Can a VFD Control a Single Phase 3hp Motor?

You can use a single phase input to a VFD to run a 3 phase motor. The drive is rated for a 3 phase input, but it will see an increase in amperage by a factor of 1.73. The rule of thumb is to double the rating of the drive when used in this manner. For more information Google single phase input to a VFD

As soon as you drop one output line on most VFD units they will shut down. The internal protection systems usually can detect a dead line condition and will take the required actions and assume one leg of the motor was lost.

Many smaller VFDs are single phase input compatible but as mentioned earlier you need to have a 3 phase motor on the output for them to work. All of the single phase input compatible VFDs should have a amps draw listing on them. Three phase input requires less amps per leg but the total power input in watts is the same.

Depending on the VFD manufacturer the single phase input amps are typically 1.5 - 2 times higher than the three phase input amps.

Unfortunately you are likely stuck with having to buy an actual 3 phase motor to fit your pump system.

Hello Sandman:
I contacted a VFD manufacture several months back concerning a problem with a well pump overloading a generator on start up. Single phase 2 or 3 hp motor and all I got was bad news. It won't work.

Like you I've seen some small variable frequency drives for half and three quarter horsepower motors but nothing any larger.

You obviously already know you can get a VFD that will accept single phase in and provide three phase out, so your panels not a problem ,but it looks like the motor will have to go.

A possible alternative, Depending on the pump you may be able to install a modulating valve in your piping and at least partially unload the motor.

Hello JRaef:

I had never considered a capacitor when dealing with variable frequency drives. Normally the voltage curve is adjusted with the frequency. With a capacitor in the circuit you're now dealing with two different types of reactance with inverse properties.

So I have a question, when dealing with a single phase (PSC ) motor does the bottom of the frequency range have to be set higher than it would be with a three phase motor?

A VFD can run with a single phase input. Most can run from a battery bank if you are considering solar.

This theory can be evaluated to see if it could work.

What you need to do is not allow the capacitor to come back into the circuit after it has been disabled by the centrifugal switch. This is if it is a cap start motor. Not sure about a cap start/run motor. The drive will provide a 120 phase shift when running. On a cap run the phase angle is 90.

You are kind of doing the reverse of a roto-phase when you run a 3ph motor from a 1ph source bringing in the capacitor after power is applied.

The motor will have to start at full speed though and then be slowed down. Removing the cap after starting can be done with relay in the start circiut or if the VFD has a programmable AUX output you can use that to control the relay. Not sure on what the minimum RPM would be though or the torque. Not sure if a drive could start the motor without the starter cap, it would probably trip on high current.

I'm sure someone has a VFD and motor they could test this on. I'm not going to do it. LOL.

You other option might be to put a temperature controller in the loop and have the pump cycle on and off with a contactor. Yes you would get spikes each time it started up, but "off" always uses less power than "on".

Actually you cannot run 3 phase motor pump by single phase VFD and you have to fix the adequate VFD for the motor pump to be in the safe side

Regards

Baldor makes a variable speed drive for small single phase motors. Check out their website @ www.baldor.com and the VS1ST line.

It's much better to replace the motor if its a standard fitting with a 3phase equivalent.

This will then facilitate better starting torque and a wider range of control which you will need if you want variable speed pumping to give energy saving.

If you go with a single phase output inverter (as Invertek, see J Raef answer) this means you may be able to use the motor but starting characteristics are not so good, its better starting under light load, also, you will need to recheck your supply as the VFD will want significantly higher supply current if at full speed than you have now (c.25A+) and the efficiency is lower than a 3phase motor!!

If you can change the motor, the inverter is lower cost and you will end up with more common, more reliable, more efficient equipment.

For the record, I've taken the above comments into consideration and:

1) approved a change-out to use the existing 208V, 1-phase INput; but with new 3-phase OUTput from a new VFD.
2) The VFD will be required to be the series optimized for this 1ph-IN/3ph-OUT (e.g. for ABB, the ACS550 series, etc.)
3) The pumps (pump motors) approved for change out with the 3-phase equivalent.
4) Same 'size' of 3hp indicates typical current in 3-phase of 11A FLA.The existing breakers were 20A, so only #12AWG cabling, but the increased input current drawn from VFD indicates only ~50% greater FLA drawn, at just under 17A, so should be OK with existing circuits, I think. Only change will be an extra conductor in the VFD flex conduit to each pump motor.

Thanks, all, for the heads up to the potential problems and solutions.