GearHeads Forum

Tmasters, not sure if you are still interested, but here is some info that I have discovered.

I have been running some of the cheap microwave synchronous motors for several months.  They are rated at 4W and get quite warm.  They still produce plenty of torque at half the rated voltage, so that is how I have been testing them.  The 12V motors are running with a 6V wall wart transformer and the 120V motors use a series resistor to cut the current in half.

These motors will randomly start in either direction, so I built a small mechanism to fix this.  The design is at https://www.thingiverse.com/thing:3548759

Trouble started when measuring the exact rotational speed to build a clock.  The motors are rated 5/6RPM, which should be 5RPM at 50Hz and 6RPM at 60Hz.  Both motors are close, but not exactly 6RPM.  One was around 5.824RPM and the other was 6.228RPM.  This may be OK for a microwave turntable, but not for a clock.

I opened one motor to see how they work.  The center hub is a permanent magnet with 4 north poles and 4 south poles.  This should rotate 15 times per second at 60Hz.  There are 5 gear sets with 12:69, 21:43, 16:35, 15:30, and 8:24 teeth.  The 12 and 69 tooth gears are very fine pitch and will likely be the first to fail.  The 21 and 43 tooth gears should wear evenly since they use a prime number of teeth, however they make it difficult to design into a clock. 

The other motor with different speeds must have slightly different gear ratios.  Both should be perfectly stable when running on AC power, but each would need a different gear ratio to drive a clock perfectly.  I think I will look for something else to drive a clock.  eBay has a few Synchron motors in the $20-30 range.

Steve

I agree, and that was my eventual plan.  But I'm still hung up on motors.  The many synchronous motors I looked at would be perfect if it were not for the fact that they all have an average rated life that is much too short for a clock that runs continuously.  So now I'm looking at a pendulum movement with a hidden magnet in the base of the pendulum and a hidden electromagnet behind the front panel.  I would use a simple microcontroller to apply tiny tweaking pulses to keep the pendulum on track.

Just a thought,  If I truly wanted to have exposed wooden gears and an accurate time piece - I would look at running th clock backwards - by that I mean I wouldn't start at the great wheel but rather start at the seconds hand and let everything else follow it.  In a normal geared clock you start with a lot of power (weight) and through gearing slowly have less and less actual power at the gear tips.  If you apply very small power from an electric movement at the seconds hand you could let there rest of the mechanism just follow along.

Tim,

Your query sparked my interest, so I ordered a couple of motors similar to these https://www.amazon.com/dp/B07G345S4H  Similar items are common on Ebay for around $3 each.

They appear to be very popular for running the turntables in microwave ovens.  The first test runs amazingly quiet with a lot of torque because of the gearing.  I may try to run one for a few weeks with a load to see if it stays quiet.

One huge disadvantage is labeled clearly in the image as CCW/CW.  The motor does not have a start capacitor to define the direction and will randomly start either clockwise or counter-clockwise.  Directional synchronous motors are a lot harder to find.  It should be a fun challenge to use a couple of one-way clutches and gears to force the output to run in one direction.

Steve

Steve - Wow, that looks like exactly what I want!  Constant RPM but no computer to deal with.  I'll look into that and check to see if it has any interesting relatives.  Thank you!

Tim

True, Sync motors are the gold standard and a lot less hassle.
 
Art

A small AC synchronous motor should have plenty of torque to run a clock.  There should be enough power to keep a few gears visibly spinning all the time.

Here is the first one that came up in my search:
https://www.amazon.com/Electric-Permanent-Synchronous-Rotisserie-Turntable/dp/B077YHG1F8

The rate is set by the line frequency which is supposed to be really accurate.

Steve

John:

  And thats a tribute to your workmanship, getting a week on a wooden clock
identifies you as a craftsman in my opinion.

Art

Most clocks that I design and build run at least 30 hours by design and half a dozen or more run a week on a winding.  Having said that they are as good barometers as they are time pieces - a change in the weather will upset them and often cause them to stop.  If I want to know exactly what time it is I look at my wrist.

Hi Tim:

  I think youll find controlling a stepper with an arduino isnt all that hard.
If you get confused as to how Im happy to help when people have questions
even on an arduino control type questions. Ive controlled a few steppers
in my time.
  Making a small arduino control a stepper has lots of examples and open
source on line. I made a marble/sand drawing video awhile back and I still
have code for making them move aorund here somewhere. :)

YaNvrNo:

>>happy new year all
>>enjoying my AZ winter holiday

    Now thats just rubbing it in as
I just finished parking the snowblower
and spreading salt.  :) ,

Happy 2019..
Art