The split-phase motor design is a type of single-phase induction motor that is commonly used for applications requiring moderate starting torque and reduced starting current. It achieves this through the use of two sets of windings: the main winding and the auxiliary (starting) winding.
Here's how the split-phase motor design contributes to better starting torque and reduced current in single-phase applications:
Phase Splitting: In a single-phase AC power supply, the voltage alternates between positive and negative cycles. In a split-phase motor, the main winding is designed to carry the current that is in phase with the supply voltage, while the auxiliary winding is designed to carry a current that is out of phase with the supply voltage. This phase shift between the two windings creates a rotating magnetic field, which is necessary for motor operation.
Starting Torque: During startup, both the main and auxiliary windings are energized. The out-of-phase current in the auxiliary winding creates a rotating magnetic field that lags behind the main winding's magnetic field. This phase difference between the two fields produces a starting torque that helps the motor overcome inertia and begin to rotate. The starting torque is higher than what would be possible with just a single winding, making split-phase motors suitable for applications that require moderate starting torque.
Reduced Starting Current: The design of the auxiliary winding is such that it has higher resistance and reactance compared to the main winding. This results in a higher impedance for the auxiliary winding, which limits the current flowing through it. As a result, during startup, the auxiliary winding carries less current than the main winding. This reduced current helps prevent excessive current draw during motor startup, which is beneficial for avoiding voltage drops and overloading the power supply.
Switching Out the Auxiliary Winding: Once the motor reaches a certain speed (usually around 75-80% of its rated speed), a centrifugal switch or other mechanism disconnects the auxiliary winding from the circuit. This is because the auxiliary winding is optimized for starting conditions and not for continuous operation. With the auxiliary winding out of the circuit, the motor operates more efficiently using only the main winding, which has lower impedance.
While split-phase motors offer advantages in terms of starting torque and reduced current, they do have limitations. They are not suitable for high starting torque applications and may experience lower efficiency compared to other motor types. For applications requiring even higher starting torque and more efficient operation, other motor designs like capacitor-start motors and capacitor-start capacitor-run motors are often used.