Ok, Tom,

Capacitance is determined by three factors:  the area of the plates, the
distance between the plates, and the dielectric.

The larger the area, or the more closely the plates are spaced, the greater the
capacitance.  Thus, as you turn the rotor plates into the stator, you decrease
the distance between the plates and thus are increasing the capacitance.  But,
as plate spacing is closer, you run the risk of shorting at high voltages.

I know there are various dielectrics which also influence capacitance.  A
dielectric is essentially an insulator between the capacitor plates; to help
prevent breakdown under high voltages.

A capacitor with plates of a given area will vary in its ability to store
electric charge depending on the material separating the plates. On the basis
of this variation, each insulating material can be assigned a dielectric
constant.  Air is defined as 1 and other dielectric materials have different
values.  There are various dielectric materials including glass, paper,
tantalum, etc.

Steve Pollo
Lansing, MI