A split-phase resistance start induction motor is a type of single-phase induction motor designed to provide higher starting torque compared to a standard single-phase induction motor. It's commonly used in applications where a higher starting torque is required but a three-phase power supply is not available.
Single-phase induction motors typically have low starting torque, which can limit their ability to start heavy loads or equipment with high inertia. The split-phase resistance start induction motor addresses this limitation by incorporating an auxiliary winding and a centrifugal switch.
Here's how it works:
Main Winding: The motor has a main winding, which is connected directly to the power supply and is responsible for producing a rotating magnetic field. However, on its own, this winding doesn't provide enough starting torque.
Auxiliary Winding: The motor also features an auxiliary winding, which is wound at a different angle relative to the main winding. This creates a phase difference between the currents in the two windings.
Centrifugal Switch: In addition to the windings, a centrifugal switch is used. This switch is mounted on the motor shaft and is designed to open at a certain speed, typically when the motor reaches around 70-80% of its synchronous speed. The switch disconnects the auxiliary winding from the power supply circuit once the motor has gained enough speed.
Starting Sequence: When the motor is initially powered on, both the main winding and the auxiliary winding receive current. Due to the phase difference between the two windings, a rotating magnetic field is created, which initiates the motor's rotation. The auxiliary winding provides the additional starting torque needed.
Centrifugal Switch Operation: As the motor accelerates, the centrifugal switch on the shaft senses the increase in speed. Once the motor reaches a certain speed, the switch opens, disconnecting the auxiliary winding from the power supply. This is done to prevent excessive current flow through the auxiliary winding during normal operation, as it's not designed to handle continuous operation.
Motor Operation: With the auxiliary winding disconnected, the motor continues to run using only the main winding. While the starting torque provided by the auxiliary winding is lost, the motor can still maintain its rotation due to the rotating magnetic field produced by the main winding.
The split-phase resistance start induction motor offers improved starting torque compared to a standard single-phase motor. However, it comes with some drawbacks, such as lower efficiency and the need for additional components (centrifugal switch, auxiliary winding), which can increase the motor's complexity and cost. It's important to properly size and configure these motors based on the specific application requirements to ensure reliable operation.