# To Vfd Or Not?



## Ouija (Jan 21, 2016)

I'm planning on buying a Precision Matthews PM-1340GT lathe and PM-935TV mill, probably both in 3-phase configuration and with the variable speed upgrades. Because of this, I suppose I don't need the extra features of a VFD, I only need to convert from single-phase in to 3-phase out. I've tried to search around and read a bit on the topic but I wanted to get some input on this specific situation.


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## ch2co (Jan 21, 2016)

A VFD basically does just that, takes 120v AC turns it into DC then chops up the DC and produces two 220-240v AC outputs making up a 3 phase output.  The variable speed is just one of the many things it can do, by varying the frequency of the AC output.  Last least thats how this old guy understands it.

CHuck the grumpy old guy


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## Ouija (Jan 21, 2016)

Gotcha. I guess what I was getting at is that perhaps I could buy a phase converter instead of a VFD? I don't know much about VFD's but I know even less about phase converters.


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## Eddyde (Jan 21, 2016)

I'm pretty sure the vary speed 3 phase machines come with a VFD as thats the only way I know to vary the speed of a three phase motor.


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## Ouija (Jan 21, 2016)

Eddyde said:


> I'm pretty sure the vary speed 3 phase machines come with a VFD as thats the only way I know to vary the speed of a three phase motor.



That had crossed my mind but I don't know anything about electrical stuff. If that is the case it would save me a lot of time and trouble and I can just order both machines wired for single phase.


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## Eddyde (Jan 21, 2016)

Or make sure the VFD can take single phase input then you have it all, Call Matt at PM, He'll give you the lowdown.


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## Ouija (Jan 21, 2016)

Eddyde said:


> Or make sure the VFD can take single phase input then you have it all, Call Matt at PM, He'll give you the lowdown.



That's probably the best course of action. I've been waiting to call him until I get the electrician out here to wire up the 220V outlet in my work room. I just want to make sure there's no issues with getting the power I need before I order the machines.


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## mksj (Jan 21, 2016)

Eddyde said:


> I'm pretty sure the vary speed 3 phase machines come with a VFD as thats the only way I know to vary the speed of a three phase motor.


Actually the variable speed version of the PM-935 3 phase machine is a mechanical vari-speed and does not have a VFD. So you would need to supply it with 3 Phase either way, unless you get a single phase motor. I prefer 3 phase.

On a lathe a VFD allows you to dial in speeds a bit easier without having to change gears on the fly. The Jog is low speed as opposed to full speed. You also get very quick braking times, usually down to 1 second vs. a long time. You can probably get more low end drive (torque) by using a sensorless vector VFD, but you have less Hp below the motor base speed. Not that it matters, the machine has plenty of power either way. Two machines, no bells or whistles, go with a factory RPC. Want a bit more control, or you considering the stepped speed PM-935 then VFDs may be the way to go. VFDs also run fine off of RPCs, so you could use both. Lots of options.


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