The field of a single-phase motor, instead of rotating, merely pulsates. No rotation of the rotor takes place. A single-phase pulsating field may be visualized as two rotating fields revolving at the same speed, but in opposite directions. It follows, therefore, that the rotor will revolve in either direction at nearly synchronous speed, if it is given an initial impetus in either one direction or the other. The exact value of this initial rotational velocity varies widely with different machines. A velocity higher than 15 percent of the synchronous speed is usually sufficient to cause the rotor to accelerate to the rated or running speed. A singlephase motor can be made self-starting if means can be provided to give the effect of a rotating field.
To get the split-phase motor running, a run winding and a start winding are incorporated into the stator of the motor. Figure 7-9 shows the split-phase motor with the end cap removed so you can see the starting switch and governor mechanism.
This type of motor is difficult to use with air-conditioning and refrigeration equipment inasmuch as it has very little starting torque and will not be able to start a compressor since it presents a load to the motor immediately upon starting. This type of motor, however, is very useful in heating equipment (see Fig. 7-10).