VFD conversion

[drwc]

I am taking the plunge and converting my Prazi lathe to cnc, including a VFD spindle drive. The drive specs can drive the motor from "0 to 6900 rpm " with apparantly a flat torque curve. My question is should I drive the spindle 1:1 off the motor or should I use a step down ratio ? I don't see using the extreme speed range as this is way more than the spindle is currently , likely max 3000 rpm.

Thanks for the advise

Wayne

[Al_The_Man]

The drive spec may be 0 to 6900rpm, but can the motor take it?
Is the motor 2 pole or 4?
With a standard 4 pole standard motor you could tentatively run at ~ 3600 rpm (120hz).
Al.

[in2steam]

I think with that size machine a DC motor in this application would be better IMHO better torque for your money, You could use a 3 phase motor but there some cases when DC is more viable and better. If your torque curve is flat that means you will have next to no power at 6900 rpms and alot at low speed three phase motors are bell shaped they have less torque at the low end and have the most effective torque near there rated speed which again drops off above rated speed(actaully it flat lines there). DC motors have fairly even torque curve, for the amount of use that the machine will recieve, a brushed motor should not be a problem.
Most cheap AC 3 phase motors can only handle about 5000 rpms motor bearing depandant.
Many people have converted there samller lathes to DC using treadmill motors which spin at a fairly high speed, search the forums.
chris

[Warpspeed]

Wayne, the problem with VFDs on something like a lathe, is that you can certainly slow down the spindle to almost zero speed, but the torque is just not there when you do so.

With a gearbox, slower speeds mean greater gear reduction and more torque multiplication. If you are swinging a really big job, and taking really heavy cuts, you need that extra drive torque that only the gearbox can provide.

A VFD will provide constant torque with constant motor current down to almost zero speed. That means available horsepower decreases with rpm. But constant torque down at very slow speed is usually not enough. You really need INCREASING torque with decreasing speed with a lathe.

A VFD on a lathe would be like driving around in only top gear in your car. While it "could" be driven around at very slow road speeds, the performance would be very poor. You really do need that gearbox in both lathe and car for exactly the same reasons.

[drwc]

Thanks guys, fortunately i didn't put out any cash yet on the drive, will likely go the DC motor route, and maintain the belt ratios for to give both speed control and power maintainance at lower speeds.

[phomann]

Hi,

To go the DC motor route, have a look at the following motors and controllers;

This controller is quite cheap and works well.
http://www.surpluscenter.com/item.as...tname=electric

As for DC motors, the following are worth a look.

May require a 2:1 pulley reduction for a lathe.
http://www.surpluscenter.com/item.as...tname=electric

May be perfect for the Prazi, or a little small, but a nice motor either way.
http://www.surpluscenter.com/item.as...tname=electric

Then when you want to control the spindle speed from your computer, have a look at the DigiSpeeds I sell. :-)

http://homanndesigns.com/store/index...&products_id=2

Feel free to contact me if you have any questions.

Cheers,

Peter.

Originally Posted by drwc Thanks guys, fortunately i didn't put out any cash yet on the drive, will likely go the DC motor route, and maintain the belt ratios for to give both speed control and power maintainance at lower speeds.