This paper analyzes and summarizes the common interference faults of frequency converter, and puts forward the corresponding solutions.
1. Introduction
As an efficient and energy-saving motor speed regulating device, frequency converter has been widely used in factories for its high cost performance. As we all know, frequency converter consists of rectifier circuit, filter circuit and inverter circuit. Among them, semiconductor switching elements are used in rectifier circuit and inverter circuit, and PWM control mode is adopted in control, which determines that the input and output voltage and current of inverter contain many higher harmonic components besides fundamental wave. These high-order harmonic components will distort the voltage waveform of the power grid, produce radio interference waves, and adversely affect the surrounding equipment, including the motor driven by the inverter. At the same time, due to the use of frequency converter, higher harmonic components will be generated in the power supply voltage of the power grid. When the thyristor rectifier equipment in the power supply works, it will lead to abnormal power supply waveform. In addition, the surge voltage caused by lightning strike, the on-off of power transformer and the on-off of power appliances will also distort the power waveform. When the grid power supply with distorted waveform supplies power to the frequency converter, it will have adverse effects on the frequency converter. This paper analyzes the above phenomena and puts forward some measures to reduce these adverse effects.
2. External interference of frequency converter
The interference of power supply to frequency converter mainly includes overvoltage, undervoltage and instantaneous power failure; Ups and downs; Peak voltage pulse; Radio frequency interference. If the power supply of the inverter is disturbed by harmonics from the polluted AC power grid without treatment, the power grid noise will interfere with the inverter through the power supply loop of the power grid. On the input circuit side of the frequency converter, AC voltage is converted into DC voltage. This is the so-called "power grid pollution" rectifier circuit. Because this DC voltage is smoothed by the filter capacitor and output to the subsequent circuit, it is actually the charging current of the filter capacitor that supplies power to the inverter, which distorts the input voltage waveform.
There are many harmonic sources in (1) power grid, such as various rectifier devices, AC /DC interchange devices, electronic voltage regulating devices, nonlinear loads and lighting devices.
These loads in the power supply network distort the voltage and current waveforms in the power grid, thus causing harmful interference to other equipment in the power grid. For example, when there is a large-capacity thyristor converter in the power supply network, because the thyristor is always turned on for part of the half period of each phase, it is easy to trap the network voltage and the waveform is seriously distorted. The rectifier circuit at the input side of the inverter may be damaged due to the large reverse recovery voltage, resulting in the breakdown and burning of the input circuit.
(2) Interference of power compensation capacitor to frequency converter
The power sector has certain requirements for the power factor of power users. Therefore, many users adopt the method of centralized capacitance compensation in substation to improve the power factor. In the transient process of switching on or off the compensation capacitor, the network voltage may have a high peak value, so the rectifier diode of the frequency converter may be broken down due to excessive reverse voltage.
(3) Interference signal radiated by power supply
Electromagnetic interference (EMI) is the electromagnetic interference caused by external noise and useless signals in the receiving process. It usually propagates in the form of a field through circuit conduction [2], that is, it radiates into the air in the form of electromagnetic waves, and its radiation field strength 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 interference suppression caused by (1) and (2) can be connected in series with an AC reactor in the input circuit of the frequency converter, and the impedance at the fundamental frequency can be ignored. However, for high-frequency interference signals with high frequency, it presents high impedance and can effectively suppress interference. The interference signal in item (3) is weakened mainly by absorption. Usually, an absorption capacitor is added to the power input of the frequency converter. You can also add a special "radio interference filter" to further weaken the interference signal.
3. Interference of frequency converter to peripheral equipment and its countermeasures
As mentioned above, the frequency converter will make the input power supply voltage produce higher harmonics. At the same time, besides the fundamental wave, the output voltage and current of the inverter also contain many higher harmonics, which will spread their energy in various ways, and these higher harmonics will adversely affect the surrounding equipment. Among them, the power supply distortion causes other equipment in the same power supply to malfunction, overheat, noise and vibration; The generated radio interference wave interferes with the TV, radio, mobile phone and other radio receiving equipment around the frequency converter, which can not work normally in severe cases; It will interfere with the external control signal of the frequency converter. After these control signals are disturbed, it is impossible to control the operation of the inverter accurately and normally, resulting in noise, vibration and heat generated by the motor driven by the inverter.
(1) Interference caused by docking in the same power supply equipment
When the inverter has a large capacity, the network voltage will be distorted, and the interference will be transmitted to other circuits through impedance coupling or ground loop coupling. In order to eliminate or weaken the interference caused by equipment connected to the same power supply, an AC reactor can be connected in series at the input end of the inverter, and a DC reactor can be inserted at the rectifier side of the inverter. You can also insert a filter at the power input of the frequency converter, as shown in the following figure 1:
LC filter is a kind of passive filter, which is composed of reactance and capacitance, and forms a * * * oscillation loop for higher harmonics, thus achieving the purpose of absorbing higher harmonics. The working principle of active filter is: by detecting the higher harmonics in the current, according to the detection results, the current with the opposite phase to the higher harmonics is input to weaken the higher harmonics.
(2) For the generated radio interference wave
At present, most frequency converters adopt PWM control mode. The output signal of the frequency converter is a high-frequency switching signal, and the output voltage and current of the frequency converter contain higher harmonics, which generate radio interference waves through electrostatic induction and electromagnetic induction. Some of these interference waves are conducted through wires, and some are electromagnetic waves and electric fields directly radiated into the air. Metal objects in the radiation field may also form secondary radiation. Similarly, the radiation outside the inverter will also interfere with the normal operation of the inverter.
Noise filter transformer can be used to suppress radio interference waves conducted by wires to form insulation for higher harmonics; A reactor is inserted to improve the impedance to higher harmonic components, and a filter is inserted at the input of the frequency converter.
It is more difficult to suppress radiated radio interference than to conduct them. The size of this radio interference depends on the structure of the inverter equipment itself and is related to many factors such as the length of the motor cable. The motor wire can be shortened as much as possible, and the wire twisting reduces the impedance; The input and output lines of the frequency converter are shielded by iron pipes; Connect the inverter housing; The input and output terminals of the frequency converter are connected in series with reactors, and a filter is inserted.
(3) For the generated noise interference
Because the inverter adopts PWM control mode, the output voltage waveform of the inverter is not sine wave, and the current flowing through the motor inevitably contains many harmonics. The harmonic frequency output by the inverter vibrates with the natural frequency of the rotor, and the noise near the natural frequency of the rotor increases. The harmonic component of inverter output increases the noise and other harmonics near the natural frequency of iron core, casing and shaft frame. Therefore, when the motor speed is controlled by frequency converter, the motor winding and iron core will produce noise due to harmonic components.
Figure 2 below shows the noise comparison between the motor driven by frequency converter and the motor directly driven by grid power supply. Usually, when the motor is driven by frequency converter, the noise generated by the motor is 5 ~ 10 dB higher than that generated by the direct driving of the grid power supply. The noise suppression measures that can be taken are:
① Choose the inverter with low noise, high carrier frequency and IGBT as the inverter module. A special motor for frequency converter is selected, and a reactor is connected in series between the frequency converter and the motor to reduce the higher harmonics generated by PWM control mode.
② Insert a filter between the inverter and the motor, which can convert the output waveform into sine wave.
(3) Choose a low noise reactor.
(4) For the generated vibration interference
When the frequency converter is used to control the motor speed, the motor will vibrate for the same reason as the noise. Especially, the pulsating torque produced by low-order and high-order harmonics brings great vibration to the torque output of the motor. If the mechanical system vibrates with this vibration, its vibration will be more serious.
Generally speaking, the following measures can be taken to reduce vibration:
① Strengthen the rigidity of mechanical structure and change rigid connection into strong connection.
② Connect a reactor in series between the frequency converter and the motor.
③ Reduce the output voltage-frequency ratio of the inverter.
④ Change the carrier frequency of the inverter.
In the process of motor speed regulation by frequency converter, if the speed regulation range is large, the * * * vibration frequency of mechanical system should be measured first, and then the frequency hopping function of frequency converter should be used to avoid these * * * vibration frequencies. If there is a margin of torque, U/f can be given smaller.
(5) Interference that leads to overheating of the motor in the control part.
Frequency converter is used to control the speed of motor. Due to higher harmonics, even if the same motor runs at the same frequency, the current of the motor will increase by 5% ~ 10%. The temperature of the motor will naturally rise. In addition, when the cooling fan of ordinary motor is installed on the motor shaft, the motor will overheat due to the insufficient cooling capacity of its own cooling fan when it runs continuously at low speed.
Countermeasures for motor overheating are as follows:
① The motor is equipped with a cooling fan, and the self-cooling type is changed to other cooling types. Increase cooling capacity at low speed.
② Choose a motor with larger capacity.
(3) Switch to variable frequency motor.
④ Change the speed regulation scheme to avoid the motor running at a low speed continuously.
With the improvement of electrical automation in the factory, all kinds of interference are increasing. Only by deeply understanding the interference problem of frequency converter and taking corresponding treatment measures can we reduce mutual harm and ensure the normal production and the stability of equipment to a greater extent.
refer to
[1] Li Zixian et al. Application, maintenance and repair of frequency converter [M]. Beijing: seismological press, 2005.
[2] Zhang. Interference sources and anti-interference measures in the use of frequency converters [J]. Metallurgical series 2007, (1) 13- 14.