Forums » Electrical Engineering

Frequency Drive as a Voltage Regulator

    • 103 posts
    January 21, 2020 7:48 PM PST

    Can this frequency drive be used for a regulated voltage supply ?  No motor attached.

    AB Powerflex70.

    • 75 posts
    February 29, 2020 6:50 PM PST

    Apologize for a late answer.  I've been working.  Ugh - what we won't do for money.

    So, you are looking at using an AB PF 70 as a variable frequency, variable voltage, UPS.  Interesting thinking.


    Disclaimer:  I have never run, nor seen any papers on running a PF 70 unloaded.  I don't know exactly what the wave forms will look like.  And I highly suspect you already know all my guesses.

    The PF 70 is a PWM drive.  The output is going to look like this:


    UPS systems are designed to be unresponsive to the load.  They have a bunch of inductance and capacitance in the output circuit. 

    VFDs are designed to drive an inductive load.  The inherent wave smoothing elements are not there.

    To get sinusoidal voltage output, you are going to need a bunch of inductance (probably series?), and likely a bunch of capacitance (parallel?). 

    UPS systems only have to run at one frequency.  One can design around a single frequency resonance.  For the VFD, one would also think the LC circuit should not resonate over the frequency range of interest.  

    Note the highly definitively engineering term, "bunch".

    Now that you know I don't have a lot of help, I am still interested in what you discover.  If you get a chance to do this, let us know what you find out about the waveforms, loads, need for reactive element in the output.


    I actually have a reason for asking.  I'm looking at using a small, 1hp, 1ph input, 3ph, 240V output to stabilize a free running induction alternator.  It is a science fair project for people living off-grid.  So, let us know how you do.


    • 103 posts
    March 2, 2020 7:30 AM PST

    The big hurdle is trying to figure out what parameters are essential to get an output, any output.  I solved the problem it was going to used for, stabilizing a CT driver,  with a 12VDC switch mode  power supply & a 12VDC to 120AC true sine inverter.  Now the only variation in current is caused by ambient temperature changes, once the equipment, conductors & test specimen are up to temperature.

    Like most other projects, the one I really want to work on gets superseded by ones I have to work on.  Actually, my load is inductive, a transformer. I was pleasantly surprised that the inverter played so well with the variable autotransformer that adjusts the voltage on the CT driver.  Now I have to upgrade the oven.  I have a pic controller thermostat used for brewing with .1 degree settings.  That should tighten up the temperature. Before I only had a two degree control. That is on hold too right now !

    I will let you know how the drive works out.