VFD rather than power amplifier?

  • Last Post 08 November 2018
Prometheus posted this 08 November 2018

So I continue sourcing components to build my electronics experimentation workbench (including the workbench itself).

I think I'm going to go with the Analog Arts SL937 oscilloscope / function generator,

But I'm coming up short on a power amplifier that can output at a sufficient frequency, voltage and wattage while driving an inductive load (most are designed solely for resistive or capacitive loads... and a tiny capacitance at that).

Most of them only have +-100 volts or less, 2 amps or less, and 100 KHz or less. I'm not so concerned about the frequency, as the inductive testing I'll be doing will be at lower frequencies.

So I got to thinking about it... a VFD is essentially a waveform generator coupled to a PWM power amplifier, and they can drive inductive loads (motors) at high voltage and amperage, at a continuously variable frequency.

So I could drive the inductive loads via a VFD and merely use the SL937 as an oscilloscope. I can pick up a sufficiently sized VFD for a few hundred dollars.

Then once I've got the VFD voltage and frequency dialed in such that the device works the way I want, I can design a standalone circuit which mimics that voltage and frequency.

The only downside is that I'd be working solely with sinewaves, rather than the arbitrary waveforms that the SL937 can generate.

What do you think?

My first idea is to create a miniaturized version of a motion-EMF generator, and package it into one of those USB charger wall warts, sans the 120 V plug blades. It'd be an "infinity battery" to charge cell phones and such.

As an aside... if you're looking for load resistors that can handle a lot of power, look for the VFD dynamic braking replacement resistors... they have them available up to 185 KW (~$1230). The 1000 W resistors should be good for our testing, and they're relatively cheap (~$79 at Automation Direct).


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  • Chris
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Chris posted this 08 November 2018

Hey Prometheus,

Re the Power Amp, I have not found anything at all in a good price range!

I use a H-Bridge, I have a thread here: Reliable and Flexible Switching System

Perhaps not the ideal solution for some, but for me, this worked out really well. It is easy to add a parallel or Series cap for resonance.

I have found, at resonance, a sine wave of sorts will be found even with a square wave H-Bridge chopping the Current.

I would start small, low power, easy to view and test, replicate those before us, learn from them first. I think your Resistor idea is good, but perhaps a little over kill for a start? Big and expensive units:



I use 5 Watt Globe's, 21 Watt Globes, 10 and 100 Watt Resistors: $4 or so.


Nice Research, its great to see such detailed research on such subjects and the sharing of it! Thanks for sharing!


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Weeks High Earners:
The great Nikola Tesla:

Ere many generations pass, our machinery will be driven by a power obtainable at any point of the universe. This idea is not novel. Men have been led to it long ago go by instinct or reason. It has been expressed in many ways, and in many places, in the history of old and new. We find it in the delightful myth of Antheus, who drives power from the earth; we find it among the subtle speculations of one of your splendid mathematicians, and in many hints and statements of thinkers of the present time. Throughout space there is energy. Is this energy static or kinetic? If static, our hopes are in vain; if kinetic - and this we know it is for certain - then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature.

Experiments With Alternate Currents Of High Potential And High Frequency (February 1892).