High-voltage motor speed control technology development content
High and low frequency converter
The inverter is a low-voltage inverter. The input side uses a transformer to change the high voltage to a low voltage. The high-voltage motor is replaced. A special low-voltage motor is used. The voltage level of the motor is various and there is no uniform standard.
This method uses a low-voltage inverter, the capacity is relatively small, and the harmonics on the grid side are large. 12-pulse rectification can be used to reduce harmonics, but the strict requirements for harmonics cannot be met. In the event of a fault in the inverter, the motor cannot be put into the power frequency grid operation, and there may be problems in some applications where the machine cannot be stopped. In addition, the motor and cable must be replaced, and the amount of work is relatively large.
Cascade speed converter
The rotor energy of the asynchronous motor is fed back to the power grid to change the rotor slip to achieve speed regulation. This speed regulation method uses thyristor technology and needs to use wire-wound asynchronous, and now almost all squirrel-cage asynchronous motors are used in industrial sites. It is very troublesome to replace the motor. The speed regulation range of this speed regulation mode is generally around 70%-95%, and the speed regulation range is narrow. The thyristor technology is easy to cause harmonic pollution to the power grid; as the speed decreases, the grid side also becomes low, and measures need to be taken to compensate. The advantage is that the frequency conversion part has a small capacity, which is slightly lower than other high-voltage AC variable frequency speed control technologies.
There is a variation of this type of speed regulation, that is, the internal feedback speed control system, which eliminates the transformer of the inverter part, and the feedback winding is directly made in the stator winding. This method needs to replace the motor, and other aspects of performance and cascade adjustment The speed is close.
The cascade speed regulating motor is affected by the rotor slip ring, can not achieve a lot of power, and the slip ring maintenance workload is also large. It belongs to the backward technology in the 1970s and 1980s, and the industrial application has become less and less.
Current source type direct high voltage inverter
In this kind of frequency converter, the input side is rectified by thyristor, the energy storage of the inductor is used, and the SGCT is used as the switching element on the inverter side, which is a traditional two-level structure. Since the device's withstand voltage level is limited, multiple devices must be connected in series. Device series is a very complex engineering application technology. In theory, reliability is very low, but some companies can achieve productization. Since the output side has only two levels and the motor is subjected to a large dv/dt, an output filter must be used. Multi-pulse rectifiers on the grid side are optional and users need to make requests for their own plant conditions. The main advantage of this type of frequency converter is that the inertial energy of the load can be fed back to the grid without the need for an external circuit.
The main disadvantage of the current source type inverter is that the power factor on the grid side is low, the harmonics are large, and it changes with the change of working conditions, which is not easy to compensate.
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