According to the capacity of the motor and the capacity of the power supply system, the direct starting capacity of the motor is determined. When the capacity of the grid is not large enough, when the motor starts directly, the voltage of the bus bar can't reach the allowed voltage fluctuation range, so we need to consider the start-up of the buck, so as to reduce the impact of the starting high current on the power grid.
There are several ways of starting, frequency conversion, and hydraulic coupler starting. It is complicated and simple, and the price of the equipment is very different.
Early subject to technical constraints, using the hydraulic coupler, with the continuous development of technology, especially the development of high frequency motor speed in recent years, the old start speed to be eliminated or will be eliminated, some of the old equipment in the transformation has been gradually replaced by high voltage frequency conversion. High voltage variable frequency speed regulating device is widely used.
Frequency converter has high and low, high and low, cascade speed control frequency converter.
The high level of type as the name implies, is inverter for low voltage converter, the input and output step-down transformer step-up transformer and high voltage power grid and electrical interface, due to low voltage converter low voltage, current but can not rise without limit, limiting the capacity of this inverter. Due to the existence of output transformer, the efficiency of the system is reduced and the area occupied is increased. In addition, the output capacity of transformer is weakened at low frequency, which makes the load capacity of frequency converter weaken when starting. 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 the inverter is also equal amplification, filter must be installed in order to apply to ordinary motor, or will cause corona discharge, insulation damage situation. If the special frequency conversion motor can avoid this situation, but the equipment is large, the land is large, the efficiency is low, the harmonics of the power grid are large and the voltage quality is poor.
The high and low frequency converter is still the frequency converter for the low voltage frequency converter, the input side uses transformer to turn the high voltage to the low voltage, the high voltage motor is replaced, the special low voltage motor, the voltage level of the motor is various, and there is no unified standard.
Because of the low voltage frequency converter, the capacity is relatively small, and the harmonic of the power grid side is larger. The 12 pulse rectifier can reduce the harmonic, but it can't meet the strict requirements of the harmonic. When the frequency converter fails, the motor can not be put into the power grid operation, and it will be problematic in some situations where there is no stop. In addition, the motor and cable are to be replaced, and the amount of engineering is larger.
Cascade inverter is 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, motor replacement is very troublesome. The speed regulating range of this kind of speed regulation is usually around 70%-95%, and the speed range is narrow. Thyristor technology is easy to cause harmonic pollution to the power grid; with the reduction of speed, the power factor of the grid side is also lower, and measures should be taken to compensate. In 70s and 80s, it was used more and now less and less.