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The disturbance produced by the frequency converter in the motor

The disturbance produced by the frequency converter in the motor


  Frequency converter is applied to various fields of industrial control due to its characteristics of saving energy, energy saving, reliability and efficiency, but it also brings some interference problems. The power supply and electrical equipment in the field will affect the frequency converter, and the high order harmonic generated by the frequency converter will also interfere with the operation of the surrounding equipment. There are three main kinds of interference caused by frequency converter: interference to electronic equipment, interference to communication equipment and interference to radio. The interference to the electronic equipment, such as computer and automatic control device, is mainly induced interference; the interference to communication equipment and radio is radiation interference. If the interference problem of the frequency converter is not well solved, not only the system can not run reliably, but it will also affect the normal work of other electronic and electrical equipment.

  Main electromagnetic interference source and way of 2 variable frequency speed control system

  2.1 main electromagnetic interference sources

  Electromagnetic interference, also known as electromagnetic disturbance (EMI), is electromagnetic interference caused by external noise and useless signal in receiving, usually through the conduction of circuit and form of field. The rectifier bridge of the frequency converter is a nonlinear load to the power grid, and the harmonics produced by it will disturb the other electronic and electrical equipment of the same grid. In addition, most inverters in the inverter use PWM technology. When they work in the switch mode and switch at high speed, a lot of coupling noise is produced. Therefore, the frequency converter is an electromagnetic interference source for other electronic and electrical equipment in the system. On the other hand, the harmonic interference in the power grid interferes with the frequency converter mainly through the power supply of the frequency converter. There are a large number of harmonic sources in the power grid, such as all kinds of rectification equipment, alternating current and direct exchange equipment, electronic voltage adjustment equipment, nonlinear load and lighting equipment. These loads can distort the voltage and current in the power grid, thus causing harm to other equipment in the power grid. If the power supply of the inverter is disturbed by the polluted AC power grid, if it is not processed, the power grid noise will interfere with the inverter through the grid power supply circuit. The interference of the power supply to the frequency converter mainly includes overvoltage, undervoltage, instantaneous power down, surge, drop, peak voltage pulse, and radio frequency interference. Secondly, the common mode interference can also interfere with the normal work of the inverter through the control signal line of the frequency converter.

  2.2 ways of electromagnetic interference

  The frequency converter can produce high power harmonics, and it has a strong interference to other equipment. The interference approach is consistent with the general electromagnetic interference approach, which is mainly composed of electromagnetic radiation, conduction and induction coupling. Specific as follows: first, on the surrounding electronic electrical equipment to produce electromagnetic radiation; the electromagnetic noise of motor direct drive motor, the iron and copper loss increase, and interference to the power supply, through the distribution network transmission to other systems; the other lines of adjacent inverter induction coupling, inductive interference voltage or current. In the same way, the interference signals in the system interfere with the normal operation of the frequency converter by the same way. The following are analyzed respectively.

  (1) electromagnetic radiation

  If the frequency converter is not in a fully enclosed metal shell, it can radiate electromagnetic waves through space. The intensity of the radiation field depends on the current intensity of the interference source, the equivalent radiation impedance of the device and the emission frequency of the interference source. The rectifier bridge of inverter is nonlinear load for power grid. The harmonics generated by it are harmonic interference to other electronic and electrical equipments connected to the same power grid. Inverter inverters mostly use PWM technology. When the expected and repeated switch mode is generated according to the given frequency and amplitude instructions, the output voltage and current power spectrum is discrete, and has higher harmonic group corresponding to switching frequency. The problem of high carrier frequency and high speed switching of field switching devices (dv/dt up to 1KV/s) is quite prominent.

  When the metal shell of the transducer has gaps or holes, the radiation intensity is related to the wavelength of the interfering signal. When the size of the hole is close to the wavelength of the electromagnetic wave, it will generate the radiation of the disturbing radiation source. Similarly, the external radiation of the inverter will also interfere with the normal work of the frequency converter.

  (2) conduction

  The above electromagnetic interference can be transmitted to other circuits through impedance coupling or grounding circuit coupling besides transmitting through the wires connected to it. Compared with the radiation interference, the path of its propagation can be far away. The typical way is: the interference signal converter connected since the industrial low voltage network generated along the distribution transformer into the medium voltage network, and the other end along the distribution transformer into civil low-voltage distribution network, the electrical equipment from the civil power distribution bus become remote victims.

  (3) induction coupling

  Induction coupling is the third way of propagation between radiation and conduction. When the low frequency interference source, interference wave radiation power supply capacity is limited, and the interference source and not directly with other conductors, but the electromagnetic interference energy can be generated inductively coupled through other conductors or conductor converter input and output wires adjacent, adjacent conductors or conductors in induction interference current or voltage. Inductive coupling can occur in the form of capacitance coupling between conductors, or in the form of inductively coupled form or capacitance and inductance, which is related to the frequency of interference sources and the distance from adjacent conductors.

  3 measures against electromagnetic interference

  According to the basic principle of electromagnetic, electromagnetic interference (EMI) must be formed.