick in and spin at around half voltage (but will draw excess current until close to working voltage), making the bulb glow a little brighter - once up at full line voltage expect the bulb to be pretty dim, it's only providing current for the fan and control circuits!

                      Dave H. (the other one)
                      Last edited by Hopefuldave; 01-13-2020, 05:06 PM.
                      Rules are for the obedience of fools, and the guidance of wise men.

                      Holbrook Model C Number 13 lathe, Testa 2U universal mill, bikes and tools

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                      • #12
                        I always power the VFDs down for safety. The VFDs power switches are something like five feet above the floor, so small children cannot reach them without a chair. So, to turn a machine on, one must turn both VFD and machine control switch on. Too many steps to be done by accident.

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                        • #13
                          You have a point with the capacitors, but I would be fearful of putting a VFD on a Variac. There are many designs out there and there is no telling just how the rest of the circuitry will react to the lower Voltage levels. It may be perfectly OK. But, on the other hand, bad things could happen.

                          I would run it on more frequent occasions, perhaps four or six times a year in stead of just two.

                          And I have seen capacitors fail with equipment that was stored for a long time. Once I had a close call with a power supply that provided about 400 VDC for tube circuits. A capacitor literally exploded while my face was only inches above it. The steel rail that the capacitors were mounted on saved my eyes: I got it on the forehead and the cheeks/chin.



                          Originally posted by Hopefuldave View Post

                          Agreed, after upwards of 6 months off-line good practise is to reform the capacitors gradually, a variac and a 60w light bulb (proper one, not a new-fangled LED or CFL) in series with the live (hot) to limit current works well - slowly ramp up the variac voltage in e.g. 5 volt steps, a half-hour apart and watch the light bulb - a second or two after each step it should dim back down to a glow-worm (a couple of watts worth of light), indicating it's pulling only a small current. After reaching 60 - 70% of the line voltage on the variac you should hear (if it's a quality VFD) the soft-start relay close as the reservoir capacitors will have sufficient voltage on 'em, and the panel should light up The fan should kick in and spin at around half voltage (but will draw excess current until close to working voltage), making the bulb glow a little brighter - once up at full line voltage expect the bulb to be pretty dim, it's only providing current for the fan and control circuits!

                          Dave H. (the other one)
                          Paul A.
                          SE Texas

                          Make it fit.
                          You can't win and there IS a penalty for trying!

                          Comment


                          • #14
                            Originally posted by Paul Alciatore View Post
                            You have a point with the capacitors, but I would be fearful of putting a VFD on a Variac. There are many designs out there and there is no telling just how the rest of the circuitry will react to the lower Voltage levels. It may be perfectly OK. But, on the other hand, bad things could happen.

                            I would run it on more frequent occasions, perhaps four or six times a year in stead of just two.

                            And I have seen capacitors fail with equipment that was stored for a long time. Once I had a close call with a power supply that provided about 400 VDC for tube circuits. A capacitor literally exploded while my face was only inches above it. The steel rail that the capacitors were mounted on saved my eyes: I got it on the forehead and the cheeks/chin.




                            It's basically what the VFD manufacturers recommend in their service manuals, to reform the capacitors - although some prefer you to access the caps direct via their DC bus connections with a current-limited DC supply, 0-1kV, which I guess cuts out both the input rectifier and the soft-start relay, so probably has more chance of the capacitors popping...

                            The issue is that the initial inrush and ripple current exceeds what the caps can handle - the effective series resistance of the capacitor then rises (causing heating) and the insulating layer degrades with time unpolarised, so the insulation breakdown voltage drops significantly - not a good combination, excess heat and internal arcing! This is probably what cost you your eyebrows... We used to do it for fun when I worked as a lab tech, a small electrolytic wired to the AC mains and taped under the expensive kit the other guy was working on - wait until he's walking out for a coffee and his back's turned, hit the switch

                            By placing a bulb in series the current is LIMITED to what the bulb consumes - so a 60w bulb on 240v limits to 250 mA, a fairly safe current (for the caps, not you!), and the variac allows a slow increase in the polarising voltage, which allows the insulating layer (an aluminium oxide, basically anodising internally) to slowly reform (hence "reforming" electrolytic capacitors) by reacting with the electrolyte BEFORE applying the full voltage - it's safe, works, almost free

                            Pretty much all modern VFDs derive their control logic supply from across the large reservoir caps that also feed the output devices (why they can stay running during brief power loss and E-stop conditions), running undervoltage won't hurt them - they have to step down the bus voltage (anything up to 800v or much higher for really BIG ones) to 5, 12, +/-24 etc. for the clever bits via an internal switch-mode PSU, the only concern might be the internal cooling fan, which is sometimes (cheap VFDs) a simple shaded-pole induction motor direct from incoming mains so is likely to overheat a little on reduced voltage - the alternative 12 / 24 vDC fans (posh 1st-world VFDs) will happily cope for a few weeks at undervoltage.

                            Dave H. (the other one) - I have a fairly good grasp of VFDs, I'm the guy getting 415v out of a 3-phase-input VFD on 240v single-phase, cost about ten pence to modify... call it 12 cents?
                            Last edited by Hopefuldave; 01-13-2020, 07:47 PM.
                            Rules are for the obedience of fools, and the guidance of wise men.

                            Holbrook Model C Number 13 lathe, Testa 2U universal mill, bikes and tools

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                            • #15
                              I'm not sure if Hopeful is agreeing with me or not. I can't imagine a VFD manufacturer recommending that you purchase a somewhat expensive auto transformer to properly form the capacitors in their units. That would almost be a kiss of death to their sales effort. "Whadda you mean, it can blow up? Well, I sure ain't gonna buy that PoS."

                              I suspect if any of them actually think that far ahead as to imagine what should be done to preserve a unit that is sitting unused for long periods, then they would add a bit of resistance in that input circuit to protect those capacitors. Something like your lamp. Or another element that has a high initial resistance and then drops down to a lower value after a few seconds. That is, IF they actually think that far ahead: I really wonder if it would actually happen. Perhaps some of the older and better players in that market.

                              Anyway, I still would recommend a shorter period for the preservative treatments.
                              Paul A.
                              SE Texas

                              Make it fit.
                              You can't win and there IS a penalty for trying!

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