Each electrical equipment has its rated power. When the rated power is exceeded, it is called overload, and the protection against this state is called overload protection. The protection against short circuits within electrical equipment is called short circuit protection. Zero-voltage protection is also called voltage loss protection. When a power outage occurs, the circuit with the above functions will automatically trip, and the electrical equipment will not start on its own the next time power is supplied. The purpose of this function is to prevent the operator from forgetting to cut off the power supply during a power outage, causing the electrical equipment to start on its own when the next call comes, causing an accident. The general contactor control circuit has this function.Today qihe smt pick and place machine sharing some tips about short-Circuit Protection or overload Protection .
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 more than ten to dozens of times the rated current. The strong electrical power generated by short-circuit currents in electrical equipment or distribution lines can cause damage, arcing, or even fires.
Short-circuit protection requires cutting off the power supply within a very short time after the 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.
Overcurrent refers to the operating state of a motor or electrical component exceeding its rated current. The overcurrent is generally smaller than the short-circuit current, within 6 times the rated current. The possibility of overcurrent in electrical circuits is greater than that of short circuit, especially when the motor is frequently started and rotates forward and reverse frequently. 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. However, the impact current caused by the overcurrent will damage the motor, and the instantaneous electromagnetic torque generated will damage the mechanical transmission. components, so cut off the power supply promptly.
Overcurrent protection is commonly implemented with overcurrent relays. Connect the overcurrent relay coil in series to the protected circuit. When the current reaches its set value, the overcurrent relay operates, and its normally closed contact is connected in series to the branch where the contactor coil is located, so that the contactor coil is de-energized. By disconnecting the main contacts of the contactor in the main circuit, the motor power is cut off in time.
Overload refers to the operating state in which 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 overcurrent operating state range. 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 short-term overload surge current or short-circuit current of the motor. Thermal relays are usually used as overload protection components. When more than 6 times the rated current passes through the thermal relay, it will take 5 seconds to operate. The heating element of the thermal relay may have burned out before the thermal relay operates. Therefore, when using a thermal relay for overload protection, a fuse must be installed at the same time. Or short-circuit protection devices such as low-voltage circuit breakers.
Pressure loss protection
If the power supply voltage suddenly disappears when the motor is running normally, the motor will stop. 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 protection link set up to prevent the motor from starting by itself or the electrical components from starting to work by themselves when the voltage is restored.
The start and stop control circuits controlled by contactors and buttons have the function of voltage loss protection. Because when the power supply voltage suddenly disappears, the contactor coil will be deenergized and automatically released, thus cutting off the power supply to the motor. When the power supply voltage is restored, the self-locking contact of the contactor has been disconnected, so it will not start on its own.
However, in circuits that use manual switches and travel switches that cannot automatically reset to control contactors, a special zero-voltage relay needs to be used. Once the power is cut off, the zero-voltage relay is released and its self-locking circuit is disconnected. When the power is restored, it will not will start on its own.
Under voltage protection
When the power supply voltage drops to 60%~80% of the rated voltage, the link where the motor power is cut off and stops working is called under-voltage protection link. In addition to the under-voltage protection function of the contactor button control method, an under-voltage relay can also be used for under-voltage protection. Set the pull-in voltage of the undervoltage relay to 0.8~0.85UN and the release voltage to 0.5~0.7UN. The undervoltage relay is connected across the power supply, and its normally open contact is connected in series to 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. Implement under-voltage protection.
Large inductance loads such as electromagnets and electromagnetic chucks, DC electromagnetic mechanisms, and DC relays will generate high induced electromotive force when switching on and off, causing electromagnetic coil breakdown and damage. Overvoltage protection usually involves connecting a resistor, a resistor string capacitor, or a diode string resistor in parallel at both ends of the electromagnetic coil to form a discharge circuit to achieve overvoltage protection.Read more: Short-Circuit Protection or Overload Protection
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