VFD and Rotary Phase Converter

MasterBlaster

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A friend has a 4 HP lathe he trying to soft-start. It’s a 240-volt, 3-phase motor so he installed a rotary phase converter in order to derive his 3-phase source. Then he had a VFD installed in attempt to soften the start. The VFD did not work and it was removed. That’s where I inherit the issue.

My memory is that VFD’s do not play well with rotary converters.

Is using an over-sized VFD a better solution? If so, what size drive is appropriate for a 4 horse motor?

Thanks!

(I hope I’ve not duplicated an earlier attempt to post this, but I don’t see my previous post.)
 
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I am assuming that the power at his location is 240 single phase
The simplest thing to do is get a VFD the can work on 240 V single phase input and supply an out of 0 to 60HZ 3 Phase output to the motor
Nothing else is necessary. The VDF will give you variable frequency 240V output directly to the motor.
They even have VFD's that can use 120V single phase in and give you 240V 3 phase out.
 
You could use a VFD directly off of single phase power to run the lathe, but you normally have to double the rating when using single phase power so it would require something like a 10 hp VFD. Most manufacturers list the derating specs when using single phase power. This site has some made specifically for single phase power.http://www.driveswarehouse.com
 
A three phase softstarter will not convert single phase to three phase so that is out.

VFD's with single phase inputs (240VAC splitphase) are readily available in 5hp output ratings. The motor would have to be 208VAC or 230VAC three phase.

If that motor is really rated 4hp, it would be an odd rascal indeed. Maybe a picture of the nameplate could be sent.

One note on using a VFD. You must do the start/stop from the VFD keypad or control terminals, not the lathe controls. The VFD simply isn't going to have the short-term overcurrent capacity necessary to provide the inrush current the motor needs for instant starting at 60hz.
 
You guys bring up some good points, thanks! I called the owner with some follow-up questions and am awaiting his response. It looks like we’ll go with a 10 HP VFD. I’ve asked for the nameplate FLA of the motor before moving ahead with the solution.

I am concerned about him stating the lathe from the controller on the lathe. I may have to re-wire those buttons so they control the VFD if he is unwilling to use the buttons on the face of the VFD.
 
"A three phase softstarter will not convert single phase to three phase so that is out."


Isent that the point of the phase converter to take single phase to 3 phase????
 
Most soft starters will not work correctly on a feed from an RPC. The "3rd phase" is coming mostly from the capacitors, which look like a short to the SCRs in the soft starter, plus the phase angles are all wrong, 2 phases are going to be 180 degrees out (from the single phase source) and many soft starters only sense one phase then time the firing from that for all three, so that oddball phase relationship results in chaotic SCR firing and generally causes the soft starter to shut down, or destroy itself eventually.

VFDs don't need the RPC and some will have similar issues with being fed by them. As the others have said, just double the VFD amp rating compared to the motor amp rating.

But also, there is no such thing as a "4HP" motor in our world, so what this is will likely be an IEC 230V 50Hz motor that is 2.2kw, which would really be 3HP, but when run at 60Hz becomes 4HP just because of the speed change (because HP is a factor of torque and speed). You will likely find that the motor FLA is closer to 3HP, and if so, there are plenty of VFDs on the market that are ALREADY designed to accept single phase 230V in and give you 3 phase 230V out, WITHOUT needing to de-rate the drive, up to and including 3HP/ 2.2kW. It's all really based on current though, so start with the actual motor FLA.
 
If you buy a VFD made for single phase operation you don't need to derate anything.
At least not the ones I've seen. For ABB inverters I'm sure of it.

PS. A lot of metal lathes have the motor running constantly and then uses some type of clutch to engage the motor.n 4HP is a small lathe though so it might not have a clutch.

Still if you want to wire it by the book, you have to change some of the electric wiring regardless.
If the lathe has an e-stop you should wire that in as well. Better VFDs have STO function that is made for use with e-stop circuits.
 
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"A three phase softstarter will not convert single phase to three phase so that is out."


Isent that the point of the phase converter to take single phase to 3 phase????

Most soft starters will not work correctly on a feed from an RPC. The "3rd phase" is coming mostly from the capacitors, which look like a short to the SCRs in the soft starter, plus the phase angles are all wrong, 2 phases are going to be 180 degrees out (from the single phase source) and many soft starters only sense one phase then time the firing from that for all three, so that oddball phase relationship results in chaotic SCR firing and generally causes the soft starter to shut down, or destroy itself eventually.

VFDs don't need the RPC and some will have similar issues with being fed by them. As the others have said, just double the VFD amp rating compared to the motor amp rating.

But also, there is no such thing as a "4HP" motor in our world, so what this is will likely be an IEC 230V 50Hz motor that is 2.2kw, which would really be 3HP, but when run at 60Hz becomes 4HP just because of the speed change (because HP is a factor of torque and speed). You will likely find that the motor FLA is closer to 3HP, and if so, there are plenty of VFDs on the market that are ALREADY designed to accept single phase 230V in and give you 3 phase 230V out, WITHOUT needing to de-rate the drive, up to and including 3HP/ 2.2kW. It's all really based on current though, so start with the actual motor FLA.

Excellent info, thanks! The 4HP info came verbally, some research shows it to be 3HP.

Properly wiring-in a VFD will be more work, but now looks like the way to go.

I really appreciate the info.
 
If you buy a VFD made for single phase operation you don't need to derate anything.
At least not the ones I've seen. For ABB inverters I'm sure of it.

PS. A lot of metal lathes have the motor running constantly and then uses some type of clutch to engage the motor.n 4HP is a small lathe though so it might not have a clutch.

Still if you want to wire it by the book, you have to change some of the electric wiring regardless.
If the lathe has an e-stop you should wire that in as well. Better VFDs have STO function that is made for use with e-stop circuits.

It does have an e-stop, and I will make sure it works. It is foot activated, and I’m not yet sure if it uses a mechanical brake after opening the motor circuit, or somehow applies DC to the motor (or both). Gotta figure that out.

Thanks for the help.
 

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