The speed control methods of high-voltage frequency converters mainly include the following:
1. Hydraulic coupling method. This involves inserting a hydraulic coupling device between the motor and the load. The speed of the load is adjusted by regulating the coupling force between the motor and the load through the adjustment of the liquid level.
2. Cascade Speed Control. Cascade speed control requires a wound-rotor asynchronous motor. A portion of the energy from the rotor windings is rectified, inverted, and then fed back to the grid. This effectively adjusts the rotor's internal resistance, thereby changing the motor's slip. Since the rotor voltage and the grid voltage are generally not equal, a transformer is needed for grid inversion. To save on this transformer, the domestic market commonly uses an internally fed-forward motor, where a three-phase auxiliary winding is added to the stator specifically to receive the rotor's feedback energy. The auxiliary winding also participates in work, thus reducing the energy absorbed by the main winding from the grid, achieving speed control and energy saving.
3. High-low method. Since high-voltage frequency conversion technology was not yet solved at the time, a transformer was used to first reduce the grid voltage, and then a low-voltage frequency converter was used to achieve frequency conversion. For the motor, there were two methods. One method was to use a low-voltage motor. The other method was to continue using the original high-voltage motor, which required adding a step-up transformer between the frequency converter and the motor.
The three methods mentioned above are all relatively immature technologies at present. Hydraulic couplings and cascade speed regulation have poor speed control accuracy, small speed range, and high maintenance workload. Furthermore, the efficiency of hydraulic couplings is somewhat different from that of frequency converters. Therefore, these two technologies are no longer very competitive. As for the high-low method, it can achieve relatively good speed control results, but compared to true high-voltage frequency converters, it has the following disadvantages: low efficiency, high harmonics, stricter requirements on the motor, and lower reliability at higher power levels (above 500KW). The main advantage of the high-low method is its lower cost.
While other high-voltage frequency converter solutions have been proposed, most lack clear feasibility or the potential to replace the three mainstream frequency converter structures mentioned above. As the cost of high-voltage frequency converters further decreases, high- and low-voltage frequency converters will exit the competition in the medium-power market, focusing instead on lower-power applications.
The main disadvantage of unit-series multilevel inverters is the complexity of the converter circuit, the large number of power components, and the slightly larger size. However, since other methods cannot meet the needs of domestic applications and the reliability of high-voltage devices is not yet very high, their competitive advantage may still be irreplaceable in the near future. Due to the issue of low output voltage, three-level inverters are mainly used in some special fields, such as rolling mills, ship drives, locomotive traction, hoists, etc. The motors in these fields are specially customized and may not be standard voltages.
At certain power levels, the replacement of traditional AC-AC inverters with three-level inverters is a technological trend. Further development of three-level inverters depends on the emergence of power devices with higher voltage withstand capabilities and further improvements in the reliability of existing products. In ultra-high power applications, i.e., power exceeding approximately 8000KW, current-source inverters (LCI load converter inverters using thyristors) remain the dominant type.
Due to the aforementioned technical characteristics, the majority of general-purpose high-voltage frequency converters currently available are unit-series multilevel frequency converters, accounting for over 70%. There are currently no fewer than twenty high-voltage frequency converter manufacturers in China, with Leadway as a representative, and they all primarily employ this circuit structure.
