The task of protecting the electrical appliance is to ensure the long-term normal operation of the motor and to avoid damage to electrical equipment, power grid and mechanical equipment caused by various faults, so as to ensure personal safety. Protection is an integral part of all automatic control systems. Discussed here is the protection of low voltage circuits. In general, there are several common protections:
Short circuit protection
Due to insulation damage of the motor windings and wires, damage to the control electronics and lines, and short-circuit faults caused by incorrect operation of the wires, the measures to quickly cut off the power supply by the protective device are short-circuit protection. Commonly used short circuit protection appliances have fuses and automatic air circuit breakers. Common short circuit protection components are fuses and circuit breakers. The fuse of the fuse is connected in series in the protected circuit. When the circuit is short-circuited or severely overloaded, it is automatically blown, thereby cutting off the circuit for protection purposes.
When the grid voltage drops, the motor runs under undervoltage. Since the motor load does not change, the motor torque decreases under undervoltage, and the stator winding current increases, which affects the normal operation of the motor or even damages the motor. At this time, the power supply is cut off by the protection device for undervoltage protection.
Loss of pressure protection
When the production machinery is working, the power grid suddenly loses power for some reason. When the power supply is restored, the protection electric appliance must ensure that the production machinery can be restarted before it can run, and it will not cause personal and equipment accidents. This protection is pressure loss (zero pressure). )protection. Electrical appliances that achieve voltage loss (zero pressure) protection have contactors and intermediate relays
Weak magnetic protection
The protective electric appliance is used to ensure that the DC motor works under a magnetic field of a certain intensity, so that the magnetic field is not weakened or disappeared, and the motor speed is prevented from rising rapidly, and even a flying phenomenon occurs. This protection is weak magnetic protection.
When the motor load is too large, the starting operation is frequent or the phase loss operation, the working current of the motor will exceed its rated current for a long time, resulting in shortened or damaged motor life. When the motor is overloaded, the measure to cut off the power supply with the protective device is overload protection.
The protective device is used to limit the starting current or braking current of the motor so that the motor operates at a safe current value without causing damage to the motor or mechanical equipment. This protection is overcurrent protection. The method of implementing overcurrent protection: connecting the coil of the electromagnetic overcurrent relay in series in the main circuit, and the normally closed contact is connected in series in the control circuit; when the overcurrent value of the motor reaches the action value of the overcurrent relay, The normally closed contact opens the control circuit to stop the motor from being disconnected from the power supply, thereby achieving overcurrent protection.
The difference between overload protection and overcurrent protection: overload protection is realized by thermal relay, which has thermal inertia. It is only used when the motor is overloaded. It is used for protection of three-phase asynchronous AC motor. Overcurrent protection is realized by electromagnetic overcurrent relay. The action is sensitive. Once the overcurrent occurs, it can be operated immediately, and the power is cut off. It is mostly used for the protection of DC motor and three-phase AC wound rotor asynchronous motor.