Welding postioner

Hi Guy's, I am in the process of making a welding positioner and using a 240dc motor from a treadmill, it hooked up to the 240v mains via a 2000w scr and bridge rectifier. The problem I have is that I can't get the motor slow enough! I want it to be variable from 3rpm to 25rpm. Any idea's?

Use a gear box for major reduction, then variable speed. What's its full voltage speed - 3000? Not realistic to have good performance of a 240 vdc motor at 3rpm.

You are dropping the speed by dropping the average voltage. You'd be better off with a 24v variable speed supply at those speeds, but you will get little hp. Also.. go to KBC's site and look at the DC motor controllers available. There are many better ways to control motor speed other then with what's little more than a light dimmer.

I used one of these Dayton AC-DC 115V Right Angle Gear Motor
To build mine. Found one on Ebay for cheap. I cut the RPM in half with chain and sprocket. Installed in an old electrical box. Works good.

Using some sort of speed reducer (gears, chain/sprocket) should also keep any electronics mainly out of harms way. Years ago I bought several new tread mill motors (such a deal) and they were 6,000 rpm. Just to try one out I put one on a small drill press using a simple rheostat (light dimmer) bridge rectifier and belt reduction through a jackshaft. It actually worked pretty well over most of its RPM range but not enough HP (rated 1/2 HP at 90 volts DC) for most of the drilling I did. It also started to overheat and as Lakeside53 has already mentioned, while I could dial the RPM way down at the lower end there was little power and it would stall easily.

You can do the math for the speed range you are looking for but be prepared to change pulley sizes etc. until you get the RPM/range you want. Using step pulleys on the gear/sprocket reduction would be a good idea.

Good luck,

Dave