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How to exclude the interference from the frequency converter

These problems often occur when you work at regular work and debug the equipment：
1 . When the converter of the heat exchange station is opened , the pressure transmitter shall jump ;
2 . using frequency converter to control the water supply , the voltage transformer is used as a signal for collecting pressure , and the voltage transformer is interfered by the frequency converter ;
3 . When the frequency converter starts up the motor , the voltage change signal is unstable and the jitter is great ;
4 . The voltage transformer ( 4 – 20mA ) shall jump after the frequency converter is started , and the integrated thermal resistance ( 4 – 20mA ) nearby is not affected , and the signal line is not shielded .
These phenomena are due to the interference of the frequency converter.
Why does the inverter produce interference
The inverter comprises a rectifying circuit and an inverter circuit , wherein the input AC power is converted into a DC voltage through a rectifying circuit and a wave loop , and the DC voltage is converted into a pulse voltage of different widths by an inverter ( called pulse width modulation voltage , PWM ) . Using this PWM voltage to drive the motor , you can adjust the motor torque and speed . This operating principle can lead to the following three electromagnetic interference :
1, harmonic interference
The rectifier circuit will produce harmonic current, the harmonic current generated voltage drop in the power supply system impedance, leading to voltage distortion, the voltage distortion for many instruments to form interference, voltage distortion is common in the top of the sine wave becomes flat. Harmonic current and voltage distortion is more serious in weak power situation the characteristics of this interference is formed using the same interference on the power grid equipment, but not between equipment and transducer distance.
2, RF conduction emission interference
Due to the load voltage of pulse shape, so the lessons from the power grid inverter current is the pulse shape, pulse current which contains a large number of high frequency components, the formation of radio frequency interference, the interference characteristic is the formation of interference to use the same power meter, and independent of the frequency converter between the instrument and the distance.
3, radiofrequency radiation interference
RF radiation interference input cable and output cable from the inverter. The inverter with input and output cable radio frequency interference current, because the cable is equivalent to the antenna will produce electromagnetic radiation, radiation interference. PWM inverter output voltage transmission on the cable, the same contains abundant high frequency components will produce electromagnetic radiation the formation, radiation interference. The characteristics of radiation interference, when the other electronic equipment near the inverter, the interference phenomenon becomes serious.
Interference processing method for frequency converter
1 . The frequency converter shall adopt single – point grounding , preferably short and thick line for grounding ; 2 . The signal lines of the sensor shall be shielded by double feet , and the shielding layer shall be grounded with the cable clamp . and 3 , adding a power filter , a filter magnetic ring or an isolator or the like on the power supply of the sensor . and 4 , the harmonic wave generated by the frequency converter is subjected to suppression processing , and the optional filtering product comprises a frequency converter input filter , a frequency converter output filter , a frequency converter input reactor , a frequency converter output reactor and the like , wherein the serial – in reactor in the input circuit is an effective method for suppressing the lower harmonic current . In addition , in order to prevent the frequency converter interfering signal and the control loop , it is necessary to supply the controller , instrument and industrial control unit with separate isolated power supply . In fact , the easiest way to live in the scene is : Keep the meter away from the converter !
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