On Fri, 14 Apr 2006 16:23:18 GMT, "Brian Sharrock"
wrote:

In the 'old' days; mains powered clocks (squirrel cage, synchronous) were
common.

Although a squirrel cage uses a synchronous stator winding, it isn't a
synchronous motor, at least not to this level of timekeeping. Their
rotors are powered by an induced current in the rotor (the squirrel cage
itself), not any sliprings or brushgear. This current is only induced if
the rotor experiences a moving or changing magnetic field - i.e. it
rotates at a different speed to the field in the stator. They can't
generate a torque unless there is some "slip", the speed difference
between synchronous speed and actual rotation. It's notable that the
more the slip, the more the output torque - so these motors can deliver
substantial power under load.

A frictionless, resistanceless, hysteresisless squirrel cage motor doing
no work would accelerate up to synchronous speed and then hold that
speed spinning freely, doing no work and producing no torque. An
idealised but possible motor might always run at a known slip which
could be compensated for by gearing. In practice such effects as
temperature and lubricant viscosity make this unpredictable, at least
for clockmaking accuracy.

Clocks use shaded pole motors, which are synchronous. As these don't
have the same increased torque response to slip they're inherently low
torque and thus only useful for clocks or other light tasks.