"Andy Dingley" wrote in message
...
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!

Er, no you wouldn't!

Truly synchronous motors have slip when they start from zero speed,
but must have zero slip when running.

They can never have zero phase lag (slip) when running or they wouldn't
turn. The lag causes the force (which constantly tries to get them back in
phase) that causes them to turn. If they were turning with zero phase lag
you might have a perpetual motion machine! This phase lag is naturally there
becasue of the load (at minimum just bearing friction) on the rotor. The
trouble is at rest, when there is no net phase lag in either direction and
therefore no force. Thus an artificial one has to be applied to ensure
starting [in the right direction].


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