Synopsis: This frequency converter is a low-voltage frequency converter. The input side uses a transformer to convert high voltage to low voltage, replacing the high-voltage motor with a special low-voltage motor. The voltage levels of these motors vary and there is no unified standard.
Based on the current market situation, it can be divided into the following categories:
hydraulic coupling
An impeller is added between the motor shaft and the load shaft, and the pressure of the liquid (usually oil) between the impellers is adjusted to regulate the load speed. This speed regulation method is essentially a slip power consumption method. Its main disadvantages are that the efficiency decreases as the speed decreases, the motor and load need to be disconnected for installation, the maintenance workload is large, and components such as shaft seals and bearings need to be replaced after a period of time. The site is generally dirty, making the equipment appear low-grade, and it is considered an obsolete technology.
Early manufacturers interested in speed control technology either did so because high-pressure speed control technology was unavailable at the time, or due to cost considerations, and thus made some applications of hydraulic couplings. Examples include water pumps in water companies, boiler feed pumps and induced draft fans in power plants, and dust collector fans in steel mills. Now, some of this older equipment is gradually being replaced during upgrades.
High-low-high type frequency converter
The frequency converter is a low-voltage frequency converter, which uses an input step-down transformer and an output step-up transformer to interface with the high-voltage power grid and the motor. This was a transitional technology when high-voltage frequency conversion technology was not yet mature.
Because low-voltage frequency converters have low voltage, the current cannot rise indefinitely, limiting their capacity. The presence of the output transformer reduces system efficiency and increases footprint; furthermore, the output transformer's magnetic coupling capability weakens at low frequencies, reducing the converter's load-carrying capacity during startup. It also introduces significant harmonics to the power grid. While 12-pulse rectification can reduce harmonics, it doesn't meet stringent harmonic control requirements. The output transformer amplifies the dv/dt of the frequency converter while stepping up the voltage, necessitating the addition of a filter for use with ordinary motors; otherwise, corona discharge and insulation damage may occur. Using a special variable frequency motor can avoid these issues, but it's less efficient than using a high-low voltage frequency converter.
High and low type frequency converter
The frequency converter is a low-voltage frequency converter. The input side uses a transformer to change the high voltage to low voltage, replacing the high-voltage motor with a special low-voltage motor. The voltage levels of the motors vary and there is no unified standard.
This approach, due to the use of low-voltage frequency converters with relatively small capacity, results in significant harmonic emissions from the power grid. While 12-pulse rectification can reduce harmonics, it doesn't meet stringent harmonic control requirements. Furthermore, when the frequency converter malfunctions, the motor cannot be connected to the mains frequency grid, causing problems in applications where shutdown is not an option. Additionally, both the motor and cables need to be replaced, resulting in a substantial engineering workload.
Cascade speed inverter
Speed regulation is achieved by feeding some rotor energy from an asynchronous motor back to the grid, thereby altering the rotor slip. This method utilizes thyristor technology and requires wound-rotor asynchronous motors, while squirrel-cage asynchronous motors are now almost universally used in industrial settings, making motor replacement extremely cumbersome. The speed regulation range of this method is generally limited to around 70%-95%, resulting in a narrow range. Thyristor technology is prone to causing harmonic pollution to the power grid; as the speed decreases, the grid voltage also drops, requiring compensation measures. Its advantage lies in the smaller capacity of the frequency converter, making it slightly less expensive than other high-voltage AC variable frequency speed regulation technologies.
There is a variation of this speed regulation method, namely the internal feedback speed regulation system, which eliminates the transformer in the inverter section and integrates the feedback winding directly into the stator winding. This approach requires replacing the motor, but its performance in other aspects is similar to that of cascade speed regulation.
Due to the influence of rotor slip rings, cascade speed-regulating motors cannot achieve high power, and the workload of slip ring maintenance is also large. They belong to the outdated technology of the 1970s and 1980s, and their industrial applications are becoming increasingly rare.
Current source type direct high voltage frequency converter
This type of frequency converter uses thyristors for rectification on the input side and inductors for energy storage. The inverter side uses SGCTs as switching elements, forming a traditional two-level structure. Due to the limited voltage withstand capabilities of the components, multiple components must be connected in series. Component series connection is a very complex engineering application technique, theoretically with low reliability, but some companies have managed to commercialize it. Since the output side only has two levels, the motor experiences a large dv/dt, necessitating an output filter. A multi-pulse rectifier on the grid side is optional; users need to specify their requirements based on their factory conditions. The main advantage of this frequency converter is that it can feed the load's inertial energy back to the grid without requiring external circuitry.
The main disadvantages of current source inverters are low power factor and high harmonics on the grid side, which vary with operating conditions and are difficult to compensate for.
Voltage source type three-level frequency converter
This type of frequency converter uses diode rectification, capacitor energy storage, and IGBT or IGCT inversion. The three-level inverter uses diode clamping, solving the problem of connecting two devices in series. Technically, it's simpler and easier than directly connecting two devices in series. At the same time, the added output level results in a better output waveform than a two-level inverter.
The main problem with this type of frequency converter is that, due to the use of high-voltage components, the output dv/dt is still quite significant, requiring an output filter. Due to limitations in the voltage withstand capability of the components, the maximum voltage can only reach 4160V. To adapt to 6KV and 10KV power grids, replacing the motor is one solution, but this makes troubleshooting the grid difficult. One workaround for 6KV motors is to change the motor connection from star to delta, thus reducing the motor voltage to 3KV. This increases the circulating current loss in the motor, and there have been cases of motor burnout in China, which may be related to this. Some companies use this type of frequency converter to achieve a high-low-high configuration, resulting in a larger capacity than when using a low-voltage frequency converter to achieve the same configuration, but the problems inherent in the high-low-high configuration still exist.
Three-level frequency converters generally use a 12-pulse rectification method.
Power module series multilevel frequency converter
This type of frequency converter uses a series connection of low-voltage frequency converters to achieve high voltage, making it a voltage source frequency converter. Its input side uses a phase-shifting step-down transformer to achieve rectification with more than 18 pulses, meeting the most stringent international requirements for power grid harmonics. Under load, the power factor on the grid side can reach over 95%. The output side employs multi-stage PWM technology, resulting in low dv/dt and fewer harmonics, meeting the needs of ordinary asynchronous motors. The output voltage of the frequency converter can be designed according to the load requirements, making it a good solution for speed regulation of 6KV and 10KV motors. The power circuit adopts a standard modular design, making replacement simple, and the components used are relatively easy to procure domestically.
This type of frequency converter uses low-voltage IGBTs as inverter components. Compared to three-level frequency converters using high-voltage IGBTs, it has a larger number of power components, but the technology is more mature. Compared to three-level frequency converters using high-voltage IGCTs, it has a larger number of power components, but the total number of components is smaller because IGCTs require very complex auxiliary shutdown circuits.
Because there are many connections between the rectifier transformer and the power module, the transformer cannot be placed separately from the frequency converter, which is not very flexible in situations with limited space.
