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Odd questions that cross my mind. Can you run a VFD off a generator?

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  • #16
    I've got a few customers that run VFD's off gensets all the time, big ones even (600 hp)
    I just need one more tool,just one!

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    • #17
      Originally posted by Paul Alciatore View Post
      First, yes you probably can run a VFD on generator power. Small generators usually produce a sine wave, just like the generators at the power plants and that should work just fine with almost every VFD. Of course, there could be the odd one that has a problem, but it would probably be hard to find that problematic one. I would totally expect it to work as long as the Voltage and available current are suitable for the VFD.

      But run a VFD from DC? It may sound OK as the power path in a VFD does first use a conversion from AC to DC. So far, so good.

      BUT, and I did capitalize that word for emphasis, there is also control circuitry in almost every VFD and that control circuitry almost certainly runs on a LOW Voltage DC which is also generated by converting the incoming AC into DC. The key word here is "LOW". The higher Voltage DC which will be used to generate the output AC waveforms is way too high to power things like ICs which are used in these control circuits. So a transformer or a separate winding on a common transformer is used to produce that low Voltage AC which is then rectified to generate the low Voltage DC (around 5 to 12 Volts) which then powers the control circuitry.

      So just placing DC on the input power cord of a VFD or even connecting it to the appropriate place inside the VFD will not allow for that conversion of the expected AC input current into the low Voltage DC that is needed to actually operate the VFD.

      In short and at best, with a DC power input, the VFD will probably just sit there and do nothing. In a worst case scenario, using DC to power a VFD could allow excessive current to flow in any transformers in the circuit and overheat those transformers, burning them out.

      It is possible for the VFC to generate that low DC Voltage without a transformer, but I can not say that any particular model does or does not do this. So, I do not recommend trying this.




      Yes, but is it a pure sine wave?? and is is clean ?? If it isn't it's liable to confuse the VFD and you may not get the desired results.

      JL..............

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      • #18
        Is it a pure sine wave? Generators and/or alternators by their nature tend to produce sine waves. It is in the nature of the rotational motion and the magnetic fields the wires rotate through.

        As to how pure those sine waves are, it is hard to say. One thing is that they will be a close approximation to a pure sine wave. By their nature they do not produce the high energy spikes that other techniques for making approximations of the sine wave can. That refers to, among other things, digitally produced approximations of a sine wave. A wire passing through a completely homogeneous magnetic field with a perfect circular motion would indeed produce a pure sine wave. But even in the generators (yes the power plants call them generators) that produce our commercial power, those magnetic fields are not completely homogeneous. I am sure that there is some variation around the edges of the fields so the Voltage induced at the center of those fields would be somewhat higher and the Voltages induced at the edges would be somewhat lower than a pure sine. Of course, this is only one example of how the generated waveform can deviate from a pure sine.

        But then there are inductive and capacitive and resistive effects that will tend to eliminate any frequencies other than the primary one so the purity of the sine wave will be better than a purely geometric analysis would predict. And these effects will be present in both large and small generators/alternators.

        If the generator/alternator uses brushes then it can also produce spikes on top of the sine wave being induced in the windings. Those spikes can have significant amounts of energy and can produce problems in the electronic circuits of a VFD. But then commercial electric power often also has spikes and other unwanted (and sometimes wanted) additions to the sinusoidal power of the grid. This can come from many sources and it is impossible to completely eliminate it.

        In practice, AC that is produced in a small generator will be no more nor less a pure sine wave as that in the large ones used to generate power for the grid. And a well designed VFD should be able to reject those unwanted electrical spikes and noise.

        Pure and clean power? Good luck, no matter what the source. About the purest and cleanest power available is a battery that is powering only one device. And even that could be subject to RFI.



        Originally posted by JoeLee View Post
        Yes, but is it a pure sine wave?? and is is clean ?? If it isn't it's liable to confuse the VFD and you may not get the desired results.

        JL..............
        Paul A.
        SE Texas

        And if you look REAL close at an analog signal,
        You will find that it has discrete steps.

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        • #19
          The term generator applies to both AC and DC generation devices. The term alternator is applied to generators that produce AC. The term dynamo is applied to generators that produce pulsating DC.

          Both alternators and dynamos fall under the more general term, generator.



          Originally posted by wmgeorge View Post
          The term 220 volt single phase should give you a clue that its an AC generator, yes it should be called Alternator. Yes there are also DC generators that use an Inverter to make the AC.
          Paul A.
          SE Texas

          And if you look REAL close at an analog signal,
          You will find that it has discrete steps.

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          • #20
            It is perhaps a semantic fallacy that cars now have alternators that produce DC. But they do so by means of three phase rectification which is built in to the assembly. Actually, even the old-time automotive generators are technically AC machines, and differ from their modern counterparts in that they use mechanical commutation to produce DC from the alternating magnetic fields in the rotor. The only true DC generator is the homopolar generator.

            How's that for thread topic drift, eh?
            Last edited by PStechPaul; 06-23-2021, 01:16 AM. Reason: link
            http://pauleschoen.com/pix/PM08_P76_P54.png
            Paul , P S Technology, Inc. and MrTibbs
            USA Maryland 21030

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            • #21
              The answer to the OP is YES! Most any useful VFD will tell you if the input bad by faulting out.

              It takes a LOT of sag to fault a VFD!

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              • #22
                The original question was can a guy run a VFD off a 220v single phase generator?

                As DC does not have phases, it's obviously an AC generator.

                If it's just a common or garden variety, turning an alternator at a governed motor RPM (to keep the frequency more-or-less correct), the output is for all practical purposes a pure sine wave. If it's an inverter generator, the output will either be a reasonably pure sine wave or that abomination a "modified sine wave".

                A modified sine wave is nothing of the sort, but rather a stepped square wave which could well upset some electronics, so should generally be avoided. The claims of safety made for them by the sellers of those things are often exaggerated.

                My own stand-by generator is an inverter type with a pure sine wave output, and I have run the single-to-three-phase VFDs for the mill and the lathe off it with no worries at all. I have also run my computer and my ham radio off it, and nothing has ever complained. You should have the VFD set up to always give you a soft start on your machines, so an inrush current on start-up will not be a problem.

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                • #23
                  I don't think the input to the VFD needs to be a perfect sine wave after all the drive changes it to a form of DC and then generates its own sine wave, and voltage for the output. I would have a concern however running electronics directly off some AC generators as when the engine speed varies so does the frequency. If you have ever heard the generator die off when it runs out of gas or stops because of Low Oil pressure?
                  Retired - Journeyman Refrigeration Pipefitter - Master Electrician - Fine Line Automation CNC 4x4 Router

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                  • #24
                    Originally posted by vpt View Post
                    I have no intentions on doing so (yet), but can a guy run a VFD off a 220v single phase generator? Like I said, I have no intentions of doing it but it is just something that crossed my mind and would be nice to know. In case there is a storm, the power is out, and a guy just has to mill something right now.
                    It will be fine as long as the generator is big enough there should be no issues whatsoever.
                    The mains power in your house is coming from a generator too, after all.
                    Cheers,
                    Jon

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                    • #25
                      The usual bunch of silly answers. Someone will be telling us about their 3 phase DC generator next.

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                      • #26
                        Since DC is straight potential, there are no sine waves to have time sequence relationships.
                        The term Phase describes time based relationships, so no way to have phased DC.
                        You can have ripple to DC current, or various levels of pulse width modulation.
                        but it is all above the zero line (on the + side).
                        Can you have phase sequenced digital (binary) square waves on multiple channels?
                        (like in stepper motors) ?? Maybe one could argue that 2 phase stepper motors fit the
                        description, because they are DC. So yes, maybe you can have 3 phase DC.
                        What say ye?

                        -D
                        Last edited by Doozer; 06-23-2021, 01:09 PM.
                        DZER

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                        • #27
                          Now the Thread goes to the Stupid mode, good bye.
                          Retired - Journeyman Refrigeration Pipefitter - Master Electrician - Fine Line Automation CNC 4x4 Router

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                          • #28
                            Excuse me ???
                            No need to be insulting.
                            I thought we were trying to quantify some nomenclature.

                            -D
                            DZER

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                            • #29
                              Not well versed in electronics but did get training in servicing vfds. They are able to approximate a full sine wave to run an ac motor so they must be able to generate below the line voltage. Is that what a bipolar transistor can do? My fluke 87 shows 700v on the dc bus of a 480v input vfd. Is that the peak volts on a 480v sine wave? If one were to run a dc supply into a vfd would it have to be 700v to manufacture a 480v rms ac sine wave?

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                              • #30
                                Originally posted by deltap View Post
                                If one were to run a dc supply into a vfd would it have to be 700v to manufacture a 480v rms ac sine wave?
                                YES. The AC voltage listed is the RMS voltage. The peak voltage of the sine wave is 1.414 times the RMS voltage: 480 X 1.414 = 678.72 If the DC bus voltage were lower the maximum RMS output voltage would be lower.
                                Another thing I have seen in all modern VFD's is that the internal control voltage is either derived from the high voltage DC bus (most common) or a separate control voltage input. If separate it is often either 120 or 240 volts AC (yes, single phase) 🙄.
                                Robin

                                Happily working on my second million Gave up on the first

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