Abstract: The basic principle of variable frequency speed control technology is based on the relationship that the motor speed is proportional to the input frequency of the working power supply: n=60f(1-s)/p, (where n, f, s, and p represent the speed, input frequency, motor slip, and number of motor pole pairs, respectively); the motor speed is changed by changing the frequency of the motor's working power supply.
(a) Purpose of variable frequency speed control transmission
my country's low energy efficiency is mainly due to its extensive economic growth model, which is characterized by an irrational economic structure, outdated technology and equipment, and low management levels. Using frequency converters (VFDs) to control the speed of mechanical equipment is of great significance for energy conservation and improved economic efficiency. The application of VFDs in labor saving, automation, improving product quality, increasing productivity, and improving product qualification rates is just as important as their application in energy conservation. Using variable frequency speed control in mechanical equipment can increase equipment lifespan, reduce noise, enable equipment miniaturization, and create a more comfortable working and living environment.
From an energy consumption perspective, speed regulation methods can be categorized into high-efficiency and low-efficiency methods. High-efficiency speed regulation refers to a constant slip rate, thus eliminating slip loss, such as multi-speed motors, frequency converters, and methods that recover slip loss (e.g., cascade speed control). Low-efficiency speed regulation methods involve slip loss, such as rotor resistance speed regulation, where energy is lost in the rotor circuit; electromagnetic clutch speed regulation, where energy is lost in the clutch coil; and hydraulic coupling speed regulation, where energy is lost in the hydraulic coupling oil. Generally, slip loss increases with the speed range; if the speed range is small, energy loss is minimal.
(II) Industry Production Scope of Variable Frequency Speed Control Applications
Variable frequency drive (VFD) technology has matured significantly over the years and has been widely adopted in industries such as steel, copper wire rolling, chemicals, fibers, automobiles, electrical machinery, machine tools, food, papermaking, cement, mining, gas, transportation, loading and unloading, factory construction, agriculture, consumer services, power, experimental research, and petroleum. VFD technology is now closely related to our production and daily lives. Currently, household appliances such as refrigerators, air conditioners, and electric vehicles already utilize related VFD technology.
(III) Characteristics of Variable Frequency Speed Control Drive
Variable frequency drives (VFDs) are used to control the speed of mechanical equipment. The characteristics of VFD speed control are that it can adjust the speed of standard motors, can continuously adjust the speed, has a small starting current, the maximum speed is not affected by the power supply, the motor can be made high-speed, small-sized, explosion-proof, and has a constant torque output at low speeds. The magnitude of acceleration and deceleration can be adjusted, and squirrel-cage motors can be used without maintenance.
(iv) Working principle of frequency converter
There are two types of frequency converter circuits: AC-AC conversion and AC-DC-AC conversion. The basic working principle is that the rectifier converts AC to DC, the smoothing circuit smooths the DC, and the inverter converts the DC into AC with an adjustable frequency. This is the basic structure of a frequency converter.
1. An AC-AC converter is a converter that directly transforms a voltage at the grid frequency into an output voltage at a frequency lower than the grid frequency without a DC intermediate link.
2. Working Principle of AC-DC-AC Frequency Converters. Currently, the most widely used type is the AC-DC-AC frequency converter. Its working principle is to first rectify the three-phase or single-phase non-adjustable power frequency power supply into DC power through a rectifier bridge, and then invert the DC power into AC power with an arbitrarily adjustable frequency through an inverter bridge to achieve stepless speed regulation. The main circuit of an AC-DC-AC frequency converter has two types: voltage-source frequency converter and current-source frequency converter. A voltage-source converter converts DC power from a voltage source to AC power; a current-source converter converts DC power from a current source to AC power. Control methods are also divided into voltage control and current control. Both methods are applicable regardless of whether the main circuit is voltage-source or current-source.
(v) Quality performance indicators of frequency converters
1. What starting torque can be output at 0.5Hz ? A good inverter can output 200% high starting torque at 0.5Hz for 22kW and below, and 180% starting torque for 30kW and above.
2. How long can the speed regulation range control accuracy be achieved, and how high can the torque control accuracy be ? High-performance frequency converters can achieve a speed control accuracy of ± 0.005 % and a torque control accuracy of ±3%.
3. Pulse conditions at low speeds. A high-quality frequency converter has only 1.5 r/min of speed pulses at 1 Hz.
4. Noise and Harmonic Interference. Noise and harmonic interference are important criteria for evaluating a frequency converter. The modulation frequency using IGBTs and IPMs is higher than that using GTOs and GTRs. Although higher-order harmonics are always present, some harmonics can be reduced with better control methods.
5. Heat generation. High heat generation in a frequency converter indicates high power consumption, low efficiency, and a poor ventilation and heat dissipation system, which will consequently affect the lifespan of the frequency converter.
