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Spectrum 125c plasma on a generator?

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  • Spectrum 125c plasma on a generator?

    I need a little technical help with a newly purchased Spectrum 125c plasma cutter. I bought the unit to make saddle cuts in fence pipe.....but I'm having a problem...the unit works perfectly on a 20 amp, 120v household outlet, but when plugged into my 5000 watt-6500 watt surge (Subaru powered) generator.....the trouble light comes on and unit will not operate. Miller website says a 2000 watt or higher generator should run this unit.
    I have checked output voltage on the generator and it is around 128v.
    Generator does NOT bog down or trip breaker when cutter is turned on.
    I'm baffled.....any help would be greatly appreciated! Thanks
    (*generator powers wire welder and other tools perfectly, it only has about 10 run hours on it)

  • #2
    See http://www.millerwelds.com/resources...rator+inverter

    Your genset may be able to supply 5kW into a slowly applied purely resistive load but it will not maintain voltage within allowable standards. And it will have wildly fluctuating voltages when a very dynamic load such as an inverter is applied (were talking switching the 5kW load on/off thousands of times per second). Those generators will be seriously derated on solid state equipment, whether it be a phase converter, computers, communications equipment,etc. Pumping out 5kW is one thing. Doing it at 120/240V +-5% while maintaining a sinusoidal waveform with no harmonics into a rapidly fluctuating load is quite another. The problem is inherent to the design of the cheap Home Depot power-outage generators. The solid state voltage regulator in a better (not necessarily bigger) generator costs about the same as the entire "get 'er done" genset. Generators are not created equal.

    Originally posted by Bodybagger View Post
    The nature of the current draw by the inverter is going to set up a nasty pattern of undercorrecting and overcorrecting by your alternator. And as soon as the voltage gets out of bounds, it's going to shut down with the error.

    The inverter, though it draws less current on average, has a bunch of inrush spikes that the alternator's VR can't deal with. The very thing that makes them have such a stable arc - the ability to VERY quickly draw more or less power as the arc requires - results in these spikes.

    The transformer machine, though it draws more current on average, draws it smoother in a way the alternator can deal with. The very thing that makes it inferior to the inverter arc - the inability to quickly draw more or less power as the arc requires- is what makes it get away just fine.

    Now it's my understanding that the Dynasty will eat these harmonics up without complaining.

    Look at the power output with an oscilloscope and you'll see harmonics develop even when the inverter is supplying a very low load. Think about how the AC voltage is produced... current flows from the VR field output through the slip rings into the rotor and creates a rotating magnetic field for the stator. Voltage depends on instantaneous load and the magnitude of the magnetic field. VR can only change the magnetic field so fast... the inverter operates MUCH faster.

    Those small generators make terrible AC, especially under varying load. It may say 5500/8500 surge, but see if it will run a 7HP air compressor. Technically, it should... But it wont.

    80% of failures are from 20% of causes
    Never compromise your principles today in the name of furthering them in the future.
    "All I ever wanted was an honest week's pay for an honest day's work." -Sgt. Bilko
    "We are generally better persuaded by reasons we discover ourselves than by those given to us by others." -Pascal
    "Since we cannot know all that there is to be known about anything, we ought to know a little about everything." -Pascal

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    • #3
      I was under the impression that the inverter cutter was fairly forgiving, I would tend to think it would work. As was stated it could be a waveform issue?

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      • #4
        The first and foremost issue would be seeing if the genset is actually putting out 120/240 with no load. It's probably not.

        80% of failures are from 20% of causes
        Never compromise your principles today in the name of furthering them in the future.
        "All I ever wanted was an honest week's pay for an honest day's work." -Sgt. Bilko
        "We are generally better persuaded by reasons we discover ourselves than by those given to us by others." -Pascal
        "Since we cannot know all that there is to be known about anything, we ought to know a little about everything." -Pascal

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        • #5
          Thanks for the information!

          I appreciate the responses to my question. I had a feeling it was a generator issue. The generator was not originally purchased to run a plasma cutter, just hoped it would. I guess NOW, I need to find out what type of generator WILL run this unit....uugghh!

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          • #6
            I agree, test the genset, make sure its working properly.

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            • #7
              Problem solved!!!!

              Thanks again to everyone who helped out! I was admiring what I thought was my new $900 paper weight and decided to go over the generator ONE more time. I broke out my volt meter again and tested the generator output on all outlets. Average was 127-129 volts. It hit me that maybe, just maybe, the voltage was TOO HIGH for the unit so I adjusted down the run speed slighly on the generator and rechecked the voltage. Once I had the output at 122 volts I plugged in the plasma cutter and SUCCESS!!!!!
              My expensive blue paperweight turned into a fully functioning plasma cutter! Thank God!

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              • #8
                Nice to see you got it working, I havnt heard of problems with those running from gensets.

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