Smoothing capacitor for 90 VDC supply?
You don't need to do anything. 

Normally 90VDC motors are run by unfiltered power from a phase control type speed controller, operated from 120VAC (or 180VDC from 240VDC). A properly designed driver will also give you significantly better torque at low speeds via IR compensation. 

Here is a typical speed controller used for this purpose. 

If you do choose to add capacitance you'll increase the voltage by approaching 40% as the ripple is reduced, and add a surge to the variac wiper which it may not like (or the fast blow fuse may blow to protect it). Peak to peak ripple is:

VRp-p = \$\frac{Tc I}{C}\$ where Tc is 1/120 or 1/100 for 60Hz or 50Hz repectively, I is the motor current and C is the capacitance in Farads. 

For example, if I = 1A and C = 330uF then at 60Hz the ripple would be 25Vp-p. You also need to ensure the ripple current rating of the capacitor is sufficient. But I don't suggest doing this.