Motor control circuits must have overload protection, short-circuit protection, and under-voltage protection. Generally, overload protection in motor control circuits is provided by thermal relays, while short-circuit protection is provided by fuses. Why, then, is overload protection in motor control circuits provided by thermal relays and not by fuses? In lighting circuits, fuses can provide both overload and short-circuit protection, so why can't they provide overload protection in motor control circuits? The following is an explanation of this issue.

Low-voltage fuses primarily consist of a fusible element, a fuse tube for installing the fusible element, and a fuse holder. Fuses are typically connected in series in the circuit. When the current through the fuse reaches or exceeds a specified value for a certain period, the accumulated energy causes the fusible element to melt, thereby cutting off the power supply and providing protection. Fuses have many parameters, such as rated voltage, rated current, breaking capacity, and time-current characteristic curves. The time-current characteristic curve shows the relationship between the current through the fuse element and the melting time of the element, exhibiting an inverse time characteristic. This means that the melting time of the fuse is inversely proportional to the current through the element—the larger the current, the shorter the melting time. The time-current characteristic curve of a fuse is illustrated below.

Generally, the relationship between the melting current and melting time of a fuse is as follows: when the current through the fuse is 1.25 times the rated current, the fuse element does not melt. When the current is 1.6 times the rated current, the fuse element melts in 3600 seconds. When the current is 10 times the rated current, the fuse element melts in 0.4 seconds. As seen in the table, the melting time is very short when the current is large, making fuses well-suited for short-circuit protection. From this analysis, it is evident that fuses are not sensitive to slight overloads—they require a long time to melt or may not melt at all. In practical applications, the overload factor of asynchronous motors should not exceed 1.2, as exceeding this would affect the motor's lifespan and reliability. This means the overload current should not exceed 1.2 times the motor's rated current. Therefore, fuses cannot serve as overload protection for motors and are generally used for short-circuit protection in motor control circuits.

When a motor overloads, if not addressed in time, the temperature of the motor windings and shaft can exceed allowable values, causing severe heating of the motor. This can damage the motor's insulation materials, accelerate insulation aging, and, in severe cases, lead to motor burnout. Thus, it is crucial to provide overload protection for motors. Because fuses are not sensitive to slight overloads and do not act quickly enough to cut off the power supply, thermal relays are typically used for overload protection in motor control circuits, while fuses are used for short-circuit protection. Each device serves its role to ensure the safe operation of the motor.