Classic AC motors: split-phase induction motor

Besides the main field winding on its stator, this squirrel-cage induction machine also has an auxiliary, or starting, winding. The starting winding is physically displaced at an angle corresponding to 90 electrical degrees from the main field winding. Additionally, the starting winding comprises fewer turns of smaller-gauge wire than the main field winding. The starting winding will have a low reactance but a high resistance relative to the main field winding. The objective is to produce a second field that is both in space and in time quadrature with respect to the main field. Such a composite field simulates that produced by a two-phase power source and rotates at a synchronous speed. The rotation of the rotor occurs only when, by one means or another, it experiences the effect of a rotating magnetic field.

Part of this objective is readily attainable. The space-quadrature requirement is realizable from the physical orientation of the starting winding relative to the main field winding. The 90° phase displacement between the currents in the windings is not attained because the starting winding is not a perfect resistance and the main field winding is not a perfect reactance. Nonetheless, the composite field has a component that rotates, and this suffices to develop starting torque in the rotor. Some where in the vicinity of about 80 percent of synchronous speed, a shaft-mounted centrifugal switch opens the starting-winding circuit. If the starting-winding were to remain connected, its torque contribution would be minimal, but its energy dissipation could lead to an unsafe rise in temperature. The equivalent circuit and the phase relationships in this motor are shown in FIG. 7.

The operating characteristics of a small split-phase induction motor are shown in FIG. 8. Notice that the speed behavior is similar to that of a DC shunt motor. As the torque demand is increased, the motor slows down and is able not only to accept higher current but to operate at a higher power factor from the AC line. Both of these changes lead to the development of greater torque in the rotor. The current induced in the rotor is largely due to its relative motion with respect to the rotating field. This relative motion is a function of the slip, which is the difference between the actual speed of the rotor and the synchronous speed. Although the mechanism is some what different, both the DC shunt motor and the induction motor slow down in order to accept more torque-producing current when the mechanical load is increased.