Basic knowledge of short circuit, overload and zero voltage protection of switching power supply

Each electrical device has its rated power. When the rated power is exceeded, it is called overload. The protection for this state is called overload protection. The protection to prevent short circuit inside the electrical device is called short circuit protection. Zero voltage protection is also called loss of voltage protection. When a power outage occurs, the circuit with the above functions will automatically trip, and the electrical equipment will not start automatically when the power is supplied next time. The purpose of this function is to prevent the operator from forgetting to cut off the power supply during a power outage , and the electrical equipment will start automatically when the power is supplied next time, causing an accident. The general contactor control circuit has this function.

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 wrong, a short-circuit fault will occur. The instantaneous fault current generated by a short circuit is dozens to dozens of times the rated current. The strong electric force generated by the short-circuit current may damage electrical equipment or distribution lines, generate arcs, and even cause fires.

Short-circuit protection requires cutting off the power supply within a very short time after a short-circuit fault occurs. The common method is to connect a fuse or a low-voltage circuit breaker in series in the line. 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 a motor or electrical component exceeding its rated current. Overcurrent is generally smaller than short-circuit current and is within 6 times the rated current. The possibility of overcurrent in electrical circuits is greater than short circuit, especially when the motor is frequently started and frequently reversed. In the case of overcurrent, if the current value can return to normal before reaching the maximum allowable temperature rise, the electrical components can still work normally, but the impact current caused by the overcurrent will damage the motor, and the instantaneous electromagnetic torque generated will damage the mechanical transmission components, so the power supply should be cut off in time .

Overcurrent protection is usually achieved by overcurrent relays. The overcurrent relay coil is connected in series in the protected circuit. When the current reaches its set value, the overcurrent relay operates, and its normally closed contact is connected in series in the branch where the contactor coil is located, so that the contactor coil is de-energized, and then the main contact of the contactor in the main circuit is disconnected, so that the power supply of the motor is cut off in time.

3. Overload protection

Overload refers to the operating state where the motor's operating current exceeds its rated current but is less than 1.5 times the rated current. This operating state is within the range of overcurrent operating states. If the motor is overloaded for a long time, 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 the motor's short-term overload impact current or short-circuit current. Thermal relays are usually used as overload protection elements.

When more than 6 times the rated current passes through the thermal relay, it takes 5 seconds for it to operate. The heating element of the thermal relay may have burned out before the thermal relay operates. Therefore, when using the thermal relay for overload protection, a short-circuit protection device such as a fuse or a low-voltage circuit breaker must be installed at the same time.

1) Pressure loss protection

If the power supply voltage suddenly disappears during normal operation of the motor, the motor will stop. Once the power supply voltage returns to normal, it may start automatically, causing damage to mechanical equipment or even personal injury. The voltage loss protection is a protection link set to prevent the motor from starting automatically or the electrical components from starting to work automatically when the voltage is restored.

The start and stop control circuits controlled by contactors and buttons have the function of voltage loss protection. When the power supply voltage suddenly disappears, the contactor coil will be de-energized and automatically released, thereby cutting off the power supply to the motor. When the power supply voltage is restored, the contactor self-locking contacts are disconnected, so it will not start automatically.

However, in circuits that use manual switches or travel switches that cannot automatically reset to control contactors, a special zero-voltage relay is required. Once the power is off, the zero-voltage relay is released, its self-locking circuit is disconnected, and it will not start automatically when the power is restored.

2) Undervoltage protection

When the power supply voltage drops to 60%~80% of the rated voltage, the link that cuts off the power supply of the motor and stops working is called undervoltage protection link. In addition to the undervoltage protection function of the contactor with button control mode itself, undervoltage relay can also be used for undervoltage protection.

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

3) Overvoltage protection

Large inductive loads such as electromagnets and electromagnetic chucks, as well as DC electromagnetic mechanisms and DC relays, will generate high induced electromotive force when switching on and off, causing the electromagnetic coil to break down and be damaged. Overvoltage protection is usually achieved by connecting a resistor, a resistor in series with a capacitor, or a diode in series with a resistor in parallel at both ends of the electromagnetic coil to form a discharge circuit to achieve overvoltage protection.