"Andy Dingley" wrote in message
...
On 18 Dec, 12:47, "Graham." wrote:

I know in its basic form a synchronous motor will run
in either direction.

Depends on the type - "synchronous" is just a characteristic of how
fast the motor runs, there are several ways to build them. Most of
these are fixed to go in one direction (either magnetically by
slugging, mechanically with a press-button ('30s clocks),
electrically, or switchably) so as to make them reliably self-
starting. If it's the simplest sort and really doesn't care, then it
needs flick-starting.

Many non-time-critical "synchronous" motors are also not quite
synchronous, as they use a little inbuilt slip to give self-starting.

It's the inbuilt slip, or phase lag, between an induction motor rotor and
the stator which gives rise to the restorative force that cases the motor to
turn - in effect the rotor is always playing "catch up" with the stator. By
definition it is then "synchronous" with the supply, speed control can be
achieved by increasing the number of poles/windings but there is an
underlying synchronism with the supply frequency in there somewhere. At rest
there is maximum phase lag equally in both directions and the forces on the
rotor are balanced - a flick will set it going either way as AD says. A
capacitor is the normal addition - this forces a small phase lag in one
direction when the motor is switched on - it starts to move and the normal
forces take over.


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)