Frequency converter is a widely used device in modern industry. If you want to realize functions such as soft stop, soft start, stepless speed regulation or special requirements for speed increase and decrease, you need a more advanced speed control device in modern asynchronous motors - a frequency converter. The main circuit of the device adopts AC-DC-AC circuit and the operating frequency is 0 ~ 400 Hz. The output voltage of the low-voltage general-purpose inverter is 380~460 V, and the output power is 0.37~400 kW. Since equipment controlled by frequency converters can significantly save electrical energy, they are widely used. However, the frequency converter produces serious harmonic currents during operation, which has become a major source of pollution in the power grid. The more common fault phenomena caused by harmonics are damage to the reactive power compensation device, tripping, and overheating of the transformer.

1、Choose a reasonable inverter

During the use of the inverter, problems such as abnormal operation and equipment failure occur. , which lead to production suspension and unnecessary economic losses, are often caused by improper selection and installation of the inverter. Therefore, we should choose an economical and practical inverter that can better meet the basic conditions and requirements of production and technology.

1.1The specified index parameters of the inverter and motor match.

As the main driving object of the motor, the type of inverter should be matched with the working parameters of the motor.

(1) Voltage matching: The rated voltage of the inverter is consistent with the load voltage of the motor.

(2) Current matching: The capacity of the inverter depends on the rated current of the inverter's continuous output. When selecting an inverter, the motor that needs to be speed-regulated must select an inverter with a continuous rated current greater than the rated current of the motor when it is working at the rated parameters, and with a quantitative margin; generally, inverters with more than 4 poles cannot be selected based on the capacity of the motor, but should be selected based on the current seat inspection standard of the motor. Even if the motor load is light and the current is less than the rated 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、Frequency converter control method

Currently, the main control methods of frequency converters are as follows.

(1) The first generation adopts U/f=C control, also known as sinusoidal pulse width modulation (SPWM) control. Its characteristics are simple control circuit structure, low cost, good mechanical characteristics and hardness, and can meet the requirements of smooth speed regulation of general transmissions. However, at low frequencies, the output voltage is low, which reduces the maximum output torque and worsens the stability at low speeds. It is characterized by the speed ratio ni being less than 1/40 without feedback device and ni=1/60 with feedback. It is suitable for general fans and pumps.

(2) The second generation adopts voltage space vector control (flux trajectory method), also known as SVPWM control. Under the premise that the three-phase waveform produces an overall effect, a three-phase modulated waveform is generated at one time, and the control is performed in the form of an inscribed polygon approximating a circle. In order to eliminate the influence of stator resistance at low speed, the output voltage and current are closed to improve dynamic accuracy and stability. Its characteristics: no feedback device, speed ratio ni=1/100, suitable for general industrial speed regulation.

(3) The third generation adopts vector control. The essence of variable frequency speed regulation vector control is that the AC motor is equivalent to a DC motor, and the speed and magnetic field components are independently controlled. By controlling the rotor flux and then decomposing the stator current, the torque and magnetic field components are obtained, and orthogonal or decoupling control is achieved through coordinate transformation. Its characteristics are: the speed ratio ni=1/100 when there is no feedback, the speed ratio ni=1/1000 when there is feedback, and the starting torque at zero speed is 150%. It can be seen that this method is suitable for all speed controls and for high-precision transmission control with feedback.

(4) Direct torque control. Direct torque control is another high-performance variable frequency speed regulation method different from vector control. The flux simulation model and electromagnetic torque model obtain the flux and torque data, compare them with the given value, generate a hysteresis comparison state signal, and then switch the switching state through logical control to achieve constant flux control and electromagnetic torque control. There is no need to imitate DC motor control. This technology has been successfully applied to AC transmission of traction electric locomotives. Its characteristics: the speed ratio without feedback device is ni=1/100, the speed ratio with feedback device is ni=1/1000, and the starting torque at zero speed can reach 150%~200%. It is suitable for large load starting and large loads with constant torque fluctuations.

3、Installation environment requirements

(1) Ambient temperature: The ambient temperature of the inverter refers to the temperature near the inverter cross section. Since the inverter is mainly composed of high-power power electronic equipment that is easily affected by temperature, the life and reliability of the inverter depends largely on the temperature, which is generally between -10℃ and +40℃. In addition, the extreme conditions that may occur in the heat dissipation of the inverter itself and the surrounding environment must be considered, 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) Vibration and shock: During the installation and operation of the inverter, care should be taken to avoid vibration and shock. Avoid loose solder joints and parts of the internal components of the inverter, resulting in poor electrical contact or even serious faults such as short circuits. Therefore, it is usually required that the vibration acceleration at the installation 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) Installation location: The maximum allowable 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 decrease, so it is generally required that the inverter be installed below 1000 m above sea level.

(5) General requirements for the inverter installation site: no corrosion, no flammable or explosive gases or liquids; no dust, floating fibers and metal particles; avoid direct sunlight; no electromagnetic interference.

4、Conclusion

The research on variable frequency speed regulation is an active and practical work in the current research of electric power transmission. The frequency converter industry has great potential and is widely used in air conditioning, elevators, metallurgy, machinery and other industries. The frequency converter motor and its supporting inverter are developing rapidly.