Mechanical Systems 369
Induction Motors
Another popular type of single-phase AC motor operates on the in-
duction principle. This principle is illustrated in Figure 14-15. The coil
symbols along the stator represent the field coils of an induction motor.
These coils are energized by an AC source; therefore, their instantaneous
polarity changes 120 times per second when 60-Hz AC is applied.
Induction motors have a solid rotor, which is referred to as a squir-
rel-cage rotor. This type of rotor, which is illustrated in Figure 14-15B, has
large-diameter copper conductors that are soldered at each end to a circu-
lar connecting plate. This plate actually short circuits the individual con-
ductors of the rotor. When current flows in the stator windings, a current
is induced in the rotor. This current is developed by means of “transform-
er action” between the stator and rotor. The stator, which has AC voltage
applied, acts as a transformer primary. The rotor is similar to a transform-
er secondary, since current is induced into it.Figure 14-14. The universal motor: (A) Schematic diagram, (8) Pictorial diagram