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DC treadmill motor question about power supply

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  • #16
    Hello. At the start of my 2x72 grinder build I purchased a scr 60, bridge rectifier and a potentiometer shown in the video in post 3. Now that I am ready to hook up the potentiometer, I have a question. The potentiometer on the scr has two leads, the new one has three terminals. Which terminals do I use. Thanks John b.


    • #17
      The circuit apparently expects a variable resistance to adjust phase angle (and thus voltage and speed). You should connect the wiper arm (probably center terminal) to one of the other terminals so that it functions as a rheostat (variable resistor). It would normally be connected so that a clockwise rotation increases speed. The reason for connecting the wiper to one of the ends is so that there will not be a momentary open circuit as the wiper traverses the resistive element. This was probably more important for low value wire-wound pots.
      Paul , P S Technology, Inc. and MrTibbs
      USA Maryland 21030


      • #18
        Wow you want to do a mag drill with it? Gonna need about 8-1 gear reduction. Every commercial drill that I have ever seen tops out at a very heavy-duty 600 RPM. And tapping would be even slower. That gear reduction would be in addition to any variable speed control. Not saying it can't be done, but it is going to require some thought and some doing.


        • #19
          The treadmill motors I've been using will run over 7000 rpm with full rectified line voltage. If the controller can handle actual DC, which would be the bridge rectifier with a filter cap, giving around 150 vdc, the no-load rpm can go a bit higher. However, a realistic rpm under load is about 4500, so a 3 to 1 reduction would give 1500 rpm as a 'working' speed. On my lathe, that's about what I get. I seldom use the full rpm, so I could have used an even higher step-down ratio. I almost never change the belt position on the pulleys- speed control is done by applying lower voltage derived from something akin to a variac. So basically I have a three-fold increase in torque automatically, and I've never had an issue with falling power at low speeds.

          What I'm suggesting is to figure out what rpm you can live with as your high speed, then use the maximum reduction ratio you can so you can still achieve that rpm. You could pick say 1000 rpm as your high speed, then use a step-down ratio of 5 to 1- that means a 5 times increase in torque at any lower rpm. That might be enough for your needs. If you need more torque, then sacrifice your high speed and use a higher step-down ratio.

          As others have suggested, you can easily smoke the motor if you're trying to use the motor directly and have it dialed in for a very slow speed. You can always use a separate fan to keep airflow up, but you're really testing the brushes at the high current levels that would be required to get a high torque output without a reduction drive. Mostly what I see is that the brush holder area isn't designed to dissipate heat very well, and the brushes can get hotter than stink if you continually push it- and the same goes for the armature. Far better to work out the maximum reduction ratio you can live with- let the motor run faster and with less current, not pushing it too close to it's rated current.

          An interesting side note- my Unimat came with what I think they called a 100 watt motor. Series motor, wound field. When I upgraded the bed I also decided to do something with the motor, so I rebuilt it to use permanent magnets (neodymium) instead of the field windings. I changed the brushes to use more of the width of the commutator, but didn't do anything to the armature except to recondition the commutator. At this point the motor must now use direct current, so I used a bridge rectifier straight from line voltage. Originally I figured the armature and the field winding would be roughly sharing the voltage applied to the motor- now the armature is getting full rectified voltage, or roughly twice what it worked with before. My worry was arcing at the commutator, but that turned out to be ok. With the stronger magnets, the back emf is higher, and that means the motor doesn't over-speed, even with the higher input voltage. But does it ever have torque now-

          I know that if I use that torque on a semi-continuous basis, the armature will overheat. I just have to be smart enough to not cook it. The main benefit I get is that the motor rpm doesn't drop nearly so far under load as it did previous to the mod. I use the same kind of power supply I use with the larger lathe, a transformer with many secondary taps, so the spindle rpm depends on the voltage I select. And as far as using that extra torque- so far I'm still using the original pulleys with the O-ring belt, and it will just slip and squeal- a change to a toothed belt and pulleys to match will be my next upgrade to this lathe.
          Last edited by darryl; 02-15-2020, 06:29 PM.
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-