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Can a VFD Control a Single Phase 3hp Motor?

07/29/2009 10:39 AM

Can a VFD be installed to control a 3hp, 208V, SINGLE phase pump motor?

I've seen the parameters and successful installations of a VFD to control a 208V, 3-phase motor with a circuit powered by 208V, 1-phase power (two pole breaker).

What I am wondering, is this:

Given an existing full breaker panel, with a 2-pole breaker serving a 208V, 1-phase pump motor that is 3hp on a geothermal wellfield loop: Can I maximize energy savings of this system by having a VFD installed to control the pump motor, rather than simply having it run full-out all the time on the contactor?

I've seen 1-phase IN/OUT VFD's for smaller motors, like 1hp, but looking at the cutsheets online for my typically specified drives on commercial jobs like the ABB-ACS800 series, Allen Bradly; PowerFlex 700 Series, Danes; VLT 4000 Series, or Yaskawa; E7 Series...they all note 3-phase in and out, and some specifically call out control of 3-phase motors from 1-phase circuit as noted above.

Has anyone tried simply using two poles of a 3-phase output from an industrial VFD?

Any particular 'strange' things need to be done to make that work?

thanks for any suggestions or feedback...

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#1

Re: Can a VFD control SINGLE phase 3hp motor?

07/29/2009 10:59 AM

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#2

Re: Can a VFD Control a Single Phase 3hp Motor?

07/29/2009 11:13 AM

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

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#3

Re: Can a VFD Control a Single Phase 3hp Motor?

07/29/2009 12:35 PM

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.

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#4

Re: Can a VFD Control a Single Phase 3hp Motor?

07/29/2009 2:55 PM

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.

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#5

Re: Can a VFD Control a Single Phase 3hp Motor?

07/29/2009 8:30 PM

As others have said, the problem is the motor, not the drive. 1 phase motors cannot inherently spin like 3 phase motors. They need a "bump" to get them going in the right direction. That is accomplished in several different ways, the most common being a starting capacitor. We call those "capacitor start" 1 phase motors. There are several versions of that with similar sounding names but they are all basically the same. The capacitor is in the circuit when the motor is at a standstill and it causes a virtual phase shift in the motor when power is applied. Then a centrifugal switch takes it out of the circuit when the motor reaches a set speed, usually 80-90% of full speed. The problem is however, if you slow that motor down, the capacitor switches back in to the circuit. VFDs use transistors on the output and to a transistor, capacitor charging looks just like a short circuit. That makes the di/dt (rate of change in current) through the transistor higher than it is designed for and it heats up abnormally. At the same time, the high amount of harmonics in the PWM output of the VFD will cause the capacitor in the motor to heat up as well. You usually end up with a race between the cap or the transistors to see who will fail first.

There are two designs of 1 phase motors that CAN be used with VFDs however; PSC and Shaded Pole. PSC stands for Permanent Split Capacitor, which has a different design that avoids switching the capacitors in and out of the circuit. The caps are also permanently in series with the motor windings which act as an inductor / filter for the harmonics, so they are more immune to the effects of the PWM power it sees. ShadedPole motors are what you see in cieling fans and such; not much power. There are 3 or 4 different manufacturers of 1 phase output VFDs to be used on these two types of motors. One of them, Invertek*, makes a version that goes as high as 3HP. They are very expensive, so make sure you have a PSC motor before you invest in one. If it's cap-start, you are S.O.L.

*Invertek brand labels that drive to several other manufacturers (Bardac, Anacon etc.) so it may appear there are more than 3, but in reality there are just several names of the same product.

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#12
In reply to #5

Re: Can a VFD Control a Single Phase 3hp Motor?

07/31/2009 5:15 AM

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?

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#6

Re: Can a VFD Control a Single Phase 3hp Motor?

07/30/2009 1:43 AM

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".

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#7

Re: Can a VFD Control a Single Phase 3hp Motor?

07/30/2009 7:18 AM

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

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Anonymous Poster
#8

Re: Can a VFD Control a Single Phase 3hp Motor?

07/30/2009 8:51 AM

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

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#9
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Re: Can a VFD Control a Single Phase 3hp Motor?

07/30/2009 11:44 AM

COMMON, COMMON MISTAKE!!!

As is the usual case whenever this subject comes up, people get confused about what is being discussed.

The OP was asking about a single phase MOTOR, not a single phase drive. That Baldor product can accept single phase power as the INPUT, but the OUTPUT is always going to be 3 phase FOR A 3 PHASE MOTOR. YOU CANNOT USE THAT DRIVE ON A SINGLE PHASE MOTOR.

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#10

Re: Can a VFD Control a Single Phase 3hp Motor?

07/30/2009 1:41 PM

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.

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#11

Re: Can a VFD Control a Single Phase 3hp Motor?

07/30/2009 3:19 PM

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.

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