Variable frequency drives (VFDs) are widely used in modern industry. Their application is increasingly widespread because equipment controlled by VFDs can significantly save energy. However, VFDs generate significant harmonic currents during operation, which has become a major source of pollution for the power grid.
To achieve features such as soft stopping, soft starting, stepless speed regulation, or special requirements for acceleration and deceleration, the most advanced speed control device for modern asynchronous motors —the frequency converter—is needed. This device uses an AC-DC-AC circuit in its main circuit, operating at a frequency of 0–400Hz. Low-voltage general-purpose frequency converters have an output voltage of 380–460V and an output power of 0.37–400kW.
1. Select a suitable frequency converter
Problems such as malfunctions and equipment failures during the use of frequency converters, leading to production stoppages and unnecessary economic losses, are often caused by inappropriate selection and installation of the frequency converter type. Therefore, it is essential to choose a frequency converter that is both economical and practical, and better meets the basic conditions and requirements of production and processes.
1.1 Matching of specified parameters between the frequency converter and the motor
Since the motor is the main driven object of the frequency converter, the type of frequency converter should be selected to match the operating parameters of the motor.
(1) Voltage matching: The rated voltage of the frequency converter matches the load voltage of the motor.
(2) Current matching: The capacity of the frequency converter depends on the rated current of the continuous output of the frequency converter. When selecting a frequency converter for a motor that needs speed regulation, it is necessary to select a frequency converter with a continuous rated current greater than the rated current of the motor when it is operating at rated parameters, and with a certain margin. For general frequency converters with 4 poles or more, the selection should not be based on the capacity of the motor, but on the current frame verification standard of the motor. Even if the load of the motor is relatively light and the current is less than the rated current of the frequency converter, the selected frequency converter should not be much smaller than the capacity of the motor.
(3) Capacity matching: Depending on the different motor load characteristics, there are different requirements for the selection of inverter capacity.
2. Control method of frequency converter
The main control methods of frequency converters currently include the following.
(1) The first generation uses U/f=C control, also known as sinusoidal pulse width modulation (SPWM) control. Its characteristics include a simple control circuit structure, low cost, and good mechanical properties and rigidity, meeting the smooth speed regulation requirements of general transmissions. However, at low frequencies, this control method suffers from reduced maximum output torque due to lower output voltage, resulting in poor stability at low speeds. Its characteristic is that the speed ratio ni is less than 1/40 without feedback, and ni=1/60 with feedback. It is suitable for general fans and pumps.
(2) The second generation uses voltage space vector control (magnetic flux trajectory method), also known as SVPWM control. It is based on the overall generation effect of three-phase waveforms, generating three-phase modulated waveforms at once, and controlling them by approximating a circle with an inscribed polygon. To eliminate the influence of stator resistance at low speeds, the output voltage and current are closed-loop to improve dynamic accuracy and stability. Its characteristics are: no feedback device is required, speed ratio ni=1/100, suitable for speed regulation in general industry.
(3) The third generation uses vector control (VC). Vector control variable frequency speed regulation essentially equates the AC motor to a DC motor, independently controlling the speed and magnetic field components. By controlling the rotor flux linkage and then decomposing the stator current, the torque and magnetic field components are obtained. Through coordinate transformation, orthogonal or decoupled control is achieved. Its characteristics are: speed ratio ni = 1/100 without feedback, ni = 1/1000 with feedback, and starting torque at zero speed is 150%. This method is applicable to all speed control applications, and with feedback, it is suitable for high-precision transmission control.
(4) Direct Torque Control (DTC). Direct Torque Control (DTC) is another type of high-performance variable frequency speed control mode, distinct from Vector Control (VC). Flux and torque data are obtained using flux simulation and electromagnetic torque models, compared with given values to generate hysteresis comparison state signals, and then switched by logic control to achieve constant flux control and electromagnetic torque control. It does not require mimicking DC motor control, and this technology has been successfully applied to AC drives in traction electric locomotives. Its characteristics include: speed ratio ni=1/100 without feedback, ni=1/1000 with feedback, and starting torque of 150%–200% at zero speed. It is suitable for heavy-load starting and loads with large constant torque fluctuations.
3. Installation environment requirements
(1) Ambient temperature: The ambient temperature of the frequency converter refers to the temperature near the cross-section of the frequency converter. Since the frequency converter is mainly composed of high-power electrical electronic equipment that is highly susceptible to temperature, the lifespan and reliability of the frequency converter depend to a large extent on the temperature, which is generally -10℃ to +40℃. In addition, the heat dissipation of the frequency converter itself and the extreme conditions that may occur in the surrounding environment need to be considered, and a certain margin is generally required for the temperature.
(2) Ambient humidity: The relative humidity of the environment around the inverter should not exceed 90% (no condensation on the surface).
(3) Vibration and shock: During the installation and operation of the frequency converter, care should be taken to avoid vibration and shock. This is to prevent the internal component connection solder joints and parts from becoming loose, which may cause poor electrical contact or even serious faults such as short circuits. Therefore, it is generally required that the vibration acceleration of the installation site be limited to below 0.6g. In special locations, anti-vibration rubber and other vibration-resistant measures can be added.
(4) Installation location: The maximum allowable output current and voltage of the frequency converter are affected by its heat dissipation capacity. The heat dissipation capacity of the frequency converter will decrease when the altitude exceeds 1000m, so the frequency converter is generally required to be installed below 1000m altitude.
(5) General requirements for the installation site of the frequency converter: no corrosive, flammable and explosive gases or liquids; no dust, floating fibers and metal particles; avoid direct sunlight; no electromagnetic interference.
4. Conclusion
Research on variable frequency speed control is currently the most active and practically valuable area of research in electrical drives. The variable frequency drive industry has enormous potential, with widespread applications in industries such as air conditioning, elevators, metallurgy, and machinery. Variable frequency speed control motors and their associated variable frequency drives will experience rapid development.
