Single-phase squirrel-cage induction motors are powered by a single-phase power supply. They can operate directly when connected to a 220V single-phase AC power supply, but certain measures must be taken to prevent them from starting. Single-phase induction motors are widely used in many household appliances such as air conditioners, refrigerators, washing machines, and electric fans.
Single-phase asynchronous motors can be classified into single-phase capacitor-run motors, single-phase capacitor-start motors, and single-phase shaded-pole motors, depending on their starting or running methods. These will be described in detail below. Single-phase asynchronous motors generally have smaller capacities and poorer operating performance.
Figure 1. Schematic diagram of a single-phase capacitor-operated asynchronous motor.
(a) Wiring diagram (b) Current phasor diagram
Figure 1 is a schematic diagram of the working principle of a single-phase capacitor-operated asynchronous motor. The stator core of a single-phase capacitor-operated asynchronous motor has two sets of windings: the main winding U1-U2 (also called the working winding) and the auxiliary winding Z1-Z2 (also called the starting winding). The two windings are spatially separated by 90 electrical degrees. A capacitor C is connected in series in the starting winding Z1-Z2 and then in parallel with the working winding, and finally connected to a single-phase power supply. Let the current flowing through the starting winding Z1-Z2 be iz, and the current flowing through the working winding U1-U2 be iu. When the power supply is connected, the currents iz and iu flowing through the two windings are 90 degrees out of phase, as shown in Figure 2.
When the two windings of the motor are connected to an AC power source, if the current is positive, the current enters from the beginning of the winding and exits from the end; if the current is negative, the current enters from the end of the winding and exits from the beginning.
As can be seen from Figure 2, at the instant t = 0, iz = 0, and no current flows through windings Z1-Z2; while at this instant, iu is negative, and the current in windings U1-U2 flows from U2 to U1. Using the right-hand rule, it can be determined that a magnetic field as shown in Figure 2 will be generated in the motor at this time, and the direction of the resultant magnetic field is downward.
As shown in Figure 2, at the instant ωt = π/2, iu = 0, and no current flows through windings U1-U2; at this instant, iz is negative, and current flows from Z2 to Z1 in windings Z1-Z2. The magnetic field distribution inside the motor at this time is shown in Figure 2, and the direction of its resultant magnetic field has rotated clockwise by a certain angle compared to the time t = 0.
Similarly, it can be seen that the combined magnetic field generated by the two currents iz and iu in a single-phase squirrel-cage induction motor is also a rotating magnetic field, as shown in Figure 2.
A single-phase squirrel-cage induction motor also has a squirrel-cage rotor, meaning the rotor windings are squirrel-cage bars connected at both ends by short-circuit rings. The squirrel-cage bars cut the rotating magnetic field in opposite directions, generating induced electromotive force and induced current. Under the influence of the rotating magnetic field, the rotor rotates due to electromagnetic force. By changing the wiring of the start and end of the working winding or starting winding with the power supply, the direction of rotation of the rotating magnetic field can be changed, thus controlling the forward and reverse rotation of the motor.
