VFD for a single phase 1.5 HP motor?

[jim foote]

Hi all,
I have an old lathe (1949)with a step pulley arrangement. I want to control the speed better. Is it possible to put a variable frequency drive on a single phase AC 1.5 motor? I'm getting conflicting ideas about this when searching the net for info. Has anyone done this? Point me in the right direction. thanks jim

 

[Jim_LS]

I've looked at single phase output VFDs for an application at work. The VFDs are expensive, have poor starting torque and only work with some types of motors, It was cheaper and more efficient for us to replace the motor with a 3 phase and run it with a standard VFD.

 

[jim foote]

poor starting torque

thanks for replying. I don't have 3 phase power here at home. Just curious why poor starting torque would be a problem, could you just wait till its up to speed?

 

[Bart Leetch]

I have a 3 phase 2 HP Balder hooked to a VFD plugged into standard 240 running a 4 stepped pulley lathe no problem. I purchased this system because it can be transferred to another lathe when & if I get one. It has plenty of starting torque because I don't try to jam a tool into the wood before it gets up to speed & it gets up to speed & torque very quickly. I usually start at real low speed & bring it up to a comfortable speed then start turning. There are thousands of these VFD systems out there in use every day, even on new lathes

While you at it go for 2 HP because you'll lose a little power in the conversion. Make sure to get a motor rated to be controlled by a VFD & insulated better than an F rating.

 

[Bill Boehme]

There are a few inverters designed for use with certain kinds of single phase AC motors (probably not your motor though). Applications for single phase inverter drive systems are rather restrictive. As already mentioned they only work where there is no starting torque load which generally means fans, dashpot actuators, and similar low torque uses. The other much more significant requirement is a constant (non varying) load -- a fan meets that requirement, but a lathe definite does not. Furthermore, if that isn't enough, there are other show stoppers why your motor would not work -- the first is the age of the motor means that the insulation class of the windings would not be able to withstand the high voltage transients produced by the inverter and secondly, the motor is not rated for the higher temperatures needed for inverter duty -- because of the slower speed, the motor's fan would not provide adequate cooling.

 

[Odie]

Jim.....

I figured we'd see a post like Bill's above ^.

I am not well versed in electrical issues, but I considered the same things you are now contemplating when I converted my lathe from single phase 1 1/2hp, to DC 1 1/2hp with variable speed drive.

At that time, I was convinced by others who know about these things, that my best bet was to forget about the old motor, and get something that was specifically designed to do what I needed my lathe to do.......and, I'm very glad I took their advice! 😀

This doesn't mean the old single phase motor can't be useful for something else. At that time, I had no idea the old motor would end up becoming one of the most useful bowl finishing tools in my shop. The old motor is now powering one of the best buffing stations ever......! 😉

Call it fate, destiny, or just plain luck.....but, this homemade buffing station is now an irreplaceable part of my shop!

ooc

 

[Dale Miner]

thanks for replying. I don't have 3 phase power here at home. Just curious why poor starting torque would be a problem, could you just wait till its up to speed?

You don't need three phase power. Supplying the three phase current is the job of the VFD. VFD's that take an input of single phase, and output three phase are common, and for most commercially available wood lathes that have a VFD, that is the type they are supplied with. Most single phase motors that would be suitable for use in a machine tool, would have a starting winding and a capacitor that are used during the startup of the motor until the motor reaches about 75% to 80% of rated speed. The starting winding serves two purposes: It provides a nudge to the motor to start rotating, and it supplies an extra boost to the startup torque. If the motor was fed current from a VFD, it would not disengage the starting winding if ran at speeds below that needed to disengage the starting winding. The capacitor and windings would become overheated and fail in short order. If the motor was ran at speeds much in excess of the rated speed, the centrifugal switch that disengages the starting winding would likely be damaged.

While Bill is correct about inverter duty motors, i have one older standard duty motor that is on a metal working machine that is being fed from a VFD and it has not given trouble. I know of several other folk that have done similar and are having good luck at it. These motors are good quality TEFC motors, so maybe that is why that dragon has not reared it's ugly head.

 

[Bill Boehme]

The primary purpose of the start winding in a capacitor start motor is to tell the motor which way to turn. And as Dale said, it also add a bit of boost to the starting torque.