Slowing down a motor

You may be disappointed when you finally get the unloaded RPM's down to where you want them. At that point, the motor may not have enough strength to do the job.

Have you tried powering the motor, temporarily, with a 3-volt DC supply from a couple of D cells, and then putting a mechanical load on the shaft to see how it performs?

I think you'd be better served by buying what's known as a gearhead motor.

http://www.allelectronics.com/...EARHEAD-MOTOR/1.html

They are available in many voltages, and speeds. Their speeds are reduced through gearing, thereby increasing their torque, rather than through reducing their input voltage. They are very powerful for their size. The motor from a Tortoise switch machine is another suggestion. It is a stall motor that can't burn out even when stalled.

Just as an aside, you should never feel that you need to compromise a piece of equipment's performance, or get frustrated, just because you don't have the correct voltage laying around. Power supplies are among the easiest things to build, and if you don't want to build one, wall-warts in every voltage and current capacity can be found in the garbage, and at thrift shops, attached to obsolete telephones, answering machines, etc.

Great answers. Thanks.  I'll try the various suggestions. Something's bound to work!

   Bob A.

Another option would be a 3A three terminal variable regulator on a simple circuit to allow you to vary the voltage with a pot.  A handful of components is all it takes, a bridge rectifier, a couple of caps, and the regulator.

I believe you'll want to investigate gearhead motors as Arthur suggests. 

It sounds like you don't want to fuss with complicated electronics like a motor speed-controller.  Controllers vary the voltage to regulate speed using a tach or sensing the motor's internal generator voltage (so-called back-emf).  This is how our engines with speed control work - they automatically apply more voltage as needed to overcome load variations such as climbing a grade or going around curves.  A motor needs more current to start it moving so starving it all the time with diodes/resistors is problematic.  

One way to look at it is a gearhead motor lowers speed sensitivity to load variations.  This likely eliminates the need for a speed-controller in your application trading mechanical complexity for electrical complexity.  You can still use a speed-controller for geared motors (as our engines do) but you probably don't need that level of control. 

In your case the trick will be to find a suitable motor.  I'd think you can find surplus in the $10-$15 range.  Since you have 12VAC available, I'd look for a DC motor in the 6-12VDC range.  You'll still need a bridge rectifier and some method of adjusting the final voltage like a 3-terminal regulator (50 cents) as John suggests.  One thing about screw drives is they rotate faster than you might think.  Let's say you have 1/4-20 threaded rod.  Suppose you want to move the carriage at, say, 1 inch per second.  That's 20 threads or revs/sec or 1200 RPM. You'll find that's pretty fast for many gear motors (often in the 10-100 RPM range).

I haven't seen the article you refer to but I end up spending more time than I would have thought fussing with mechanical issues such as coupling the motor to the rod, positioning limit switches to sense end-of-travel, aligning the guide-rail for the nut, etc.

Given the fact that the motor is already running on a worm drive, he may have plenty of torque and just need a way to control the voltage to the motor, hence the regulator.  While a gearhead motor may be a slick solution, my sense is the thing is already mechanically working, so just adding a few parts may solve the problem.

If you use the adjustable regulator circuit use the 3 amp LM350,not the LM317 and use a heat sink. 

Dale H

You asked about a dropping resistor.  If you use a resistor, you create a "constant current" setup.  A motor works much better in a "constant voltage" mode because the motor usually needs extra current when starting the load into motion.  The voltage regulator suggestions would fit this mode better.

If the motor is small, I don't think you are going to get enough torque to do the job reliably.  All Electronics has several gear motors in their catalog.  Note that the no-load speed is faster than what you will get with a load.  You can get a nice motor for $15!

Yep, I did specify a 3A part.  There is also a 5A three terminal variable regulator, LM338,  for the really big jobs.

Check out the Linear Technology LT1083 for REALLY big jobs.  I have had good luck with them.  Hint: You can get free samples from their website.

That is a serious regulator. 7.5A!

I just created an account and asked for two samples, we'll see how it goes.

OK, guys, I took what appeared to be the simplest advice, and went to my favorite local place for such things (Hobby Engineering, in South San Francisco), and bought a small metal gear motor (Tinkering Supplies H04468-01L) that runs 240 RPM at 6V DC, which I compute (with 20 threads per inch in my threaded rod) to allow the crane to move at a stately one inch in five seconds.  With the low gearing (100:1), it should have ample power to the shaft.  I'm waiting for some glue and paint to dry to try everything out, but it looks promising.  Will let you know how it came out. Thanks again.

   Bob A.

Sounds good, I didn't realize you were willing to tear the old motor out.

How did you couple the motor's 3mm D-shaft to the 1/4" rod?

Well, you'll get the benefit of a flexible coupling which I suppose is what a real coupler should do.  You know the coupler is out there somewhere but by the time you find it or pay several times more for shipping than the part, etc. etc. you just want to be done with it!  So here's one rigid coupler I made with a basic drill and tap set a few years back using 1/2" acrylic rod for a 1/4-20 thread like yours. The coupler was tapped on the right; I thought I'd need to glue it in but it held tightly.  On the left, the motor shaft was also D-notched so there's the set-screw. 

Stan - Great idea!  If my jury-rigged assemblage doesn't hold, I may well try your idea! Thanks.

   Bob A.