Frequency converters are widely used equipment in modern industry. If you want to complete the functions of soft stop, soft start, stepless speed regulation or special requirements for speed rise and fall, you need the more advanced speed control equipment in modern asynchronous motors - frequency converters. The main circuit of the equipment is AC-DC-AC circuit, and the operating frequency is 0 - 400 Hz. The output voltage of the low-voltage universal inverter is 380-460 V, and the output power is 0.37-400 kW. Inverter-operated devices are widely used because they offer significant energy savings. However, the inverter has become the primary source of pollution in the power grid due to serious harmonic currents during operation. The more common fault phenomena caused by harmonics are damage to reactive power compensation equipment, tripping, transformer overheating, etc.
1. Choose a reasonable inverter
During the use of the inverter, there are problems such as abnormal operation and equipment failure. , resulting in production shutdown and unnecessary economic losses, is often caused by improper inverter selection and installation. Therefore, we should choose an inverter that is economical and useful and can better meet the basic conditions and requirements of production and skills.
1.1 The specified index parameters of the inverter and the motor are matched.
As the primary drag object of the motor, the type of inverter should match the operating parameters of the motor.
(1) Voltage matching: The additional voltage of the inverter is the same as the load voltage of the motor.
(2) Current matching: The capacity of the inverter depends on the additional current output of the inverter. When selecting the inverter, the motor that needs to regulate the speed must select the inverter with additional current greater than the additional current when the motor is operating with additional parameters, and there is a quantitative margin; Generally, the inverter with more than 4 poles can not be selected according to the capacity of the motor, but should be selected according to the current seat inspection specification of the motor. Even if the motor load is light and the current is less than the additional current of the inverter, the selected inverter cannot be much smaller than the motor capacity.
(3) Capacity matching: According to the different load characteristics of the motor, there are different requirements for the selection of inverter capacity.
2. The control method of the inverter
At present, the primary control methods of the inverter are mainly as follows.
(1) The first generation selects U/f=C control, also known as sinusoidal pulse width modulation (SPWM) control method. Its characteristic control circuit is simple in structure, low in cost, good in mechanical characteristics and hardness, and can meet the requirements of stable speed regulation of general transmission. However, at low frequencies, the output voltage is low, which reduces the large output torque and worsens stability at low speeds. It is characterized by the fact that the speed ratio Ni of the non-reactive equipment is less than 1/40, and Ni = 1/60 when there is a reaction. Suitable for general fans and pumps.
(2) The second generation selects voltage space vector manipulation (magnetic flux orbit method), also known as SVPWM manipulation method. Under the premise that the three-phase waveform is fully functional, the three-phase modulated waveform occurs at one time, and is controlled by the method of approximating the circle with the internal polygon. In order to eliminate the effect of stator resistance at low speeds, the output voltage and current are closed to improve dynamic accuracy and stability. Its features: non-reactive equipment, speed ratio NI=1/100, suitable for general industrial speed regulation.
(3) Three generations of vector control. The essence of frequency conversion speed regulation vector control is that the communication motor is equivalent to the DC motor, and the two weights of speed and magnetic field are controlled independently. After manipulating the rotor flux, and then differentiating the stator current, the two weights of torque and magnetic field are obtained, and the orthogonal or decoupled manipulation is completed through coordinate transformation. It is characterized by a speed ratio of NI=1/100 when there is no response, a speed ratio NI=1/1000 when there is a reaction, and a starting torque of 150% at zero speed. It can be seen that this method is suitable for all speed control, and is suitable for high-precision transmission control with reaction.
(4) Direct torque control. Direct torque control is another high-performance variable frequency speed regulation method that is different from vector control. The flux and torque data are obtained from the flux simulation model and the electromagnetic torque model, which are compared with the given values, and the hysteresis comparison condition signal is generated, and then the constant flux flux control and electromagnetic torque control are completed by switching the status through logical manipulation. There is no need to imitate the DC motor control, and this skill has been successfully applied to the communication transmission of traction electric locomotives. Its characteristics: the speed ratio of non-reaction equipment is Ni = 1/100, the speed ratio of reaction equipment is Ni = 1/1000, and the starting torque at zero speed can reach 150% - 200%. It is suitable for heavy load starting and large load with stable torque fluctuation.
3. Device environmental requirements
(1) Ambient temperature: The ambient temperature of the inverter refers to the temperature adjacent to the cross-section of the inverter. Since the drive is primarily composed of high-power power electronics that are susceptible to temperature, the lifetime and reliability of the drive is highly dependent on the temperature, generally between -10°C and +40°C. In addition, it is necessary to consider the extreme conditions that may be presented by the inverter itself and the surrounding environment, and the temperature generally requires a certain margin.
(2) Ambient humidity: The relative humidity of the environment around the inverter does not exceed 90% (no condensation on the surface).
(3) Oscillation and impact: In the process of installation and operation, attention should be paid to prevent oscillation and impact. Prevent serious faults such as poor electrical contact and even short circuits caused by loose solder joints and parts of the internal components of the inverter. Therefore, it is generally required that the oscillation acceleration at the device site should be limited to less than 0.6 g, and anti-seismic measures such as shockproof rubber can be added in special places.
(4) Device location: The maximum promised output current and voltage of the inverter are affected by its heat dissipation capacity. However, if the altitude exceeds 1000 m, the heat dissipation capacity of the inverter will be reduced, so the inverter is generally required to be installed below 1000 m above sea level.
(5) The general requirements of the inverter for the device site: no corrosion, no flammable and explosive gas or liquid; Dust-free, floating fibers and metal particles; Protect from direct sunlight; There is no electromagnetic interference.
4. Conclusion
The study of variable frequency speed regulation was an active and useful operation in the research of electric transmission at that time. The inverter industry has great potential and is widely used in air conditioning, elevator, metallurgy, machinery and other industries. Inverter motors and their supporting inverters are developing rapidly.
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