Section 16.1: Single-Phase Induction Motors cont.
Electrical Method(s) cont.
In either the split-phase motor or capacitor-start motor, the starting and running (main) windings can be designed so as to produce winding mmfs that are nearly 90° (electrical) apart and as equal in magnitude as desired so as to produce an ersatz two-phase motor having starting torque
N.B.: If the winding mmfs aren’t equal*, a rotating net mmf is produced nevertheless, but it “wobbles,” i.e.; its magnitude varies with time
* Typically, FST ¹ FMAIN for economical reasons. Unlike the relatively expensive
main winding, which is designed to withstand full-load running duty, the starting
winding need only be designed for (brief) startup duty¾so it’s less expensive to
produce. The same economics applies to the capacitor-start motor’s capacitor
design requirements as well.