A common phenomenon of frequency converter interference
1. As soon as the inverter of the heat exchange station is opened, the pressure transmitter jumps randomly;
2. In the water supply controlled by the inverter, the pressure transformer is used as the signal to collect the pressure, and the pressure transformer is interfered by the inverter;
3. When the inverter starts the motor, the voltage transformer signal is unstable and beats badly;
4. The voltage transformer (4-20mA) jumps randomly after the inverter is started, while the nearby integrated thermal resistance (4-20mA) is not affected, and the signal line is not shielded;
These phenomena are due to the interference of the frequency converter.
Why do AC drives interfere?
First of all, you should know that the inverter is used to change the frequency. The inverter includes a rectifier circuit and an inverter circuit, and the input alternating current is converted into DC voltage through the rectifier circuit and flat-wave circuit, and then the DC voltage is converted into pulse voltage of different widths (called pulse width modulation voltage, PWM) through the inverter. By using this PWM voltage to drive the motor, you can adjust the torque and speed of the motor.
This principle of operation causes the following three types of electromagnetic interference:
1. Harmonic interference
The rectifier circuit will produce harmonic current, which produces a voltage drop on the impedance of the power supply system, resulting in the distortion of the voltage wavetype, and the distorted voltage forms interference for many meters, and the common voltage distortion is the top flattening of the sine wave. When the harmonic current is constant, the voltage distortion is more serious in the case of a weak power supply, and this interference is characterized by interference to equipment that uses the same power grid, regardless of the distance between the equipment and the inverter.
2. Radio frequency conduction emission interference
Because the load voltage is pulsed, the inverter draws current from the grid is also pulse, and this pulse current contains a large number of high-frequency components, forming radio frequency interference, which is characterized by interference to the meter using the same power grid, and has nothing to do with the distance between the meter and the inverter.
3. Radio frequency radiation interference
RF radiation interference comes from the input and output cables of the frequency converter. When there is a radio frequency interference current on the input and output cables of the inverter, because the cable is equivalent to an antenna, electromagnetic wave radiation will inevitably be generated, resulting in radiation interference. The PWM voltage transmitted on the output cable of the inverter also contains abundant high-frequency components, which will generate electromagnetic wave radiation and form radiated interference. Radiated interference is characterized by the phenomenon of interference becoming severe when other electronic devices are in close proximity to the frequency converter.
How to solve the interference problem?
1. The inverter should be grounded at a single point, and it is better to use a short and thick wire for grounding;
2. The signal line of the sensor is shielded with both feet, and the shielding layer is grounded with a cable clamp.
3. Install a power filter, filter magnetic ring, or isolator on the power supply of the sensor for isolation.
4. Suppress the harmonics generated by the inverter, and the optional filter products are: inverter input filter, inverter output filter, inverter input reactor, inverter output reactor, etc. Serial insertion of reactors into the input circuit is an effective way to suppress lower harmonic currents.
In addition, in order to prevent the inverter from interfering with the signal and control loop, it is necessary to use a separate isolated power supply for the controller, instrument and industrial computer.
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