Re-purposing a commercial VFD as AC signal source - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: December 19, 2016, 01:05:24 pm
content: I have a requirement to provide a low frequency (20 to 300Hz) AC sine wave at around 5kW.I thought about buying a commercially available 3 phase drive, ripping out the control logic, but using the high voltage / current stuff as a base?Typically, these drives rectify the 3phase to an intermediate DC link (at around 600V) then chop that up at between 10 and 1 KHz across 3 half bridges.I could just use two of those half bridges to generate the AC sine wave, using a suitable output inductor (which will be quite large for ~30A)Anyone done this before??

author: jpittner
author_rank: Contributor
timestamp: December 19, 2016, 01:16:08 pm
content: Hi,I have experimented exactly in this direction at the 1.5kW level.However, the VFDs send PWM modulated signals directly to the motor,not "smoothed" low freq. AC. You will need a 3-phase power low pass filterto get it. The VFD from aliexpress I got actully in the user manual writesthat use of such a filter is not recommended, but it worked anyway.There was no need to modify the control hardware of software of the VFD,just add the external filter. However, the AC was acceptable but not ideally shaped sine.To get a better sine form, rather than to reverse engineer the chinese VFD,I eventually constructed my own AC source, open hardware/open source, if interested seehttp://www.pittnerovi.com/jiri/hobby/electronics/frequency_changerJiri

author: max_torque
author_rank: Super Contributor
timestamp: December 19, 2016, 06:31:09 pm
content: Yes, as i mentioned, and output filter (inductor) will be necessary to filter the PW waveform towards a sinusoidal form. That inductor of course is subject to the usual constraints / compromises between size,cost,loss and the maximum harmonic content that can be tolerated in the outout waveform.As i need variable frequency at variable voltage, that rules out std VFDs, as they run output voltage coupled to frequency. I was going to keep the rectifier, DC link, 3 phase IGBT bridge (just using two halves out of 3) and gate drive circuitry etc, but add my own controller at the PWM signal level.

author: jbb
author_rank: Super Contributor
timestamp: December 19, 2016, 07:14:59 pm
content: HiDo you need constant voltage across that frequency range? If so, you'll need some custom control. Possibly (if the drive uses a recognisable micro) you can just reprogram the existing controller. Please beverycareful if you go this route. The following should be considered:Main DC capacitor will stay charged for a while after AC input removedThe internal microcontroller is almost certainly at mains potential. I suggest anisolatedprogrammer & comms system. Otherwise you could get shocked through your test PC. "Oh, I'll just use a laptop to float it" is aterribleidea.If at all possible, do all testing over 60V (whether at the input, DC link or output!) with the case closedOvercurrent protection is criticalOvervoltage protection on the main DC link is recommendedHowever, you might be able to put the drive into constant Volts per Herz mode, where U = k*f (k is a constant), aka constant flux mode.Either way, if you add an LC filter to the output to get something more like a sine wave, make sure that the resonant frequency of the filter is not too close to the switching frequency (or any harmonics). Also add some high value discharge resistors (like 100kOhms +) to the filter caps in your LC filter.jbb

author: max_torque
author_rank: Super Contributor
timestamp: December 19, 2016, 08:57:16 pm
content: I plan to start testing with a "dummy" isolated DC link, at around 30v, with a very small amount of capacitance, with current limiting resistors feeding the bridges to ensure that code snafu's that lead to un-expected switch timing (shoot through!) do not result in mega-death!Once basic switch waveforms have been validated, i can step up the voltage and current to look at things like dead time and switching frequency / losses etc. I will probably run the drive standard and log those parameters to get an idea of what the original controller was doing regarding timing.