On 18 Dec, 13:33, "Bob Mannix" wrote:

It's the inbuilt slip, or phase lag,

Slip isn't just phase lag, it's a slower speed too. Otherwise you've
built a perpetual motion machine!

Truly synchronous motors have slip when they start from zero speed,
but must have zero slip when running. This does make them more
complicated to manufacture, as they generally require some separate
starting device. OTOH, a simple fan motor can use a slug (a shorted
turn of thick copper around half the winding, inductively producing a
phase lag).

Practice for clocks was often to use manual start buttons. In the GPO
("Class B" clocks?) this "bug" was documented as a "feature". As the
clocks weren't self-starting after a power outage, they would be
obviously stopped rather than just set slow.