High voltage motor speed control technology
High voltage motor control technology can be divided into the following:
High frequency converter
The inverter for low voltage converter, the transformer high voltage input side to low-voltage, high-voltage motor will be replaced by low pressure, special motor, motor voltage level is varied, there is no uniform standard.
This approach due to the low voltage inverter, the capacity is relatively small, the grid side harmonicis large, the 12 pulse rectifier can reduce the harmonic, but can not meet the stringent requirements of the harmonic. When a fault occurs in the inverter motor, not into the power gridoperation, in some applications cannot stop where there will be a problem. In addition, the motor and the cable will be changed, a large amount of engineering.
High level inverter
The inverter for low voltage converter, the input and output step-down transformer step-up transformer and high voltage power grid and electrical interface, this is a transitional technology at the time of high voltage frequency conversion technology is immature.
Due to low voltage converter low voltage current, but can not rise without limit, limiting the capacity of this inverter. Because of the existence of output transformer, reduce the efficiency of the system,covers an area of increasing; in addition, reduced output transformer at low frequency when the magnetic coupling ability, so that the inverter load capacity decreased at startup. The harmonics on power system, if the 12 pulse rectifier can reduce the harmonic, but can not meet the stringent requirements of the harmonic; output transformer step-up at the same time, dv/dt production of theinverter is also equal amplification, filter must be installed in order to apply to ordinary motor, or will cause corona discharge, the insulation damage condition. If the motor special can avoid this situation, but as the level of the inverter type.
Hydraulic coupler
Join in between the motor shaft and the impeller shaft load, adjust the impeller between liquid(usually oil) pressure, to adjust the speed of the load. This control method is essentially the slip power consumption practices, its main drawback is that with the decrease of speed efficiency is more and more low, need to disconnect the motor and load installation, maintenance workload, over a period of time need to seal, bearings and other parts replacement, the scene is generally dirty,equipment is of low grade, is out of technology.
Early interest in regulating technology manufacturers, or because there is no pressure control technology can choose, or take into account the cost factor, there are some applications ofhydraulic coupler. Boiler feed water pump water pump, such as power plant water company and fan,steel plant dust blower etc.. Now, some of the old equipment in the transformation has been gradually replaced by high voltage frequency conversion.
Current source converter
This converter, the input side of the thyristor rectifier, using inductive energy storage, use SGCT asthe inverter side switch element, two level structure of the traditional. Due to the voltage level of the device is limited, must use multiple devices in series. Devices in series is a very complexengineering application technology, the theory of reliability is very low, but some companies can do production stage. Because the output side only two level, the larger dv/dt motor bearing, output filter must be used. The grid side of the multi pulse rectifier as optional, the user needs for their own situation factory requests. The main advantage of this converter is the need for external circuit canload inertia energy back to the grid.
The main drawback of current source converter is low power factor, harmonic, and change with the changes of working conditions, not compensation.
Cascade inverter
The asynchronous motor rotor energy feedback to the grid, thus changing the rotor slip speed control, the speed control by SCR technology, requires the use of winding type asynchronous motor, and now almost all the industrial field of squirrel cage asynchronous motor, replace the motor very trouble. This speed range speed control mode in general about 70%-95%, narrowspeed range. SCR technology is easy to cause the harmonic pollution of power grid; when the speed decreases, power factor is low, need to be taken compensation. The utility model has the advantages of smaller capacity than other parts of inverter, high voltage VVVF technology slightly lower costs.
This way there is a speed change form, namely the internal feedback speed control system, save the transformer inverter, the feedback winding directly on the stator winding, this approach toreplace the motor performance, and other aspects of the cascade control approach.
On effect of motor speed control by rotor ring, can not do a great power, the slip ring maintenance workload is large, the backward technology belongs to the seventy or eighty's, the industrial application has become less and less.
Power module series multilevel inverter
The transducer adopts low-voltage inverter series means to achieve high voltage is the voltage source inverter. Its input side adopts phase shifted step-down transformer, 18 pulse rectifier modeabove, to meet international power harmonics up to the most stringent requirements. With the loadwhen the power factor can reach more than 95%. Multistage PWM technology, on the output side of dv/dt is small, less harmonic, meet the needs of ordinary induction motor. According to the output voltage of the inverter load design needs, is a better way to resolve 6KV, 10KV motor speed control. Power circuit using standard modular design, simple replacement, the devices used in the domestic procurement easier.
This converter adopts low voltage IGBT as inverter components, compared with the three level inverter with high voltage IGBT, a relatively large number of power components, but the technologyis relatively mature. Compared with the three level inverter with high voltage IGCT, a relatively large number of power components, but the total number of components is less, because IGCT requiresvery complex auxiliary shutdown circuit.
Because of the rectifier transformer and power module connection more, so the transformer can not be separated with the inverter, in the limited space of the occasion is not very flexible.
Three level voltage source inverter
This converter uses diode rectifier, capacitor, IGBT or IGCT inverter. Three level inverter, usingdiode clamp, solves the problem of the two devices in series, technically simple and direct than the two devices in series easily, at the same time, increase an output level, the output waveform of two level.
The main problem of this converter is: because of the use of high-voltage devices, the output side of the dv/dt is still more serious, need to adopt the output filter. Due to the limitation of the breakdown voltage, the maximum voltage can only meet the needs of 4160V, 6KV and 10KV power grid, replacement motor is a kind of practice, but to the power grid failure caused by bypass more trouble. There is an alternative approach for the 6KV motor, the motor is a star type connectioninstead of angle connection, so that the motor voltage is 3KV; this approach makes the circulation loss increased motor, has already burned motor case, there may be related to this. There are companies in this way so that the high level inverter, capacity than the original low voltage inverter to achieve low high way, but some problems still exist in high level mode.
Three level inverter using 12 pulse rectifier mode.
