Repulsion-Induction Motor, also known as the repulsion-start induction-run (RSIR) motor, is a type of single-phase induction motor that combines features of both repulsion motors and induction motors. It's designed to provide higher starting torque than a regular split-phase induction motor.
The speed-torque characteristic of a repulsion-induction motor is unique due to its operational principle, which involves starting in the repulsion mode and then transitioning to the induction mode once it reaches a certain speed. Here's how the speed-torque characteristic generally looks for a repulsion-induction motor:
Starting Phase (Repulsion Mode): During starting, the motor operates in the repulsion mode. In this mode, the rotor windings are connected directly to the AC power source, and they develop a high starting torque due to the interaction between the stator and rotor windings. This high starting torque allows the motor to overcome initial inertia and accelerate the load.
Transition Phase: As the motor gains speed, centrifugal force acts on the rotor segments. These segments are attached to the rotor and are mounted at an angle. Due to this arrangement, as the motor speeds up, the rotor segments tend to move away from the stator windings. This gradually shifts the motor into the induction mode.
Induction Phase (Induction Mode): Once the motor reaches a certain speed, the rotor segments move far enough away from the stator windings, and the motor operates as a standard induction motor. The rotor's squirrel-cage windings interact with the rotating magnetic field produced by the stator windings, generating torque and maintaining continuous operation.
The speed-torque characteristic curve would generally show the following features:
Starting Torque: At the beginning of the curve, the motor exhibits a relatively high starting torque due to the repulsion mode operation. This enables the motor to start loads that require higher initial torque.
Transition Point: There's a point on the curve where the motor transitions from the repulsion mode to the induction mode. This transition point is influenced by the design of the rotor segments and the speed at which centrifugal force becomes significant.
Stable Operation: Once in the induction mode, the motor operates similarly to a regular single-phase induction motor. The torque decreases as the speed increases, and the motor settles into its steady-state operating point.
It's important to note that the exact shape of the speed-torque curve can vary based on the design of the motor, the arrangement of rotor segments, and other factors. Repulsion-induction motors are used in applications where high starting torque is required, such as for driving heavy loads or in situations where there's a need to accelerate the motor quickly.