Gramps (Customer) asked a question.

May 26, 2021 at 3:19 PM
How to use a VFD to control a step down transformer driving a coil.

Ideally, we want to very the frequency of a magnetic coil on the fly!

  • 14 answers
  • 492 views

  • PouchesInc (Customer)

    That doesnt sound very safe how you have it written in the thread title. The most likely outcome would be the VFD throws a fit of errors.

    Why cant you just use a frequency generator to send different Hz to the coil? If the frequency generator cant handle the amperage load, you could use something more like an audio amplifier to do it since that is what a speaker is.

  • Adisharr (Customer)

    Like PouchesInc mentioned, that's not ideal. You may have better luck with a single phase output drive if you need the high voltage.

  • Gramps (Customer)

    Pouchesinc, Thanks for replying.

    Yes that would certainly work. Do audio amps put out 5 volts at 3 amps​?????

    We thought it would be easier to tune down a $50 VFD.​

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  • Adisharr (Customer)

    Do you know what your impedance will be at the frequencies you'll be running?

  • Gramps (Customer)

    Adisharr,

    I think calculating impedance is above my pay scale . Operational frequencies would be 5 hz to 500 hz​

    It appears that hz determines the impedance. Correct?

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    • Adisharr (Customer)

      I'm pretty rusty myself - depending on the amplifier, it will be rated for a minimum impedance which is usually 4 ohms (sometimes 2). The DC resistance for a speaker will be lower than it's impedance so to be safe it would be good to know the DC resistance of your transformer input side.

       

      It might be worth just trying a cheap one out to see what you get in case you blow it.

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  • Gramps (Customer)

    " in case you blow it."

    LOL!​

    The ohmage is 5 ohms per coil pair.

    Transformer input resistance is ​ 4 Ohms on the primary side(110 volts)

    Could the amplifier could drive it directly? Most speakers are 4 ohms these days.

    According to some ads I looked at, a 100 watt amplifier would be pushing about 7 amps flat out so 3​ amps should be mid range.😎

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    • Adisharr (Customer)

      I'm from the 'let's see what happens' group haha :)

       

      I think you'd be ok with that amp Gramps - sounds like a decent safety factor.

    • PouchesInc (Customer)

      Most audio amps do not use a linear potentiometer for controlling the output level, but rather an "audio taper" pot, also called logarithmic. So 50% on the volume knob may really only be 10% output level of the amp.

      lin_log_actionYou may want to connect your coil to the system, make sure the volumes and levels of everything are turned all the way down, and then use a multimeter on the wires to the coil to check the voltage. Play around with the output level of the signal generator since it says 2-10v, and play around with the amplifier as well to get it to the voltage you need/want. Remember that an audio amplifier has a voltage gain stage as well as a power stage with the current. Often if you set the amp to "low gain" it will not do any voltage gain on the signal, but you'll have to play around with it all to find those levels that work right.

      Expand Post

  • Gramps (Customer)

    Now to find an oscillator that will match the input.

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