What device protects motors from overheating by controlling electrical current?

The thermal overload relay is designed specifically to protect electric motors from overheating due to excessive current. When a motor experiences a current that exceeds its rated capacity, the thermal overload relay will detect this increase in temperature and disconnect the motor from the power supply, thus preventing damage due to overheating.

This device operates based on the concept of thermal expansion, where an increase in temperature causes a bimetallic strip inside the relay to bend. If the temperature rises too high, this bending action trips the relay, cutting off power to the motor. This proactive measure ensures the motor operates within its safe limits, prolonging its lifespan and maintaining efficiency.

In contrast, other devices listed may serve different functions: a ballast is typically used in lighting applications to regulate current; a switch is a simple on/off mechanism; and a circuit breaker protects electrical circuits by stopping overloads but does not specifically tailor its operation for motor protection. Thus, the thermal overload relay's unique capability to monitor and respond to the thermal conditions of a motor is what makes it the correct choice in this context.