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Not sure where to find a 60 to 120 rpm AC motor

I'd like to add a small motor to a hand cranked coffee grinder (Hario Skerton Coffee Grinder).  It's conical burr style grinder that produces a consistent particle size (much more so than the mini blender, whirlly style grinder).  I'll expand on why that's important when I build the thing.  

My problem is that is takes 10 minutes or so to grind enough coffee for two people. I'd like to put a motor on it.  I tried spinning it with a battery powered drill, but the RPMs were too high.  The burr spun faster than coffee beans could feed.  I'd like a motor in the 60 to 120 rpm range.  I still need to determine the torque required.  Additionally, I'd like to plug this right in to a 110 outlet.  Does anyone have a suggestion on an appliance I could scavenge or the type of motor I should look to buy?  
Thanks,
Mike

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Toga_Dan2 years ago

agree on the cordless drill. Iirc, its possible to power em direct from their charger if batt fails. Maybe possible to add a second frill gearbox to get lower speed with lotsa torque.

... um... thats "drill" gearbox, not frill.

The battery powered drill seems to me like it would be a good match.

One of the nice things about old cordless
drills is they can be found second-hand for very low prices. I mean
once the battery fails, or gets lost, they're pretty much garbage to
most people.

I
actually built a phonograph record player one time, driving it with
what was formerly a cordless drill. However, this trick required a
speed sensor and feedback, for to keep the angular speed constant, at
33+1/3 rpm, or whatever speed it was supposed to be for that kind of record. That is to say, using feedback, it is possible to make a cordless drill turn at an
arbitrary constant speed, and I think that kind of control could maybe solve your "RPMs
were too high" problem.

I think I left myself some hand-drawn notes for the
circuit diagram for this drill powered phonograph. If you are interested, I
could share a picture of the notes, and also pictures of the artifact
itself. I think they're in the same box, somewhere around here...

MikeM50 (author)  Jack A Lopez2 years ago
Jack thank you very much for the great reply. I would appreciate the photos when you have a minute.

Did you check ebay?, it only took a minute and here is a 60 rpm AC motor, $30

Screenshot.png

I found the artifact, but I didn't leave myself any notes. So figuring out how I did this will take a little longer.

I found a drawing for the circuit for just the PWM (pulse width modulation) for driving the drill motor. Actually, I think this circuit was drawn for a similar, but earlier project, but I'm using basically the same circuit here. It is big MOSFET being driven by a circuit built around a SG3524, which is a PWM driver IC.

This might be enough to get you started. Or you might not want to even know the details of how I was doing the speed sensing and feedback, because there ?might? be an easier way.

I mean you might be able to drive the drill motor with PWM to throttle the power, but without feedback. It kind of depends on the mechanical load presented by the coffee grinder, and also the mechanical advantage of the gearbox. If the motor is turning at a speed so slow it is close to stalling, then it probably will stall without help from the feedback. So for that case, the feedback is needed, to keep kicking the motor, to keep it moving.

Conversely, if the gearbox has large mechanical advantage, so the motor can turn really fast while the output shaft turns really slowly, and it will just ride through the little bumps.

Either way, you need the motor to keep moving without stall, so you can leave it unattended for like 10 minutes, while it grinds your coffee.

pwm-drill-phonograph-record-player-3.jpgpwm-drill-phonograph-record-player-2.jpgpwm-drill-phonograph-record-player-1.jpgpwm_drill_motor_driver.jpg

go to ebay and type in "geared motor"