Disk size for 3600 RPM sync TTY motor

Dear List:

A machinist-friend of mine just finished making me a perfectly balanced
1/2" thick by 8.75" diameter lexan disk and mandrel to mount it on my
3600 RPM synchronous TTY motor.  I was planning on having two 3.5" long
by 5/8" copper pipe electrodes going through the disk (1.5" on either
side), arcing along their edges like a cylinder gap.  The disk is indeed
perfectly balanced, but the motor just doesn't seem to have enough oomph
to get it to sync, even without the copper pipes.  It appears I'm going
to have to scale down the disk diameter, and probably thickness too.

I was wondering what size disks others have been using with 3600 RPM TTY
sync motors?  Is the problem one of rotating mass, where it just can't
get over the startup hump, or is it air drag, where any attempt at such
prominent rotating electrodes would be too much drag for this motor?

Regards, Gary Lau
Waltham, MA USA