Stoke Green Cricket Club Clock

This is a 'Gents of Leicester' mains operated timepiece with a 23" face. It had been repaired previously but had only worked for six months or so and had been stored for several years since.

The synchronous rotor runs at 200rpm and drives the minute hand via a 12000:1 three worm train. It is not self starting and requires the hand adjustment knob to be pulled and released to flick the rotor in the correct direction.

I found that the motor would run perfectly well once started, however it would start very rarely.

A first impression was that there was a considerable amount of grease on the train which meant the rotor was not very free to rotate- it would stop in less than one revolution from the flick start with no power. Cleaning the train and re-oiling freed the rotor up and it would spin for several seconds with no power. Great - this didn't help the starting performance however...

So was the motor itself faulty ? A previous repairer had shimmed out end float in the rotor shaft - I couldn't see why, because the shaft is biased to one end by a leaf spring. Maybe he was repositioning the rotor relative to the pole pieces?

My first thought was that perhaps the rotor had become demagnetised - but it soon became clear this was a reluctance motor with a simple soft iron rotor. So maybe something had become magnetised ? Ran a demagentiser over the rotor and pole pieces - no difference.

Next the coil - perhaps a shorted turn was sucking some of the driving power ? Took off the original windings (estimated at ~25000 turns) and replaced with 1600 turns of 0.25mm wire. This allowed a wider range of operating conditions to be tried - all to no avail. At higher magnetising currents the rotor just started dancing... What was left ???

Looking at the rotor assembly again I noticed something unexpected - there was something just visible in the very small gap between the rotor and the die cast flywheel. (Click on image for close-up.) Under magnification it appeared to be a single thin 'U' shaped wire spring with one end turned into the rotor and the other turned into the flywheel. Both the rotor and the flywheel were absolutely rigidly attached to the shaft, which up to that point had seemed quite reasonable - but - the presence of the spring indicated that they might be intended to move relative to one another...

Pressing the assembly apart required some force- the rotor came off relatively easily, but the flywheel was very stubborn. This revealed a plain area of the shaft where the flywheel should have been free to rotate, however it was 'glued' tightly in place with an unidentifiable white substance, presumably ancient 'lubricant'.


Cleaning up and reassembling the shaft with the flywheel now partially decoupled from the rotor completely cured the starting problem. I guess the secret is that the spring allows the rotor to 'cog' independently of the fly wheel, allowing it to synchronise more easily. A normal synchronous motor with a magnetised rotor doesn't require this additional complication so there must be a good reason to use the reluctance motor - perhaps lower overall manufacturing cost, or no worries about long term demagnetisation...?

The only problem is that the clock now runs at 15 Vac ! Rewinding the 25000 turns of fine 'hair' wasn't a real option so a transformer has been incorporated into the case.