I have some questions about capacitor start (and run as well) motors. Does the capacitor do anything besides create a phase difference between the start and run windings? Also, if the mfd rating changes will the phase angle change also? Thank you.

The answer to both questions is likely yes, but understand that it is a very dynamic situation with switched windings and capacitors.

To a significant degree, angles would vary with the infinite selection of motors and associated capacitors. Initially, one could take measurements on a loaded PSC {capacitor-run} motor and construct a simple phasor diagram based on various terminal voltages. For a starting capacitor and associated switching, it gets more complicated—for everything happens so fast, so peak measurements would not necessarily be the same as coincident measurements. Even with simpler PSC motors, at starting electrical characteristics will change over rotor acceleration, even though there is no discrete electrical switching.




[This message has been edited by Bjarney (edited 05-25-2003).]

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 1Ø 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 µF), 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" ...)

Scott35