Solutions to de-rating requirments for single-phase input to VFDs

Generally, VFDs that allow single-phase input require de-rating the power controlled by the VFD to a value specified by the manufacturer which I've seen quoted between 50% to 66% of nameplate. In short, your 10 HP rated VFD shouldn't be hooked up to a motor larger that between 5 HP and 6.6 HP (which you can't find anyway!) depending on the manufacturer's de-rating spec.

The de-rating is necessary because the bridge diodes in the input section aren't sized large enough to put the entire load through: a full-wave bridge for single-phase uses only 4 of the 6 diodes provided for the normal 3-phase input. Also, the smoothing capacitors would be sized too small.

So, my thought is to add an external full-wave bridge sized to handle the max output rating of the VFD. Additional smoothing capacitance can be calculated and added externally. This new external input section would be wired into the VFD across the DC bus. The remaining issue is the phase-loss circuitry. I've seen several (older) VFDs where the phase-loss circuits were CTs clamped on the input phases and no discrimination was done in terms of timing checks to make certain that 3 actual phases were present. So, the CTs could be moved over to the new input section and one input doubled up with 2 CTs on it and the other input monitored by the remaining CT.

Does this seem like a workable scheme?

Thanks!
Dave

Have a look here, your not the first one to consider it.

http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/how-do-i-supply-dc-one-leg-vfd-320898/

SAF Ω

LowEnergyParticle said:

Does this seem like a workable scheme?

Click to expand...

It has been.

But times have changed.

- con: More new VFD are 'optimized' (economically more than electrically..) to where they aren't meant for de-rating OR DC bus powering AT ALL.

- pro: at the same time, the 1-P need has been recognized as a large enough market segment that MOST major-makers now have models specific to 1-P operation right out of the box. AND 'a few' makers don't stop at 2 HP, but go to 15 HP or so.

Advantage to NOT "DIY" is that the makers have more field-failure-mode feedback and a larger staff & R&D budget to correct issues over many, many units than the average one-man-band. Hard to compare to that, even if your design seems brilliantly executed.

So THEIR product has a warranty, retains any UL/CS/TuV ratings and any other 'compliance' that a DIY haveth not. Extra costs are spread out over time, and the 'waranteed' item should have more reliably predictable time over which to spread them.

That might affect your insurance coverage, etc.

JM2CW, but it seems the time is past to bother oneself with one-at-a-time DIY for 1-P operation of VFD's. "Used" ones were already a dice-roll, what with heavily-overbuilt ones mostly gone by now and capacitors not lasting forever.

More gain to be had putting your scarce time and other resources into other needs that have NOT yet been addressed with mass-market solutions.

Bill

MOST small VFD's.. 5 hp and under, are available in single phase input/no derate needed models.

Have been for probably 10 years..

Derate is ONLY needed on VFD 3 phase models, that were intended for 3 phase only installations. And some of those, have phase loss protection, and will be unhappy on single phase input.

Single phase ONLY input VFD's, are available (no derate) to 50 horsepower. http://www.polyspede.com/wp-content/uploads/2015/05/spedestarb.pdf

Thank you all for your replies! It seems that I've dated myself somewhat. A little market research shows single-phase only VFD's up to 5 HP are indeed thick on the ground, and some manufacturers (Note abarnsley's link above to Polyspede) go much higher. Monarchist's post is well taken: in 2016 it seems that the kind of mod I had in mind no longer makes economic sense.

I appreciate the insight, and will continue with pricing single-phase input VFDs.

Best regards to all,
Dave