376 Electrical Power Systems Technology
internal features of capacitor motors are similar to split-phase induction
motors. All induction motors have squirrel-cage rotors that look similar to
the one shown in the illustration. You can also clearly see that capacitor
motors have a centrifugal switch assembly, start windings, and run wind-
ings. Also, notice the location of the starting capacitor. The wiring diagram
of a capacitor-start, single-phase induction motor is shown in Figure 14-
- Notice that, except for a capacitor placed in series with the start wind-
ing, this diagram is the same as for the split-phase motor. The purpose of
the capacitor is to cause the current in the start winding to lead (rather
than lag) the applied voltage. This situation is illustrated by the voltage/
current curves shown in Figure 14-20. The current in the start winding
now leads the applied voltage, because of the high value of capacitance in
the circuit. Since the run winding is highly inductive, the current through
it lags the applied voltage. Note that the amount of phase separation now
approaches 90°, or an actual two-phase situation. The starting torque pro-
duced by a capacitor-start induction motor is much greater than that of a
split-phase motor. Thus, this type of motor can be used for applications
requiring greater initial torque. However, they are somewhat more expen-
sive than split-phase AC motors. Most capacitor motors, as well as split-
phase motors, are used in fractional-horsepower sizes (less than one hp).
Another type of capacitor motor is called a capacitor-start, capaci-
Figure 14-18. Voltage/current relationships in a split-phase induction motor