Synchronous Motor
Built by Dick Watson in October 2005
As used to drive a digital clock in Meccano Magazine Quarterly volume 59, issue 2, April 1974.
This is an 8-pole motor which runs at exactly one quarter of the speed of the two pole alternators which produce mains electricity — that is a quarter of 3000 RPM or of 50Hz (50 RPM or 12.5Hz). The pointer on the right is geared down by a factor of 750 so that it rotates once per minute (there is a pair of Stokys 6:1 gears in the train).
This motor is quite powerful and had no trouble turning the drums carrying the numbers intermittently as required.
The instructions had an error. The eight poles of the rotor (Rod and Strip connectors) should alternate N S NS etc. every 45. This means that, at any moment the four coils of the stator, which are 90, should all have the same polarity, changing from N to S 50 times per second. During this time the rotor will move so that the poles nearest the coils change as well, i.e. through 45. The missing four poles of the stator are called “virtual poles”. If the wiring instructions in the article are followed, the poles of the stator alternate as would be needed for a 4-pole motor.
The motor would be much more powerful with extra coils to provide eight strong poles for the stator, instead of four strong and four weak ones.