Variable Speed Control Question 1HP

[Dusty Sawyer]

I'm looking for a way to add some kind of speed control to my existing Lathe. Right now I can run between 400 and 1600 RPM on my Powermatic Model 45 lathe which employes sheeves. In order to turn large items on the outboard I really need to get down to about 75 to 150 RPM to be safe.

I've looked for motors with a much lower speed and have considered changing the pulley sizes on the motor and arbor where the drive sheeve is located, but at best I will still only get the RPM down a little bit because of clearance issues where the pullies are located.

So I am left with the option of either adding a second arbor to the drive mechanism where I can employ larger pullies to bring the speed down, or using a Variable Speed Phase Converter. Now, the arbor solution I can work out on my own pretty well. But I am wondering if anyone has experience with the VS Phase Converter ? ... they range in price from 120 on up and I may need to change my single phase motor out from a single to 3 phase for this to work.

Any suggestions?

Thanks,

 

[DIYGUY]

You should not need to do a phase conversion in order to use a VFD motor control. I suggest you cruise on over to OWWM.org and review the many posts others have made on this issue. The folks there are as nice as the ones here and there is a lot of expertise and material available for you.

In general I think you are going to give up a lot of torque if you try to slow down your motor, but never having done it I could be proven wrong. There are many, many variables involved here. I strongly suggest you enlist the aid of either a professional or at a minimum someone who is well versed.

Keep on turning ...

See if this helps:
http://www.owwm.org/viewtopic.php?t=35264&highlight=motor+speed+reduce

And from another post in the same vein on this site :
<snip>
As Bob V points out with the lathe, VFD's are not a direct replacement for pulleys or gear trains. Shaft horsepower is the product of torque x RPM. If you have a machine and you have a choice of two motors, both rated at the same output HP, but differ in speed, e.g. 3450 and 1725 RPM, it will make no difference which one you choose. If you pick the 1725 RPM motor, you will need a pulley that is twice the diameter as what you would need with the 3450 RPM one. The torque output (at rated speed) of the 1725 RPM motor is twice that of the 3450 RPM motor. Once you go through the speed reduction of your pulley system, assuming we have the same speed at the driven shaft, you still have the same HP available at that final output shaft and the same torque (minus the losses from the drivetrain, e.g. pulleys, belts, bearings, etc.) HP at the output shaft will be the same as the driving motor (minus losses), torque depends on the drivetrain speed ratio.

Now if we take a VFD and use it to control the output shaft speed, we have a different scenario. In general, a VFD running in the sub-100% range operates in "Constant torque" mode. If we run the motor at 25% of rated speed, the output torque is constant but the shaft HP is 25% of rated. If instead you use a drivetrain to accomplish the speed reduction, the HP available is constant and the torque at the driven shaft is 400% of what the motor is rated at. Torque relates directly to the cutting force generated. Power relates to how fast you can do the cutting.

This is also why an electric motor is rated for HP, not torque. They are directly related through shaft speed, but they are not interchangeable concepts.
</snip>

There is the answer to the torque issue I posed.

 

[Dusty Sawyer]

Thanks Mark!

 

[WoodWrangler]

As Scott mentioned above, I do have one on my bandsaw for the power feeder. It's 3-phase, but after being wired into that can now run off the shared power of the motor (220v). The PM3520b has one too. Unfortunately, that's about all I know. NCWWer "woodydiver" hooked me up a while back. He's not on much, but I'm sure would answer any questions you have if you drop him a PM.