Starting method of asynchronous motor with external frequency-sensitive resistor

(I) Formula

The external resistor is frequency sensitive and has stable automatic resistance changing performance.

What does a frequency sensitive resistor look like? A reactance with an iron core.

The motor is powered but not rotating yet, and the rotor winding is inducted.

The induced current has a large frequency and passes through the largest frequency-sensitive resistance.

As the rotor speed increases, the current frequency decreases.

The frequency-sensitive resistance gradually decreases and the machine is turned off after starting.

The manufacturing configuration is relatively complex and requires special information
　　
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The appearance of the frequency sensitive resistor used in conjunction with the wound rotor three-phase asynchronous motor is basically the same as that of a reactor with an iron core, but the material used for the iron core is iron plate, and there is no insulation treatment between the sheets, which is completely different from the ordinary reactor with an iron core. The figure below shows a form of appearance. There are three coils wound on the iron core, and the three-phase winding is connected in a star shape. In order to adapt to different motors or different starting performance requirements of the same motor within a certain range, the winding can be led out with 2 to 3 intermediate taps.

The characteristic of frequency-sensitive resistor is that when AC passes through its winding, the alternating magnetic flux in the core will generate eddy current inside it, which will generate heat loss, and the magnitude of this loss is proportional to the square of the power-on frequency. This makes the frequency-sensitive resistor equivalent to a resistor whose resistance is proportional to the square of the power-on frequency. The name of frequency-sensitive resistor can be understood as the relationship between resistance value and frequency is very "sensitive".

When the three-phase AC of a certain frequency (such as 50Hz) is first introduced into the three-phase winding of the stator of the wound rotor three-phase asynchronous motor, the rotor has not yet rotated. At this time, the rotor winding will cut the magnetic lines of force in the air gap at the same speed as the stator rotating magnetic field, generating an induced current with the same frequency as the stator power supply. When this current passes through the winding of the frequency-sensitive resistor, it will generate the maximum heat loss in its iron core, which is equivalent to the maximum external resistance. As the rotor starts and accelerates continuously, the frequency of the induced current in the rotor winding will also decrease, so that the heat loss generated in the iron core of the frequency-sensitive resistor will also decrease accordingly, which is equivalent to the external resistance decreasing. When the motor is started, the speed of the rotor will be close to the speed of its stator rotating magnetic field. At this time, the frequency of the rotor current drops to near zero, and the resistance of the frequency-sensitive resistor will also drop to the minimum value. The three-phase winding of the frequency-sensitive resistor is short-circuited with a switch, and its function ends here, that is, the mnemonic "get off the job after starting is completed".

From the above process, we can know that the frequency sensitive resistor is a starting resistor that automatically adjusts its resistance value during the starting process. Due to its excellent performance, it is widely used.

Frequency-sensitive resistors are manufactured by specialized manufacturers and have a complex matching relationship with electric motors. Generally, it is necessary to find dedicated technical information (such as larger electrician manuals, etc.) to solve the problem.