How fast does a flip clock motor spin?

Here's a random question to ponder:

How fast does a flip clock motor spin? This question recently came up among members of FlipClockFans.com. Let's see if we can figure this out.

The Japanese Copal II motors are used probably in the majority of vintage flip clocks. These are a type of AC (alternating current) motor known as a synchronous motor. What this essentially means, is that the speed of the rotor (or turning part of the motor) is in sync with the frequency of the electrical current being used to run the clock. In the US and Canada, the frequency of household current is 60 Hz or 60 cycles per second. In the vast majority of the rest of the world, the frequency used for home current is 50 Hz.

The formula for calculating a motor's synchronous speed in revolutions per minute (or RPM) is as follows:

120 x the result of the frequency in Hz divided by the number of poles of the motor.

While many synchronous motors use a low number of poles (2-8) the copal motors have taken one winding (coil of wire) and arranged 18 interdigitated steel poles pieces around this coil. The bases of the steel projections are arranged so that they pick up flux from either one side or the other of the coil in an alternating fashion creating 9 pole pairs.
For more details about how small synchronous motors work check out this all about circuits article. They have a really good illustration of how the poles are created.

The following is just a video of the motor in action

So, in the USA, to figure the speed in RPM you would divide the frequency (60) by the number of poles (18) to get 3.333. Multiply this by 120 and you'll get 400 RPM (Revolutions per minute). Divide 1800 by 60 to get the revolutions per second. In this case, you end up with 400 divided by 60 to get 6.67 spins per second. If you look at the video of one of these little mighty Copal motors in action, it looks believable. Doing the calculation for people running clocks with 50hz current yields the result of 50/18 = 12.5 times 120 = 333.33 RPM or divide that by 60 to get 5.56 revolutions per second.

So what are we going to do with this little piece of trivia? Maybe bore our uninitiated family and friends further with our fascination with flip clocks. That's what they deserve for not loving these little guys.

So bore on Flip Clock Fans.


Sync. Speed (in RPM) = 120 x Frequency (Hz) / # poles