In circuit design, it is important to pay attention to methods and approaches. Select different design schemes based on different circuit environments to achieve the expected results. In the process of designing circuits, it is necessary to understand some proprietary terms. This article will explain the differences between short-circuit protection, overload protection, and zero voltage protection!

        1. Short circuit protection

        When the insulation of electrical appliances or wiring in the electrical control circuit is damaged, the load is short circuited, or the wiring is incorrect, short-circuit faults will occur. The instantaneous fault current generated during a short circuit is tens to tens of times the rated current. The strong electric force generated by short-circuit current in electrical equipment or distribution lines may cause damage, arcing, and even fire.

        Short circuit protection requires cutting off the power supply in a very short time after a short circuit fault occurs. The common method is to connect fuses or low-voltage circuit breakers in series in the circuit. The operating current of the low-voltage circuit breaker is set to 1.2 times the starting current of the motor.

       2. Overcurrent protection

        Overcurrent refers to the operating state of an electric motor or electrical component that exceeds its rated current. Overcurrent is generally smaller than short-circuit current, within 6 times the rated current. The possibility of overcurrent in the electric 5-gas circuit is greater than that of short circuit, especially when the motor is frequently started and frequently reversed. In the case of overcurrent, if the current value can be restored to normal before reaching the maximum allowable temperature rise, the electrical components can still work normally. However, the impact current caused by overcurrent will damage the motor, and the instantaneous electromagnetic torque generated will damage the mechanical transmission components. Therefore, the power supply should be cut off in a timely manner.

       Overcurrent protection is commonly achieved through overcurrent relays. Connect the overcurrent relay coil in series in the protected circuit. When the current reaches its set value, the overcurrent relay will activate, and its normally closed contacts will be connected in series in the branch where the contactor coil is located, causing the contactor coil to be powered off. Then, the main contact of the contactor in the main circuit will be disconnected, causing the motor power supply to be cut off in a timely manner.

       3. Overload protection

         Overload refers to the operating state of an electric motor where the operating current exceeds its rated current but is less than 1.5 times the rated current, and this operating state is within the range of overcurrent operating state. If the motor operates under long-term overload, the temperature rise of its winding will exceed the allowable value and the insulation will age or be damaged. Overload protection requires instantaneous action without being affected by short-term overload surge current or short-circuit current of the motor, and thermal relays are usually used as overload protection components.

       When more than 6 times the rated current passes through a thermal relay, it takes 5 seconds to activate. It is possible that the heating element of the thermal relay has been burned out before the thermal relay is activated. Therefore, when using a thermal relay for overload protection, short-circuit protection devices such as fuses or low-voltage circuit breakers must be installed at the same time.

       1) Voltage loss protection

       If the power supply voltage suddenly disappears during normal operation of the motor, the motor will stop running. Once the power supply voltage returns to normal, it may start on its own, causing damage to mechanical equipment and even personal accidents. Voltage loss protection is a protective component set up to prevent the motor from starting or electrical components from working automatically when the voltage is restored.

       The start and stop control circuit controlled by contactors and buttons has a voltage loss protection function. Because when the power supply voltage suddenly disappears, the contactor coil will cut off power and automatically release, thereby cutting off the power supply to the motor. When the power supply voltage is restored, the contactor will not start on its own because the self-locking contacts have been disconnected.

       But in circuits that use manual switches and travel switches that cannot reset automatically to control contactors, specialized zero voltage relays are required. Once the power is cut off, the zero voltage relay is released, its self-locking circuit is disconnected, and when the power is restored, it will not start on its own.

       2) Undervoltage protection

       When the power supply voltage drops to 60%~80% of the rated voltage, the step of cutting off the motor power and stopping operation is called the undervoltage protection step. In addition to the undervoltage protection function of the button controlled contactor, undervoltage relays can also be used for undervoltage protection.

       Set the pull in voltage of the undervoltage relay to 0.8~0.85UN and the release voltage to 0.5~0.7 UN. The undervoltage relay is connected across the power supply, and its normally open contacts are connected in series in the contactor coil circuit. When the power supply voltage is lower than the release value, the undervoltage relay acts to release the contactor, and the main contact of the contactor disconnects the motor power supply to achieve undervoltage

Protection.

      3) Overvoltage protection

       Electromagnets, electromagnetic suction cups, and other large inductive loads, as well as DC electromagnetic mechanisms, DC relays, etc., can generate high induced electromotive force when turned on or off, causing electromagnetic coil breakdown and damage. Overvoltage protection usually involves connecting a resistor, resistor series capacitor, or diode series resistor in parallel at both ends of an electromagnetic coil to form a discharge circuit and achieve overvoltage protection.