Phase loss protection of three-phase squirrel-cage asynchronous motor After reading the 18th edition of your newspaper this year, "Another Law on the Protection of Phase Loss of Motors", the author feels that there is something wrong with it. According to this method, if the circuit is out of phase before the motor is started, the motor cannot be started, and it can indeed play a certain protective role. But the article said: "Press the start button ..., the motor rotates. If the C-phase fuse is blown, the KQ is immediately released, and the KM is also powered off and disconnected for protection." The actual situation is: click here Method connection, as long as the motor rotates, no matter which phase fuse is blown out, the phase failure accident will occur, and KQ will never be released. Therefore, the KM will not be disconnected when the power is turned off, and the motor will continue to operate in the absence of phase until it is burnt out, and the motor cannot be protected at all! Most of the small and medium-sized three-phase motors currently used by industrial and mining enterprises are squirrel-cage rotor three-phase asynchronous motors. This motor has a history of more than 200 years. The reason why it can last forever is because of its excellent performance, large starting torque, simple structure, easy production, and low price. Electric Ceramic Cooktop,Ceramic Cooktop,Electric Stovetop,Electric Cooktops Ceramic Stove Shaoxing Haoda Electrical Appliance Co.,Ltd , https://www.hotplates.nl
In the future, I have read the discussion articles about the lack of phase protection of motors in the 13th edition of the 20th edition, the 28th edition of the 18th edition, the 29th edition of the 18th edition, the 34th edition of the 18th edition, and the 37th edition of the 18th edition. The best of all the specific methods proposed in the article can only be achieved: for example, the motor has been out of phase before starting. It can play a protective role when starting the motor. But the motor operation interruption phase will not work at all, and it will still burn the motor! Although some articles confirm the lack of this method, the analysis is not in place and no real reason has been found. Now the author will talk about the principle and structure of the three-phase asynchronous motor. Analysis on the problem of phase failure protection of three-phase asynchronous motors!
Squirrel-cage rotor three-phase asynchronous motors, when three-phase alternating current is passed through the stator coil, the stator core will generate a strong rotating magnetic field. This magnetic field will push the rotor core to rotate. The squirrel bar embedded in the rotor core is like the secondary coil of the transformer, which will induce the alternating voltage corresponding to the stator. Because the two ends of the squirrel cage are closed, it is like a short circuit in the transformer secondary coil. Will produce a large short-circuit alternating current. This alternating current causes the rotor to generate an additional alternating magnetic field. This magnetic field interacts with the magnetic field of the stator, which strengthens the electromagnetic force between the stator and the rotor, which increases the torque of the rotor and stabilizes the torque. Because the additional magnetic field of the rotor rotates at a high speed relative to the stator coil, the stator coil cuts the additional magnetic field of the rotor and the stator coil generates an additional electromotive force. This electromotive force is also called back electromotive force. Because of the existence of this back electromotive force, the squirrel-cage rotor three-phase asynchronous motor (the stator coil AC impedance is relatively small) has an important characteristic: that is, during the initial start-up, when the rotor is not rotating, the back electromotive force is 0, and the starting current is large , The starting torque is also very large, the starting current can reach about 7 times the normal working current. After starting, after the rotor rotates normally, the back electromotive force is established, which offsets most of the input voltage (can reach about 85%). Therefore, the current drops significantly (to about 1/7 of the starting current), which is the normal working current under the rated load of the motor.
It is also the existence of this back electromotive force that makes the line voltage of A, C or B, C at the motor end when the motor in operation is off in one phase (such as phase C), nor is it half of 380V. It's about 350V. At this time, the phase C voltage at the motor end can reach about 185V (the line voltage and phase voltage at this time depend on the size of the motor, the weight of the load and other specific circumstances). Think about the voltage of about 350V applied to the 380V relay that has been pulled in. Will it be released? Of course not! Therefore, the relay KM in the article of the "18th edition of the" Motor Phase Loss Protection Another Law "will never be released. However, due to the lack of one-phase power supply, the current of the other two phases will increase significantly, greatly exceeding the rated current. After a long time, the motor will burn the two-phase windings due to overcurrent! However, if the motor lacks one-phase power at the start of the motor, the motor will not start. If phase A or phase B is missing, relay KM cannot be closed due to power failure. If the phase C is missing, press the start button SB2, because the lack of phase C electrical relay KQ can not pull in, KM will not pull in, the motor can not start. Therefore, only in this case. This line can play a role in protecting the motor.
For the same reason, when the three-phase motor with star connection is interrupted during operation, due to the existence of back electromotive force, the midpoint voltage of the star winding of the motor does not fluctuate much, and the voltage between the zero line and the zero line is very small, which cannot drive the relay. As for the method of artificially finding the midpoint with the capacitor or resistance star connection method, the principle is the same as that of connecting the center point of the motor star winding to control the relay. It is impossible to directly protect the motor with the relay when the motor operation is interrupted.
In the 70s and 80s of the last century, an enterprise produced such a principle that the potential of the center point of the star-connected motor will change when the phase is broken, and a special protector is produced to protect the motor. Although an electronic device is used to amplify the amount of change in the midpoint of the star connection. Can play a certain phase protection. But if the sensitivity is lowered, it will not work. If it is adjusted up, the unbalanced fluctuation of the three-phase voltage, the change of the connection node of the transmission line and the continuous change of the load. Once the electronic device is amplified, even the continuous phase will produce a malfunction. Therefore, the effect is greatly reduced, and the utilization rate is not high!
As for phase A in the 20th edition of the 13th phase, phase A in the 18th edition of the 29th phase, phase A or B phase in the 18th edition of the 34th phase, if the phase has been disconnected before the motor is started, pressing the start button is a forced start of the phase failure. If the motor is larger, there will be a lot of current. When the switch or relay cannot start the motor and cut off the power supply, a large arc will occur, which is very likely to cause danger and damage the equipment or hurt people!
These models use star-point connection relays to protect the motor's broken (lack of) phase circuit when the motor operation interrupts the phase. As mentioned earlier, the voltage increase between the midpoint and the neutral is very small. If a high voltage relay is connected between the midpoint and the neutral to protect the motor, it will not work at all. If the phase is missing before the motor works and it just meets the forced start conditions, such as forced start. The midpoint voltage will be high. Sensitive relays with low voltage will burn out! Therefore, the use of these circuits in practice will bring great uncertainty and danger!
The principle of the motor protector currently used is to monitor the change of the three-phase current of the motor. As long as one phase is disconnected, the input current of this phase will be greatly reduced to 0, and the protector can immediately shut down the motor. For example, the JD-5B motor comprehensive protector produced by Taihua Electric Co., Ltd. in the early 1990s is very good. It is suitable for wide current, and the setting current is between 8A and 200A. If threading multiple times, the minimum current can be used below 4A. Not only can display and protect the broken (missing) phase, but also can display and play overload protection, and the price is not high.
With the rapid development of science and technology, many of the new motor protector control parts have used single-chip microcomputers, and digital circuits have been applied to motor protection. The effect is of course different, but the price is not ordinary.