Which of the following motor types has the highest starting torque?

The capacitor start, capacitor run (CSR) motor is recognized for its high starting torque due to the presence of capacitors in its design. When the motor starts, the capacitor provides a boost in phase shift, allowing the motor to develop a significant starting torque that is essential for applications requiring heavy load starting. This initial torque surpasses that of other motor types because it effectively increases the phase difference between the current in the start winding and that in the run winding.

As the motor accelerates and reaches near operational speed, the capacitor remains in the circuit, permitting efficient operation and maintaining good torque characteristics during running conditions. This dual capability is essential in scenarios where immediate and strong torque is needed for starting mechanisms, such as in compressors or fans.

By comparison, other motor types such as the split-phase motor, permanent split capacitor motor, and shaded pole motor have lower starting torques. The split-phase motor relies on the resistance and inductive reactance for phase difference, resulting in generally weaker starting torque. The permanent split capacitor motor provides a moderate starting torque but does not match the performance of a CSR motor. The shaded pole motor is known for its simplicity and low cost but offers very low starting torque suitable only for light loads.