VFD 1 or 3 phase?

OK silly question of the week..

I have a single phase 240Vac 2hp motor on a woodlathe. I want to be able to vary the speed without dropping the tourqe. Do I need a 240V three phase motor or is there a single phase VFD?

First look up toruqe and interstand it's twisting effort indepenent of power. Second motors are essentilly constant torque devices.

If you want to slow your lathe yet keep its stock removal capacity (HP at the tool), you need to have a mechanical speed reduction to do so. You can't simply slow the motor.

Halve the motor speed and you halve the motor HP; there is no escaping this because it's tied to the laws of physics. An alternative is to determine the amount of HP you need at minimum motor speed (say 1 HP at 175 motor shaft RPM) and get a motor that develops 1 HP when dialed down to 175 RPM (6 hz) then to scale the motor HP at line sync (1750 PRM using the example: 10 HP).

Ignorance of physics will not only kill you but waste your time and money.

To answer the question as asked:

YOu need a 2 hp 3phase motor and a single phase input, 3 phase output VFD then think about Forrest Addy's response. If you have a belt drive now you will still need it. You can select a speed range to work in and use the VFd within this range. And example: for spindle turning you can set the lathe belts to give about 2400 rpm then set the VFD for 90 Hz max output and you can vary your speed all over the place. Second example: for bowl turning you can set the belt for about 600 rpm or maybe 900 rpm and vary the speed with the VFd within this range. this is how I use a VFD on my lathe. The VFD will not totaly replace the belt drive as you can see whenever you look at a OneWay lathe. it has a VFD and a belt drive for the exact reasons Forrest Addy explained.

Bruce Norton
Kingsport, Tn

I am doing a similar VFD conversion on my old Clausing metal-cutting lathe. Here is a speadsheet I did, showing the HP available at the spindle at various speeds. I am using a 7.5 HP 4-pole three-phase motor (1800RPM).



My lathe's original mechanical variable-speed drive had a 1HP motor, so I would have had a constant 1HP across my original spindle speed range, which was 360-2000RPM in open belt, and 52-280RPM in back gear. Actually rather less than 1HP, due to the rather inefficient movable-pulley belt drive system.

As you can see in the above chart, I would still have at least the original 1HP over my entire original spindle speed range. I realize that showing constant HP above the 60Hz base speed is incorrect. Most VFD and motor combinations will show a gradual reduction in motor HP above 60Hz. My setup would still be way over 1HP at the max needed frequency of 112Hz or so.

Darron

WOW! great stuff thanks everyone for clearing this up. Now a very quick question, is there a way to reduce the speed of a normal bench grinder (we are 240V 1phase here @ 50hz)I just want to slow it down a bit. PWM has been mentioned. I cant change the motor of course to a VFD controlled 3ph so am stuck with the 1ph but I am sure a speed controler can be added to drop the overall speed to around 1425rpm

There are a few VFDs available for 1 phase motors, but they do not work on all 1 phase motor types, and unfortunately the types they do not work on are the ones typically found on machine tools. Capacitor start motors cannot be controlled by VFDs because the capacitors tend to get into a race with the VFD transistors to see which one can let their magic smoke out fastest. 1 phase VFDs can only be used on PSC (Permanent Split capacitor) and Shaded Pole motors, both of which are suitable for things like pumps and fans only.