Why is a thermal overload relay used with single-phase motors?

A thermal overload relay is essential in single-phase motors primarily because it serves to protect the motor from overheating that arises due to overcurrent conditions. When a motor experiences excessive load or a fault condition, the current flowing through the windings can increase beyond the motor's rated capacity. This excessive current generates heat, which can damage the motor's insulation and potentially lead to catastrophic failure.

The thermal overload relay monitors the temperature of the motor or the current flowing through it. When it detects that the current exceeds a preset limit for a specific duration, it interrupts the power supply to the motor, thereby preventing further overheating. This intervention helps prolong the motor's lifespan and ensures reliable operation, making it a critical component for safeguarding single-phase motors against thermal damage.

The other options do not accurately describe the primary function of a thermal overload relay in this context. For instance, while improving motor efficiency, preventing electrical noise, or enhancing torque characteristics can be important in motor design and operation, these are not the main roles of a thermal overload relay. Its explicit purpose focuses on safeguarding the motor from overheating due to conditions of overcurrent.