High-voltage motor speed control technology
Voltage source type three-level inverter
This type of frequency converter uses diode rectification, capacitor energy storage, IGBT or IGCT inverter. The three-level inverter form adopts the diode clamp method to solve the problem that the two devices are connected in series. It is technically easier than direct connection between the two devices. At the same time, an output level is increased, so that the output waveform is better than two. Good.
The main problem with this type of frequency converter is that due to the use of high voltage devices, the dv/dt on the output side is still severe, requiring an output filter. Due to the limitation of the withstand voltage level of the device, the maximum voltage can only be 4160V. To adapt to the needs of the 6KV and 10KV power grids, it is a practice to replace the motor, but it is more troublesome to the grid when the fault occurs. There is a workaround for the 6KV motor, which is to change the motor from star connection to angle connection, so that the voltage of the motor becomes 3KV; this method increases the circulating current loss of the motor, and there are cases in the country where the motor is burned. It is possible to be related to this. Other companies use this kind of frequency converter to realize the high and low modes, so that the capacity is larger than the low-voltage mode of the original low-voltage inverter, but the problems still exist in the high-low mode.
Three-level inverters generally use 12-pulse rectification.
Power module series multi-level inverter
This type of frequency converter uses a low-voltage inverter in series to achieve high voltage, and is a voltage source type inverter. Its input side uses phase-shifting step-down transformer to achieve more than 18 pulses of rectification, meeting the most stringent international requirements for grid harmonics. When the load is loaded, the grid side power factor can reach more than 95%. Multi-stage PWM technology is adopted on the output side, which has small dv/dt and less harmonics, which meets the needs of ordinary asynchronous motors. The output voltage of the inverter can be designed according to the needs of the load, which is a better way to solve the 6KV, 10KV motor speed regulation. The power circuit adopts standard modular design, and the replacement is simple. The devices used are also easy to purchase in China.
This kind of frequency converter uses low voltage IGBT as the inverter component. Compared with the three-level inverter using high voltage IGBT, the number of power components is large, but it is technically mature. Compared to three-level inverters using high-voltage IGCT, the number of power components is larger, but the total number of components is smaller, because IGCT requires a very complicated auxiliary shutdown circuit.
Since there are many connections between the rectifier transformer and the power module, the transformer cannot be placed separately from the inverter, and it is not very flexible in places where space is limited.
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