1. Short-circuit protection When a short circuit occurs […]
1. Short-circuit protection When a short circuit occurs, the main circuit is cut off by the fuse of the fuse FU. In order to expand the protection range, the fuse is as close to the power supply as possible in the line, and is generally installed directly under the knife switch.
2. Overload protection Overload protection is achieved through thermal relay FR. In the case of long-term overload of the motor, the FR action disconnects the control circuit, de-energizes the contactor KM coil, cuts off the main circuit to stop the motor, and realizes overload protection of the motor.
3. Under-voltage protection and loss (zero) voltage protection Under-voltage protection and loss (zero) voltage protection are realized by the electromagnetic mechanism of the contactor itself. When the power supply voltage is severely undervoltage or lost (zero) due to some reason, the armature of the contactor is automatically released, the main contact and the self-locking contact return to the normally open state, and the motor stops working. When the power supply voltage returns to normal, the contactor coil cannot be energized by itself, and the motor can be restarted only after the operator presses the start button SB2 again. The electrical control circuit has the following three advantages after it has undervoltage and loss (zero) voltage protection capabilities:
(1) It can prevent the motor from running at low voltage.
(2) It can avoid grid voltage fluctuation caused by multiple motors starting at the same time.
(3) It can prevent equipment and personal accidents caused by the electric motor suddenly starting and running when the power is restored.
It can be seen from the analysis of the electrical control circuit that the basic method of electrical control is to issue a command signal through a button, and the contactor controls the control object by controlling the input energy. The relay is used to measure and reflect the control process. In the individual quantities. For example, the thermal relay can reflect the temperature change of the controlled object, and send out control signals at the appropriate time to enable the contactor to achieve various necessary controls on the main circuit.