Adding to Bjarney's excellent posts, here are a few simple items:
1: MFD size is a figure derrived from the needed XC (Capacitive Reactance) in series with the XL (Inductive Reactance) of the aux. (start) winding - in order to achieve a given phase lag (or lead) as viewed by the main (run) winding.
With Cap. Start scenarios, the result of the start winding will have a larger level of current flowing in the start circuit.
In order to get the 1Ã˜ rotor to spin off from static stand still, the normally stable magnetic field on the run winding needs to be reduced at one point - which is what's done by the start winding.
Changes in Capacitor MFD size affects the overall X of the start circuit. Too much will draw an excessively large start current plus put the phase offset too far away from an effective level - ending up with an uneven or difficult start with a loaded rotor.
Too small of start cap. size affects the starting torque and may keep a loaded rotor from running out of locked rotor.
2: For start/run or PSC (Permanent Split Capacitor) motors, the running cap. will be sized just large enough to allow a constant and low level of current to flow in the aux. winding(s). Run caps are typically small sizes (like no bigger than 5.0 ÂµF), and result in a slight running phase offset.
For further data, check out a good Electrical Engineering Handbook, where you will find all kinds of geeky information!
If members wish to submit text data regarding this, send it to me and I'll post it soon (when I get finished "Putting Out Fires"