One of the heavy load starting thermal relay protection circuits

The thermal relay is an overload protection relay. Its two or three heating elements are connected in series to the main circuit of the motor
. When the motor is overloaded, the current flowing through the heating element increases and causes it to heat up. The bimetallic sheet close to the heating element is heated
and bent in a direction. Until the release is pushed open, the coil of the contactor loses power and trips, thereby achieving the purpose of protecting the motor.
    The so-called heavy load refers to the load with large inertia moment (such as blower, hoist, air compressor, etc.), and the starting time of the motor
is long (more than 5s). In order to prevent the thermal relay from operating during startup, the following circuit can be used. Figure 4-10 shows a thermal relay protection circuit
    with a saturated current transformer . The thermal relay is connected through the saturated current transformer . The start-up time is 20 to 30 seconds, and can be up to 40 seconds.