In addition to meeting the control requirements of the production process of the controlled equipment, the three-phase asynchronous motor control circuit must also take into account the reliability of the circuit's failure and abnormal working conditions
. Because when these situations occur, the current will increase, the voltage and frequency will decrease or increase, and the device will be damaged. Therefore, the protection link in the control circuit is an indispensable part of the motor control system. Commonly used protection circuits include short circuit protection, overload protection, overcurrent protection, voltage loss protection and undervoltage protection. 1, short circuit protection In motor control systems, the most common and dangerous faults are various forms of short circuits. If the insulation of electrical appliances or circuits is damaged, control appliances and circuits malfunction, or operation or wiring errors are caused, short circuit accidents may occur. When a short circuit occurs, the instantaneous fault current generated in the line can reach more than ten times the rated current . Excessive short-circuit current will cause damage to electrical equipment 15 or power distribution equipment, and even cause arc failure. cause fire. Therefore, when a short circuit occurs in the circuit, the power supply must be disconnected quickly and reliably, which requires the short circuit protection device to have instantaneous action characteristics. The common method of short circuit protection is to use fuse and low voltage circuit breaker protection device. 2, over-current protection over-current protection is a current-type protection that is different from short-circuit protection. The so-called overcurrent means that the motor or electrical component operates in a state where the
exceeds its rated current, which is generally smaller than the short-circuit current and does not exceed 6 times the rated current. This kind of overcurrent is more likely to occur during the operation of the motor than a short circuit, especially for motors that start frequently, rotate forward and reverse, and repeat short-duration operations.
over-current protection is commonly implemented by over-current relays. Usually over-current relays are used in conjunction with contactors , that is, the over-current electrical coils are connected in series to the protected circuit. When the circuit current reaches its setting value, the over-current relay action, and the normally closed contacts of the current relay are connected in series in the contactor coil circuit, causing the contactor coil to be de-energized and released, and the main contact of the contactor is disconnected to cut off the motor power. This current protection link is often used in the control circuits of DC motors and three-phase wound rotor asynchronous motors. 3, overload protection Overload means that the motor operates at a current greater than its rated current, but the overload current exceeds the rated current by a smaller multiple. Usually within 1.5 times the rated current. There are many reasons for motor overload, such as sudden increase in load, phase loss and operation, and reduction in grid voltage. If the motor is overloaded for a long time, the temperature rise of its winding will exceed the allowable value, causing the insulation material to become brittle, aging, and shorten its life. In severe cases, the motor will be damaged.
overload protection device is required to have inverse time characteristics and will not be affected by the short-term overload surge current or short-circuit current of the motor and act instantaneously, so thermal relays are usually used for overload protection. When more than 6 times the rated current passes through the thermal relay, it will take 5 seconds to act. In this way, the heating element of the thermal relay may be burned out before the thermal relay acts. Therefore, when
uses a thermal relay for overload protection, it still A short-circuit protection device with a fuse or voltage-resistant circuit breaker must be installed. Since overload protection characteristics are different from overcurrent protection, overcurrent protection methods cannot be used for overprotection.
4, voltage loss protection When the motor is working normally, if the power grid suddenly loses power for some reason, the power supply voltage drops to zero, the motor stops, and the moving parts of the production equipment also stop. Because under normal circumstances it is impossible for the operator to turn on the power switch in time. If no measures are taken, the motor will automatically start running when the power supply is restored, which may cause personal and equipment accidents, and cause grid overcurrent and instantaneous network degradation. The protection set up to prevent the motor from starting on its own or the electrical components from putting themselves into work when the voltage is restored is called voltage loss protection.The start and stop controlled by contactors and buttons have the function of voltage loss protection. At this time, because when the power supply voltage disappears, the contactor will automatically release and cut off the power supply to the motor. When the power supply voltage is restored, because the self-locking contact of the contactor has been disconnected, it will not start on its own. If instead of using a button, a manual switch or a form switch that cannot be reset automatically is used to control the contactor, a special zero-voltage relay must be used. Once the power is cut off during work, the zero voltage relay is released, and its self-locking circuit is disconnected. When the power supply voltage is restored, it will not start by itself.
『 statement : This article comes from the Internet. It is not an original article on the hotel engineering exchange platform. The source will be indicated. The reprinted content is for learning and communication purposes only. The views stated do not represent the position of the hotel engineering exchange platform. The copyright belongs to the original author. If there is any infringement, please contact us to delete it. Thank you
