Variable frequency drives (VFDs) are widely used in modern industry. If you want to achieve functions such as soft stop, soft start, stepless speed regulation, or have special requirements for speed adjustment, you need a VFD, a relatively advanced speed control device for modern asynchronous motors. The main circuit of this device uses an AC-DC-AC circuit, with an operating frequency of 0~400 Hz. Low-voltage general-purpose VFDs have an output voltage of 380~460 V and an output power of 0.37~400 kW. Because VFD-controlled equipment can significantly save energy, it is widely used. However, VFDs generate significant harmonic currents during operation, becoming a major source of power grid pollution. Common fault phenomena caused by harmonics include damage to reactive power compensation devices, tripping, and transformer overheating.
1. Select a suitable frequency converter
Abnormal operation and equipment failure during the use of frequency converters can lead to production stoppages and unnecessary economic losses. These problems are often caused by improper selection and installation of the frequency converter. Therefore, we should choose an economical and practical frequency converter that better meets the basic conditions and requirements of production and technology.
1.1 Matching of specified parameters between the frequency converter and the motor.
As the main drive device of the motor, 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 is consistent with the load voltage of the motor.
(2) Current matching: The capacity of the frequency converter depends on the rated current of its continuous output. When selecting a frequency converter, the motor requiring speed regulation must have a continuous rated current greater than the rated current when the motor is operating at its rated parameters, with a certain margin. Generally, frequency converters with 4 poles or more should not be selected based on the motor's capacity, but rather on the motor's current standard specifications. Even if the motor load is light and the current is less than the frequency converter's rated current, the selected frequency converter should not be much smaller than the motor's capacity.
(3) Capacity matching: Different requirements are required for the selection of inverter capacity according to different load characteristics of motor.
2. Control method of frequency converter
The main control methods for 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 include a simple control circuit structure, low cost, good mechanical properties and rigidity, and it can meet the requirements of smooth speed regulation in general transmissions. However, at low frequencies, the output voltage is low, which reduces the maximum output torque and deteriorates stability at low speeds. Its characteristic is that the speed ratio ni without feedback is less than 1/40, and ni=1/60 with feedback. It is suitable for general fans and pumps.
(2) The second generation adopts voltage space vector control (magnetic flux trajectory method), also known as SVPWM control. Under the premise of the overall effect of the three-phase waveform, a three-phase modulated waveform is generated at one time, and the control is performed by approximating a circle with an inscribed polygon. In order to eliminate the influence of stator resistance at low speeds, the output voltage and current are closed to improve dynamic accuracy and stability. Its characteristics are: 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 the DC motor, and the speed and magnetic field components are controlled independently. By controlling the rotor flux linkage and then decomposing the stator current, the torque and magnetic field components are obtained, and orthogonal or decoupled control is achieved through coordinate transformation. Its characteristics are: the speed ratio ni=1/100 without feedback, the speed ratio ni=1/1000 with feedback, and the starting torque at zero speed is 150%. It can be seen that this method is applicable to all speed control and is suitable for high-precision transmission control with feedback.
(4) Direct Torque Control. Direct torque control is a high-performance variable frequency speed regulation method that differs from vector control. Flux and torque data are obtained from flux simulation models and electromagnetic torque models, compared with given values, and a hysteresis comparison state signal is generated. Then, the switching state is achieved through logic control, realizing constant flux control and electromagnetic torque control. This technology has been successfully applied to AC drives in traction locomotives without needing to mimic DC motor control. Its characteristics include: a speed ratio of ni=1/100 without feedback and ni=1/1000 with feedback; and a starting torque of 150%~200% at zero speed. It is suitable for heavy load starting and heavy loads with 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 power electronic equipment that is susceptible to temperature, the lifespan and reliability of the frequency converter depend to a large extent on the temperature, which is generally between -10℃ and +40℃. In addition, extreme situations that may occur in the heat dissipation of the frequency converter itself and the surrounding environment must be considered, and a certain temperature margin is generally required.
(2) Ambient humidity: The relative humidity of the environment around the inverter shall 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. Loosening of internal component solder joints and parts can lead to serious malfunctions such as poor electrical contact or even short circuits. Therefore, the vibration acceleration at the installation site is generally required to be limited to below 0.6 g. In special locations, anti-vibration measures such as anti-vibration rubber can be added.
(4) Installation location: The maximum allowable output current and voltage of the frequency converter are affected by its heat dissipation capacity. However, if the altitude exceeds 1000 m, the heat dissipation capacity of the frequency converter will decrease, so it is generally required that the frequency converter be installed below 1000 m altitude.
(5) General requirements for the installation site of the frequency converter: no corrosive, no flammable or 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 an active and practical area of study in electric drive technology. The variable frequency drive (VFD) industry has enormous potential and is widely used in industries such as air conditioning, elevators, metallurgy, and machinery. Variable frequency motors and their associated inverters are developing rapidly.
