[time-nuts] Building a mains frequency monitor

[Ron Bean]

>> A more modern name for a synchronous motor is a permanent magnet stepper
>> motor.  Any PM stepper, and a couple of microfarad capacitor becomes a
>> synchronous motor when connected to the power line.

I've seen a data sheet for a motor that's actually spec'd for both uses.
I don't know how common that is.

>The old pre-digital way of generating a rotating field for a synchronous
>motor was a shaded pole.  They worked well for low torque applications like
>clock motors.  I saw plenty of them when I was a kid.  The rotor was hidden
>in a package with the first layer of gears.  I assume it was a permanent
>magnet but never took a package apart to verify that.

You're thinking of the Telechron clocks (also branded GE, Revere, and 
Herschede). Some good pics of the motors here:

http://telechronclock.com/
https://clockhistory.com/telechron/

Also, lots of pics of the clocks themselves:
http://www.telechron.net/main.htm

There are people who rebuild the rotors, for a fee (usually they'll swap 
yours for a rebuilt one). I've fixed one clock this way, and I have a 
couple of others that need it.

The other major manufacturer was Sessions, they had a bayonet system 
that allowed you to replace the whole motor easily (not including the 
gears), but they weren't as durable as the Telechrons.

A friend of mine has an early Sessions clock where the motor is held in 
with #5 screws. I'd never seen these before, it took me a while to find 
the right wrench to take it apart.