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Common protection methods for motors

Article Source:Thermal protectorAuthor:Kangding MetalPopularity:Published time:2019-10-14 17:38

Motor protection is a comprehensive protection for the motor. That is, when the motor has overload, phase loss, stall, short circuit, over voltage, under voltage, leakage, three-phase unbalance, overheating, bearing wear, and eccentricity of the stator and rotor, alarm or protection;
Devices that provide protection for the motor are motor protectors, including thermal relays, electronic protectors, and smart protectors. At present, large and important motors generally use intelligent protection devices.


 
Motor protector classification

1, 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. When the insulation of the motor windings, the insulation of the wires are damaged, or when the electrical circuit fails, for example, the main contact of the forward-rotation contactor is not opened and the main contact of the reverse contactor is closed, a short circuit occurs. At this point, a large short-circuit current is generated in the circuit, which will cause excessive heat generation, which will damage the insulation of the motor, electrical appliances and wires. Therefore, the power must be turned off immediately when a short circuit occurs. 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. Circuit breaker (commonly known as automatic switch), which has short circuit, overload and under voltage protection. Generally, fuses are more suitable for systems that require less motion accuracy and less automation. When a short circuit occurs, it is likely to cause a phase fuse to blow, resulting in single-phase operation; However, as long as the circuit breaker is short-circuited, it will automatically trip, and the three-phase circuit will be cut off at the same time. The circuit breaker has a complicated structure and is widely used in occasions requiring high requirements.


2, undervoltage protection
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. The electrical appliances that implement undervoltage protection have contactors and electromagnetic voltage relays. Fuse and thermal relays cannot be undervoltage protected because the motor operates under undervoltage. The increase in the stator windings is not sufficient to operate the fuses and thermal relays, so the two appliances cannot be undervoltage protected. When the motor is running, if the power supply voltage disappears for some reason, the protection of the motor self-starting to prevent the power supply from being restored is called zero voltage protection, and the zero voltage protection relay KHV is often used for zero voltage protection. When the motor is running normally, excessive reduction of the power supply voltage will cause some electrical appliances to be released, causing the control circuit to malfunction and may cause an accident. Therefore, it is necessary to cut off the power supply when the power supply voltage drops below a certain allowable value, which is undervoltage protection. Undervoltage protection Common electromagnetic undervoltage relay K When the motor is running normally, the excessive reduction of the power supply voltage will cause some electrical appliances to be released, causing the control circuit to malfunction and may cause an accident. Therefore, it is necessary to cut off the power supply when the power supply voltage drops below a certain allowable value, which is undervoltage protection. The undervoltage protection is commonly implemented by the electromagnetic undervoltage relay KV. The coil of the undervoltage relay is connected across the two phases of the power supply. When the motor is running normally, when the undervoltage fault or zero voltage occurs in the line, the coil KV of the undervoltage relay is energized, its normally closed contact opens, the contactor KM is released, and the motor is cut off. For the circuit where the button is activated and has a self-locking link, it has zero voltage protection function, and it is no longer necessary to consider zero voltage protection.


3, 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 appliance must ensure that the production machinery can be restarted after it is restarted, without causing personal and equipment accidents. This protection is a loss of pressure (zero pressure) protection. The appliance that realizes the loss of pressure (zero pressure) protection has a contactor and an intermediate relay.

4, 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. A field weakening relay (ie, an undercurrent relay) is connected in the DC motor excitation circuit to achieve weak magnetic protection. Undercurrent relay works: During the starting and running of the DC motor. When the excitation current value reaches the action value of the undercurrent relay, the relay pulls in, so that the normally open contact serially connected in the control circuit is closed, allowing the motor to start or maintain normal operation; However, when the excitation current decreases or disappears, the undercurrent relay is released, its normally open contact is opened, the control circuit is cut off, the contactor coil is de-energized, and the motor is de-energized.