Improved ways of making a sync motor?

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Original poster: "Colin Dancer" <Colin.Dancer-at-dataconnection-dot-com> 

I've been pondering sync motor modification for a while and wondered whether
anyone has had thoughts along the same lines or has expert knowledge?  My
goal is a physically smaller/lighter motor for a given sync output power.

Normally I've converted induction motors to sync operation via the
"standard" method of milling two flats (for 2pole motors) into the rotor.
This reduces the thickness of some of the aluminium "windings", but I
believe the major action is through removal of material from the iron
laminations.  For the bench grinders I normally convert, the rotor is 45mm
diameter and the optimum seems to be flats about 20mm across.  This is
measured by checking the minimum voltage required to achieve sync.

These motors have all locked up nicely, but their holding power is obviously
far down on their original non-sync operation. The result is that for a
given disk size you end up needing a motor which is physically much larger
than a motor originally designed for sync operation.  I'm guessing that the
reduced power is partly due to the removal of core excitation due to cutting
of flux lines due to slip, and partly due to reduction of the
"active"/"magnetized" area of iron on the outer edge of the rotor.

I also know that most "real" sync motors have slip rings providing DC to the
rotor windings effectively generating fixed N/S poles.  On the smaller units
the rotor may even be a permanent magnet.

Thinking about this, a couple of ideas have come to mind.

* Does it help to cut deeper but narrower length-ways notches rather than
wide flats?

* Similarly, can you get a higher power motor by cutting two deeper notches
/ holes in the middle of the rotor to fix the poles by effectively cutting
one or two of the aluminium rotor windings on each side of the rotor rather
than be removing large amounts of iron?

* Can you fix the poles by embedding a couple of high power rare earth disc
magnets on either side of the rotor (subject to ensuring they stay attached
at high speed to a hot rotor ;-) ?

* Alternatively can you convert an alternator into a sync motor, by feeding
DC to the rotor winding (as normal via the slip rings) and AC to the stator
windings (rather than driving the alternator and taking AC from the stator
winding)?  Depending on the design I could quite believe this setup might
not start in a predictable direction, unless there are multiple stator
windings and you "phase" one set relative to the other using a run
capacitor.

I've got a couple of motors to experiment on, but wondered whether anyone
have initial words of wisdom?

Colin.