View Full Version : Advice Requested
09-14-2002, 12:24 PM
Hi to all! I'm the new kid on the block! http://bbs.homeshopmachinist.net//smile.gif I ask a lot of dumb questions and this is the first: I have a bench top knee mill with a 1/2 horse AC motor. Speed changes are via step pulley. The problem is that even with the belts not tensioned it take an act of Congress to move from pully to pully. I was wondering if there was some way I could add a variable spped control and set the belts on the highest spindle speed. I've asked several people about this,, some say it can be done, some say it can't. any thoughts and info as to what I need to do exactly would be greatly appreciated. Keep in mind that the only thing I know about electricity is is bites!
You'll need a 3-phase motor and a VFD (variable frequency drive). See assorted VFD notes in the archives. What you propose will work, most of the time. You'll still likely need to change belt position occasionally to get really low speeds, but it ought to reduce the amount of belt changing you need to do.
(n.b. you can't put a VFD on a single-phase motor.)
It shouldn't be that difficult to change belts though. I assume you know enough to turn the pulley as you're moving the belt from step to step. By chance is there enough take-up to let you buy a slightly longer belt, so you can get a bit more slack when you release the tension?
[This message has been edited by SGW (edited 09-14-2002).]
09-15-2002, 07:20 AM
Thanks SGW! I had suspicioned that would be the case. yes, I do turn the pulleys when changing but the belts are super tight. It might be more practical to search for one of those kits where you make up your own belts. That should allow me enough slack to make this a bit easier. Incidently, a Dremel tool has variable. Is there some type of device in that control that converts ac to dc and then back? Sounds like it's time for me to go to the local college and pick up a course in motor control. Thanks again guy!
Dremel tools and such have what are known as universal motors (with brushes). The speed of those can be fairly easily controlled.
An induction motor, on the other hand, is locked to the AC line frequency, Totally different animal.
09-15-2002, 12:53 PM
You can get drives that will control single phase motors fine, baldor makes several as do others. They are not inexpensieve though.
I think you could control speed fine this way but you might have alot less torque on the high speed end than you would like for some jobs.
Power belt sells 5' strips that you can add or remove links from to make the exact length you need. maybe that and a little extra motor take up is all you need.
Good luck, Rick
What happens to the starting circuit on a single-phase motor when you slow it down? Don't they cut out based on speed?
09-15-2002, 05:52 PM
That is right. The switch does drop upon speed reached at a given point. When a single phase motor is controlled by a drive the motors used don't normally have starting windings.
Many people choose to use three phase motors powered by drives using single phase.
I hope that will help, Rick.
09-16-2002, 09:03 PM
Thanks for all the feed back guys! I think I'll save myself a lot of grief if I use Chip's advice and switch to power belts. Thanks again to all!