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

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  • #46
    Originally posted by David Powell View Post
    Something no one has so far mentioned.
    What happens when someone uses a small generator, barely up to the job, with the usual poor governors, couples it up to their vfd, gets her running with no load and then begins to actually work the machine they are powering.?
    I have absolutely no experience with this, but I do have some experience with a very large lathe driven by an Hydraulic motor, initially powered by a gas engine with a rough governor driving a pump
    . Everything rolling round nicely, engage feed, tool begins to cut, chuck slows down, after a couple of seconds delay governor opens wide, old lathe takes off like a bat out of hell, sure made screwcutting big threads fun !
    We later put a 20 hp electric motor to drive the pump, That was much better.
    Regards David Powell.
    Cheap vfds do not completely remove the dc bus input voltage from the output voltage, so as the input voltage drops the motor will draw.. Either more power (current is conserved) if the motor is overloaded.. Or it will draw less power (if the motor load is below nameplate hp.) The effect isnt much, but what im saying is the vfd wont add instability to the system.

    the vfd will remove the generator's varying rpm from the driven load, which should greatly smooth out a bad governor.

    small generators achieve regulation by saturation, so you should not see more than 260vac at any time*, which means the vfd should not trip on overvoltage when suddenly turning off the motor. (Coast to stop). (Obviously, regenerative braking requires a brake resistor)

    *A typical 5kw generac generator (brushed) dumps 600 watts into the rotor btw. (One of the reasons they suck up 1/3rd gallon an hour of gasoline at no load)

    a vfd which has a rather low voltage trip threshold pulling the generator down to 55hz and 200 volts will likely produce a beat frequency in the driven motor. Or a 17.5hz frequency due to single cylinder 4 stroke engine.
    Last edited by johansen; 06-24-2021, 08:18 PM.


    • #47
      One thing to remember about using DC to supply a VFD is that you need a lot of DC voltage, 1.414 times the AC output plus a little. Most small Solar and wind generators are way to low voltage. Most are either 12, 24 or 48 volt. You could design a system that would do it, but it would be special.

      Happily working on my second million Gave up on the first


      • #48
        Another thing to remember is that some generators do not produce a good waveform. It may read right with a true RMS meter, but it may be a somewhat flat-topped waveform that does not have the right peak voltage that a true sine wave would have.

        Also, the way a VFD draw current is not like other loads. The VFD rectifiers draw nothing when the generator voltage is low during each cycle of the AC. But once the generator voltage exceeds the bus voltage, there is a heavy current drawn from the generator during the time that the generator output is above bus voltage. There is thus a pulse of high current, during which all the power that comes from that cycle is drawn. That puts a premium on having the correct peak voltage, but also tends to "clip off" the top of the waveform, voltage-wise.

        Smaller generators have a fairly high series impedance. With that pulsed high current, a generator may not produce the peak voltage that is seen unloaded. To hold up to that sort of current draw requires a considerably larger generator.

        What it comes down to is the VA capability of the generator. The pulse current is somewhat equivalent to a bad power factor, which draws excess VA over the real power produced. The larger generator is needed to produce the excess VA.

        So the needed generator may be larger than expected.
        CNC machines only go through the motions


        • #49
          Thank you all! AC current generator. Good things to know, never know when a guy will need to turn or mill something in an outage. Yes I know where power in the lines comes from but a gas powered generator is a bit different. I know many electronics warn against using a charger off a small gas powered generator. Sometimes I like to ask before learning the hard way.


          • #50
            Originally posted by vpt View Post
            Thank you all! AC current generator. Good things to know, never know when a guy will need to turn or mill something in an outage. Yes I know where power in the lines comes from but a gas powered generator is a bit different. I know many electronics warn against using a charger off a small gas powered generator. Sometimes I like to ask before learning the hard way.
            The main problem with small generators is the reaction to load and voltage regulation. Electronic loads such as inverter welders and vfd's, will have a very variable load pattern and the if the regulator doesn't react quickly, over voltage situations will cause the protections on the load to "overwork" itself to failure. A generator should be much bigger than the load to handle this.
            Helder Ferreira
            Setubal, Portugal


            • #51
              Originally posted by vpt View Post
              .................. I know many electronics warn against using a charger off a small gas powered generator. Sometimes I like to ask before learning the hard way.
              Chargers for Lead acid batteries are a bad one.... high power chargers such as found in many transformer based inverters for solar. (many will do a charge if AC is supplied to their output).

              The battery voltage is essentially a constant. No matter what current the charger can provide, it is generally much less than what would change voltage (the battery can supply hundreds of amps).

              So, at the peak of the AC, the generator voltage is "clipped" to a flat-top, screwing up the regulator, and making meters read wrong. Charge current is OK, because a small generator is essentially acting as a current source during that time.

              Charging the bus capacitors of a VFD is similar, only the voltage can be changed, just not instantly.

              Bigger generator = more available current, less voltage issue, better performance.
              CNC machines only go through the motions


              • #52
                Only issue I have with all that has been discussed is whether the dc generator is an ac generator at heart. I say no. It may generate a pulsating voltage as it commutates, but the voltage does not alternate. The more poles, the smoother it is usually, but the voltage never changes direction. True, the homopolar generator would be the smoothest as there is no commutation, but it would not be immune to voltage fluctuations as these would occur whenever there is a change in the loading.

                For relevance to the OPs question, the only thing I can see that would prevent a vfd from working from a 'generator' would be if the vfd uses a transformer to provide the low level dc voltage to operate its control circuitry, and the 'generator' has only a high dc output. It is a common thing in microwave oven control panels, audio equipment with remote controls, kitchen appliances with remote controls, etc. to have a small power transformer to provide a low dc voltage to operate the control circuitry. I can't say that it would be uncommon for a vdf to have this low level power supply, but I will say that it's unlikely for a low control voltage to be derived directly from a high voltage dc source. That usually means that a good sized power dropping resistor would be needed. This kind of thing is not common anymore. I would want to know beforehand whether the vfd needs a 'normal' ac input before applying just a high voltage dc source to it.

                Of course, it's assumed that any 'generator' that would be considered to power a vfd would have an ac output- where is there a generator that would have a high dc output, but not an ac output? A scientific instrument perhaps, but nothing that the average person is going to try powering a vfd with.
                Last edited by darryl; 06-25-2021, 09:04 PM.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                • #53
                  At the present time, all the VFDs which I have detailed information about, use an SMPS to derive the control voltages, operating directly off the DC bus. Consequently, such VFDs are perfectly capable of operating with a DC source.

                  The SMPS is normally a low power flyback type supply providing the voltages to operate the user controls and the drive signals for the power devices. The two are isolated so that the user control section does not use a common supply with the switching signal processor and drive circuitry.

                  The total volume occupied by the SMPS is normally far smaller than an iron transformer power supply would occupy, and it is cheaper to make.
                  CNC machines only go through the motions


                  • #54
                    Here is an old VFD that has a conventional AC powered low voltage supply. It's probably around 1990 vintage. I bought it for about $50 on eBay, and the control panel stopped working so it was not repairable (nor worth repairing). The IGBTs and capacitors are probably worth what I paid, and the big heat sink may be useful, but it is relatively huge and massively heavy compared to a modern 5 HP VFD.

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                    Paul , P S Technology, Inc. and MrTibbs
                    USA Maryland 21030


                    • #55
                      Yes, in that and earlier time periods, the transformer was common. With one-chip flyback supplies becoming common, often (Power Integrations) needing only a very few external components, it has become quite rare to see an iron transformer used for the control power.

                      The entire control power supply can usually occupy less volume than just the filter capacitor on that transformer supply.
                      CNC machines only go through the motions