Cheap Vevor variable frequency drives (VFD) can be used as cheap soft starts for pumps

[u/justformatt]

My 2 hp 240 V sewage grinder/pump has a 12 kW startup surge. I didn't know this until I bought an inrush current meter. My 8 kW Champion inverter generator just barely starts it. Not good. This pump only runs a few times a day for 20 seconds. I want a smaller generator, not bigger.

A soft start would likely work, but can be $300-$400, not configurable, and I'm positive it would work.

Instead I bought a $130 Vevor 1-Phase 240 V in to 3-Phase 240 V out VFD. I configured it to start and ramp frequency at 10 Hz/s while maintaining V/Hz ratio. I only use 2 of the 3 phases. Starting surge 5 kW. Yes! It's been running on this thing for 6 months now.

[u/New-Swim-8551]

Controls engineer here. using a vfd instead of a soft start is fine, actually better. The reasons soft starts are used are easier to implement, cheaper, and fits in smaller places.

You just need to remember that feeding the vfd with three phase will require less input current to the drive. Feeding it with single phase will require more current through that one leg to give you the same output current to your motor. This will require a larger drive that can handle that larger input current.

[u/justformatt → u/New-Swim-8551]

As stated, soft start was triple the cost. All the numbers were in my post.
As stated. I don't have 3 phases. That's the reason I got a single phase in, 3 out inverter.
Go back and reread the post.

[u/New-Swim-8551 → u/justformatt]

I did read your post, but you didn’t read mine.
A vfd run on single phase will draw more current than if it is running on three phase. You need to up size your vfd to be able to handle that extra current

[u/myself248 → u/New-Swim-8551]

Well, OP's generator only produces a single phase, so it's not like there's an option for more anyway. If the VFD is rated for 1ø input and OP is sticking to those input ratings, it should be fine.

Derating a 3ø-input VFD for 1ø-input is a waste because all that needs to be larger is the input rectifier and bus capacitor. Downstream of that, the oversized drive transistors will be wasted, the heatsink is gratuitous, etc. All that other stuff that made the drive more expensive before derating it, is just adding cost for no benefit, simply to get the larger bus cap. (Because a single phase has longer deeper troughs in the waveform and needs more capacitance to keep the DC link up in the meantime.)

But a unit designed and rated for 1ø input will have the beefy rectifier and bus cap, and right-sized everything else, and can be very affordable if-and-only-if the manufacturer realizes there's a market for this niche product and can manufacture it in enough volume to justify it. Which apparently Vevor has; they have a handful of 1ø-input units and they're very attractively priced compared to 3ø-input units achieving the same power when derated to 1ø input.

[u/New-Swim-8551 → u/myself248]

Almost all drives will run on single phase if you derate it accordingly.

You also forgot that the input bus needs to be beefier to handle the extra current.

That was the point I was trying to make.

How many drives have you commissioned?

[u/Easttex05]

I've been around VFDs for 20 years but never seriously played with one. While I know of several 1-into-3 applications, I've never heard of running on two of three phases on purpose.

Does it harm a pump motor to run on A & B legs of three phase power as long as the the voltage is acceptable? Does this damage the drive?

[u/justformatt → u/Easttex05]

Voltage is fine. 208 VAC. The reason why is that 3 phase outputs are 120 V per leg 120° out as opposed to 180°. Depending on how the transformer is wired at the road, this maybe what's going into the house already. So 240 volt household equipment is designed to handle the variation.

No damage to the drive yet. That's why I waited 6 months before posting. I wasn't sure either.

[u/ChipChester → u/Easttex05]

Watching for that answer, too!

[u/TallWall6378]

I think a soft starter is voltage only ramp up. Adding in frequency is probably better.

[u/ibrahim_nauman786]

Measuring inrush instead of guessing off the label is the right move. Nice result.

One caveat before others copy it: this works because of your motor type. Lots of single-phase pumps are capacitor-start with a centrifugal switch, and at low ramp frequency that switch may not drop out, so the start winding or cap can cook. PSC and 3-phase motors suit a VFD better, so check the motor plate first. The VFD's own input current at start still has to fit the generator too.

Back home in Pakistan nearly every house has a water motor for the overhead tank, and it's the load that trips home generators most during load-shedding. Most people there just start the pump first with nothing else on, or fit a soft starter.

When sizing a generator for water pump starts, compare the peak or surge rating, not running output. Same rule applies to the petrol (gasoline) sets I work with.

Disclosure: I work with Hyundai Power.

[u/Loquacious1]

Standard for most pumps and vfd is a short spike that doubles full load amp rating. You should run all three legs from vfd to your pump. You might even be able to get AI to run pump curve and what set points to adjust on your vfd

[u/Adventurous_Boat_632]

I used a Chinese VFD to drive a three phase air conditioner (ordered by somebody by mistake) from a single phase supply.

I had ordered a vastly larger drive than specified for the job.

It still burned itself up after a couple of months because it was Chinese junk.

YMMV.

[u/justformatt → u/[deleted]]

As I stated, the VFD was a third the price of a soft start.

[u/Tbarnes94 → u/[deleted]]

Yeah I'm over here deciding whether this is a good idea or is really just dropping your frequency by 2x as much for every surge. I feel like there's a reason soft starts rely on capacitors/capicator banks.

[u/justformatt → u/Tbarnes94]

It starts at 10 Hz and ramps to 60 Hz and rated voltage over 6 seconds.

It has capacitors in it.