Final answer:
Not all overload relay units have heaters. Some do, while others do not. The purpose of the heater is to provide thermal protection to the motor.
Step-by-step explanation:
Not all overload relay units have heaters. Some overload relay units have built-in heaters, while others do not. The purpose of the heater in an overload relay unit is to provide thermal protection to the motor against overloads and excessive current. The heater elements are designed to heat up and cause the relay to trip when excessive current flows through the circuit.