The frequency-sensitive varistor is actually a special three-phase iron core reactor. It has a three-column core, and each column has a winding. The three-phase winding is generally connected in a star shape. The impedance of the frequency sensitive rheostat significantly changes with the change of the current frequency. When the current frequency is high, the impedance value is also high, and when the current frequency is low, the impedance value is also low. This frequency characteristic of the frequency sensitive rheostat is very suitable for controlling the starting process of the asynchronous motor. At startup, the rotor current frequency fz is maximum. Rf and Xd are the largest, and the motor can obtain larger starting torque. After start-up, as the speed increases, the rotor current frequency gradually decreases, and both Rf and Xf automatically decrease. Therefore, the motor can obtain a constant torque characteristic approximately, and the stepless start of the motor is achieved. After start-up, the frequency sensitive rheostat should be short-circuited.
Start circuit principle
1, automatic control
(a) Close the air switch QF to turn on the three-phase power supply.
(b) SA board to the automatic position, according to SB2 AC contactor KM1 coil power and self-locking, the main contact is closed, the three-phase power to start the motivation stator to start. (At this time, the frequency sensitive varistor is in series with the rotor circuit).
(3) At this time, the time relay KT is also energized and starts timing. After the set time, the normally closed contact of KT's time-delay closed is closed. The intermediate relay KA coil circuit is connected. The KA's normally open contact is closed, making the contactor KM2 coil loop possible. Electricity, KM2's normally open contact is closed, the frequency sensitive rheostat is short-circuited and the start-up process is completed.
(4) The thermal relay for line overload protection is connected to the secondary side of the current transformer because of the large motor capacity. In order to increase the sensitivity and reliability of the thermal relay, it is connected to the secondary side of the current transformer.
(5) In addition, during the start-up period, the normally closed contact of the intermediate relay KA short-circuits the thermal element of the relay in order to prevent the thermal element from malfunctioning due to a large starting current. During normal operation, the KA normally closed contact opens and the thermal element is connected to the secondary circuit of the current transformer for overload protection.
2, manual control
(A) Close the air switch QF to turn on the three-phase power
(b) Move SA to manual location
(3) Pressing the start button SB2, the coil of the contactor KM1 is energized, sucked and self-locked, and the main contact closure motor starts with the frequency sensitive rheostat.
(d) When the speed is close to the rated speed or when the current meter is observed near the rated current, press the button SB3 intermediate relay KA coil and the self-locking, KA's normally open contact closes the KM2 coil circuit, the KM2 normally open contact will be closed Frequency-sensitive rheostat short-circuit resection.
(e) KA's normally closed contact opens, and the thermal element is connected to the secondary circuit of the current transformer for overload protection.
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