Drive motor speed control device as an energy efficient, because of its high performance and low cost, at the factory test transformer has been more widely used. As we all know, the inverter is composed of a rectifier circuit, a filter circuit, and an inverter circuit. The semiconductor switching elements are used in both the rectifier circuit and the inverter circuit, and the PWM control method is used in the control. This determines that the input and output voltages and currents of the inverter include many high voltages in addition to the fundamental wave. Subharmonics. These higher harmonic components will cause the distortion of the voltage waveform of the power grid, resulting in radio interference waves, which will adversely affect the surrounding equipment, including the driving object of the inverter test transformer . At the same time, due to the use of frequency converters, components of the higher harmonics will be generated in the power supply voltage, and when the thyristor rectifier devices in the power supply system work, they will lead to abnormal power waveforms. Test transformer In addition, due to the lightning strike or the opening and closing of the power transformer, the surge voltage generated by the opening and closing of the electric power appliance will also cause the power supply waveform to be distorted. When such a waveform distortion power supply to the inverter is supplied, it will Has a bad influence on the inverter. The article analyzes these phenomena and proposes measures to reduce these adverse effects.
2. External interference to the inverter
The interference of the power supply to the inverter mainly includes overvoltage, undervoltage and instantaneous power-down; surge, drop; spike voltage pulse; radio frequency interference. If the power supply of the inverter is disturbed by the harmonic dry test transformer from the polluted AC grid, the grid noise will interfere with the inverter through the power circuit of the grid if it is not processed. On the input circuit side of the inverter, the AC voltage is changed to a DC voltage. This is the rectifier circuit often called "grid pollution." Since this DC voltage is output to the subsequent circuit after being smoothed by the filter capacitor of the test transformer , the power supply to the inverter is actually the charging current of the filter capacitor, which causes the input voltage waveform to be distorted.
(1) Various rectifying devices, AC-DC interchangeable devices, electronic voltage adjusting devices, non-linear loads, and lighting equipment are included in the power grid. A large number of harmonic sources These loads in the power supply network cause waveform distortion in voltage and current 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 equipment in the power supply network, the thyristor of the test transformer is always turned on during part of each phase half cycle, so it is easy to make the network voltage appear notch and the waveform is seriously distorted. It may cause the rectifier circuit on the input side of the inverter to be damaged due to the occurrence of a large reverse recovery voltage, and the transformer is tested and the input circuit is broken down and burned.
(2) the power compensation capacitance of the inverter power sector interference have certain requirements on the power factor of the power units, test transformer this reason, many users are lumped capacitance compensation method to improve power factor at the substation. During the transient process of compensation capacitor input or output, the network voltage may have a very high peak. As a result, the rectifier diode of the inverter may break down due to the excessive reverse voltage.
(3) Interference signals transmitted by the power supply radiation Electromagnetic interference (EMI) is the electromagnetic interference caused by external noise and unwanted signals in the receiving test transformer . It is usually transmitted through the circuit and transmitted in the form of a field [2] ie electromagnetic wave The way to radiation in the air, its radiation field strength depends on the current intensity of the interference source, the equivalent radiation impedance of the device, and the transmission frequency of the interference source.
For the interference suppression produced by (1) and (2), the AC reactor can be connected to the input circuit of the inverter, which is insignificant for the impedance at the fundamental frequency. However, for high-frequency high-frequency dry test transformer signal interference, it presents a very high impedance and can effectively suppress the effect of interference. The interference signal for item (3) is mainly weakened by absorption. Inverter power input, usually with absorption capacitors. A special "radio interference filter" can also be added to further weaken the interference signal.
3. Inverter interference with peripheral equipment and countermeasures <br> As mentioned above, the inverter can generate higher harmonics of the input power supply voltage. At the same time, in addition to the fundamental wave, the output voltage and current of the test transformer inverter also contains many higher-order harmonic components that will propagate their energy in various ways. These higher harmonics will bring about the surrounding equipment. Bad influence. Among them, the distortion of the power supply causes other devices in the same power supply to malfunction, overheating, noise, and vibration; the generated wireless interference wave brings to the radio receivers such as transformers , radios, and mobile phones around the inverter. Interference, if it is serious, it cannot work properly; it interferes with the external control signal of the inverter. After these control signals are disturbed, it cannot accurately and normally control the operation of the inverter, causing noise, vibration and heat generation of the motor driven by the inverter. .
(1) Interference brought about by the same power supply device When the capacity of the inverter is large, the network voltage will be distorted, and the interference will be transmitted to other circuits through impedance coupling or ground loop coupling. To eliminate or reduce the interference caused by equipment connected to the same power source, test the transformer by connecting the inverter's input terminal to the AC reactor and insert the DC reactor on the rectifier side of the inverter. It is also possible to insert a filter on the power input of the inverter, as shown in Figure 1 below:
The LC filter is a passive filter, which consists of a reactance and a capacitor to form a resonant circuit for higher harmonics, so as to achieve the purpose of absorbing higher harmonics. The working principle of the active filter is to attenuate the higher harmonics by detecting higher harmonics in the current, and by inputting a current having a phase opposite to that of the higher harmonic components according to the detection result.
(2) At present, most frequency converters use PWM control methods for the generated radio interference waves. The inverter output signal is a high-frequency switching signal. The output voltage and output current of the inverter contain high-order harmonics, and radio interference waves are generated through electrostatic induction and electromagnetic induction. Some of these interference waves are conducted through wires, and some of them radiate directly into the air by electromagnetic waves and electric field test transformers . The metal objects in the radiation field may also form secondary radiation. In the same way, the radiation outside the inverter can interfere with the normal operation of the inverter.
The suppression of the radio interference wave conducted by the wire can be implemented by a noise filter transformer, which insulates the high-order harmonics, and inserts a reactor to increase the impedance to the high-order harmonic components. The filter is inserted at the input of the inverter.
The suppression of radiated radio interference waves is more difficult than transmitting radio interference waves. The size of this radio interference depends on the structure of the inverter installation itself, and on many test transformer factors such as the length of the motor cable. Can shorten the electric wire of the electric wire as far as possible, the electric wire adopts double-strand twist measure, reduce the impedance; Inverter input, output line are put into iron tube shield; Connect the frequency changer casing well; Inverter input, output end connect reactor in series, insert the electric-wave filter .
(3) Noise interference generated Since the inverter adopts the PWM control method, the output voltage waveform of the inverter is not a sine wave, and the current passing through the motor inevitably contains many harmonics. The harmonic frequency of the inverter output is in resonance with the natural frequency of the rotor. The noise around the natural frequency of the rotor increases, and the harmonic components output from the inverter make the core, casing, and shaft frame harmonics near the intrinsic test transformer frequency. Increased noise. Therefore, when the inverter is used to control the motor speed, the motor windings and the core generate noise due to the harmonic components.
Generally, when the inverter is used to drive the motor, the noise generated by the motor is 5 to 10 dB higher than the noise generated by the direct drive of the power grid.
The measures that can be taken to suppress noise are:
1 Select the test transformer low-noise frequency converter with high carrier frequency with IGBT and other inverter modules. The inverter-specific motor is selected and a reactor is connected in series between the inverter and the motor to reduce the higher harmonics generated by the PWM control method.
2 Insert a filter that converts the output waveform into a sine wave between the inverter and the motor.
3 use low noise reactors.
(4) When the frequency converter is used to control the motor speed by the vibration interference generated, the same noise causes the motor to vibrate. In particular, the pulsating torque generated by the higher harmonics of the lower order brings about great vibration to the torque output of the motor. If the mechanical system resonates with this vibration, its vibration becomes even more severe.
You can usually take the following measures to reduce vibration:
1 Strengthen the rigidity of the mechanical structure and change the rigid connection into a strong connection.
2 Inductor reactor 3 is connected in series between the inverter test transformer and the motor to reduce the output voltage-frequency ratio of the inverter.
4 Change the carrier frequency of the inverter.
When the frequency converter regulates the motor speed, if the speed range is large, the resonance frequency of the mechanical system should be measured first, and then the frequency hopping function of the inverter is used to avoid these resonance frequencies. If there is a margin in the rotation, U/f can be given less.
(5) For the interference caused by the overheating of the transformer in the electric test of the control unit, the inverter is used to control the motor speed. Due to the higher harmonics, even if the same motor is operated at the same frequency, the motor will increase by 5%. ~10% of current. The motor temperature naturally increases. In addition, the cooling fan of an ordinary motor is mounted on a motor shaft. When the motor is continuously operated at a low speed, the cooling of the cooling fan is insufficient and the motor is overheated.
There are several countermeasures for overheating the motor:
1 It is equipped with a cooling fan for the motor, and it is cold type for the cold type. Increase the cooling capacity when running at low speeds.
2 choose a larger capacity motor.
3 Use inverter-specific motors.
4 Change the speed regulation scheme to avoid continuous low speed operation of the motor.
With the increased automation plant electrical test transformer, various disturbances are increasing, only a deep understanding of the interference of the inverter, and take appropriate measures to deal with, to be able to reduce the harm to each other between each other, to a greater extent To ensure the normal production and equipment stability.
