I might be buying an old 12x36 lathe very cheap. I would like to power its 3 phase 3 hp motor with a VFD. 220 single phase to 220 3 phase.

On a lathe, should I oversize the VFD or buy one rated for 3hp?

Oversize by 2, is the rule of thumb.

If its rated for a single phase input and comes from a reputable company I wouldn't oversize.

We are not talking phase converter. I'm looking at a vfd.

I have three machines on VFDs.  Never used a phase converter.  My 2X rule of thumb statement comes from other machinist’s.  Suit yourself.  A 6HP or even 10HP VFD is cheap as chips.

My research is pointing to using a 3hp rated vfd is fine if it is designed to run off single phase or 3 phase.

If you run a 3 phase to 3 phase vfd off single phase you need to oversize it by double.

I'm deciding if I want to oversize to a 4hp or 5hp rated vfd for extra safety margin. The difference in price does matter right now as I wasn't planning on a project right now.

I'm running VFDs on two mills.  I oversized one by 2 and matched the other one.  I like the performance of the oversized one better.  It can change directions much faster.

Are the settings for the ramp up speed the same?

No, I can ramp up and down faster on the oversized one.

If I ramp down my lathe too fast it will unscrew my chuck....

I think I'm going with a 5hp rated vfd just for a bit of safety margin. It will not draw more than 16-18 peak amps so I'm running 12-2 for the supply on a 20 amp breaker.

Just found this on vfd dot org.



Variable frequency drives also optimize motor starting characteristics. Variable frequency drives bring motors up to full speed quickly and by drawing only 100% to 150% of full load amps (FLAs). This ability to start at normal FLA is very important if the power supply cannot withstand the normally six times FLA across-the-line starting draw, or even the 350% FLA soft-start device current. Variable frequency drives do this by managing the magnetic flux of an induction motor. Magnetic flux is directly proportional to the voltage and inversely proportional to the frequency. By keeping the flux constant, the inrush current does not exceed the FLA rating of the motor, and full torque is maintained. This is a significant improvement on a soft-start, which has significant voltage drop problems and cannot start under full load.

That brings up a good point.  Is your new lathe powered by an induction motor?

As far as I know.

That's best.  Will give you a lot more range with the VFD.

I asked the electricians at work this very question. He told me that a VFD  should be close in size but never under. VFDs can be oversized but it can cause problems of you aren't careful. It depends on how you set them up and the features you use.

If you ran a 5 hp VFD on a 3hp motor, you still set the the same nameplate current on the smaller motor. So the VFD will still do its job and protect the motor from over current. The probelm is you can set a higher current, and if you fuse for the VFD size you do not have adequate circuit protection for the wiring in the smaller motor.


I set this guy up on my South Bend. Brand new old stock on ebay for $50 VFDs are dirt cheap these days.

Attached File


Attached File

My Monarch is a 3hp motor with a 3hp VFD and I've never tripped it out with the exception of slamming the clutch down in a high gear.  That's bad practice regardless, so I don't do it.

Given that most VFDs will tolerate a short overcurrent situation, I would (and have)  go size for size at this HP rating.   I don't think the additional current from the 5hp will provide any gain for the added cost.  If you need to up-size down the line then select a unit to fit that combo.

If it's a clutch lathe, that's great, use the clutch to ease into things.  If it's clutchless, like an SB, then set your spindle ramp to keep startup current moderate.  I don't use the VFDs for decel on any of my manual machines, so I can't speak to tuning that.  

I also don't vary the Freq, set it at 60hz and let it roll.  Yes, it will allow you to get a perfect SFM but in reality that's moot at this level of work.  Most machines were designed with speed selections to cover the bases of their capacities from 60hz.