Figure 4-10.Rotor currents in a single-phase ac induction motor.
Now, if the rotor is rotated by some outside force (a twist of your hand, or something), the push-pull
along the line in figure 4-10, view A, is disturbed. Look at the fields as shown in figure 4-10, view B. At
this instant the south pole on the rotor is being attracted by the left-hand pole. The north rotor pole is
being attracted to the right-hand pole. All of this is a result of the rotor being rotated 90º by the outside
force. The pull that now exists between the two fields becomes a rotary force, turning the rotor toward
magnetic correspondence with the stator. Because the two fields continuously alternate, they will never
actually line up, and the rotor will continue to turn once started. It remains for us to learn practical
methods of getting the rotor to start.
There are several types of single-phase induction motors in use today. Basically they are identical
except for the means of starting. In this chapter we will discuss the split-phase and shaded-pole motors; so
named because of the methods employed to get them started. Once they are up to operating speed, all
single-phase induction motors operate the same.
What type of ac motor is most widely used?
Split-Phase Induction Motors
One type of induction motor, which incorporates a starting device, is called a split-phase induction
motor. Split-phase motors are designed to use inductance, capacitance, or resistance to develop a starting
torque. The principles are those that you learned in your study of alternating current.
CAPACITOR-START.The first type of split-phase induction motor that will be covered is the
capacitor-start type. Figure 4-11 shows a simplified schematic of a typical capacitor-start motor. The
stator consists of the main winding and a starting winding (auxiliary). The starting winding is connected
in parallel with the main winding and is placed physically at right angles to it. A 90-degree electrical