The overload relay is an electronic control component for overheating or overload current protection.
The main function of the overload relay is to protect the motor against overload damage. When the motor is mechanically overloaded, the current flowing through the motor is greater than the rated current. If the motor's supply voltage is too low or lacks phase, it can also cause the motor to be overloaded. When the supply voltage of the motor is too low, the current of the motor increases under the same load condition; when the motor is out of phase, the motor can continue to operate, but the current on the remaining two-phase winding of the motor increases sharply.
If the current flowing through the motor windings is too long, the motor will overheat, causing damage to the motor insulation and even burning the windings. In order to protect the motor from overcurrent damage, the overload relay is connected in the power supply circuit. The operating current of the overload relay is set to a certain value. When the motor current exceeds the value, the control relay contact is disconnected after a period of delay. The specific delay time is determined by the characteristics of the relay, and the overload relay has a reverse current characteristic, that is, The relay action time decreases with increasing current, which means that the relay has a longer delay of the action time when it is slightly overloaded, and the relay has a shorter delay time and almost immediate action when it is heavily overloaded. However, the overload relay does not provide short circuit protection because the overload relay operates for a relatively long time. In the control circuit, the contact of the overload relay is connected in series with the contactor coil. When the contact of the overload relay is disconnected, the controller is powered off and the power supply of the motor is disconnected. For large motors, the overload relay can be indirectly connected to the motor supply circuit via a current transformer. The overload relay can be reset automatically or manually. The manual reset can be reset by pressing the reset knob. Below we will discuss various types of overload relays.
