From the perspective of the market situation, the high-voltage motor speed regulation technology can be divided into the following types:
Fluid couplings
An impeller is added between the motor shaft and the load shaft to adjust the pressure of the liquid (generally oil) between the impellers to achieve the purpose of adjusting the load speed. This speed regulation method is essentially a slip power consumption type approach, its main disadvantage is that as the speed decreases, the efficiency is getting lower and lower, the motor needs to be disconnected from the load for installation, the maintenance workload is large, and after a period of time, it is necessary to replace the shaft seal, bearing and other components, and the site is generally dirty, and the equipment grade is low, which is an elimination technology.
In the early days, manufacturers who were more interested in speed regulation technology had some applications for hydraulic couplings because there was no high-voltage speed regulation technology to choose from, or because of cost considerations. For example, the water pump of the water company, the boiler feed pump and induced draft fan of the power plant, the dust removal fan of the steel plant, etc. Nowadays, some old equipment has been gradually replaced by high-voltage frequency conversion in the transformation.
High-low-high frequency converters
The inverter is a low-voltage inverter, which uses an input step-down transformer and an output step-up transformer to realize the interface with the high-voltage power grid and motor, which is a transitional technology when the high-voltage inverter technology is not mature at that time.
Due to the low voltage of the low-voltage inverter, the current cannot rise unlimitedly, which limits the capacity of this inverter. Due to the existence of the output transformer, the efficiency of the system is reduced, and the footprint is increased, and the magnetic coupling ability of the output transformer is weakened at low frequency, so that the load carrying capacity of the inverter is weakened at start-up. The harmonics of the power grid are large, if the 12-pulse rectification can be reduced, but can not meet the strict requirements of harmonics; the output transformer is boosted at the same time, the inverter produces DV/DT is also equally amplified, and the filter must be installed to be suitable for the ordinary motor, otherwise there will be corona discharge, insulation damage. This can be avoided by using a special inverter motor, but it is better to use a high-low inverter.
High and low type inverter
The inverter is a low-voltage inverter, and the input side uses a transformer to change the high voltage to low voltage, and the high-voltage motor is replaced, and a special low-voltage motor is used, and the voltage level of the motor is varied, and there is no unified standard.
This method is due to the use of low-voltage inverter, the capacity is also relatively small, the harmonics on the power grid side are larger, and 12 pulse rectification can be used to reduce harmonics, but can not meet the strict requirements of harmonics. When the inverter fails, the motor cannot be put into operation of the power frequency grid, and there will be problems in some occasions when it cannot be shut down. In addition, the motor and cable have to be replaced, and the amount of work is relatively large.
Cascade speed control inverter
The energy of part of the rotor of the asynchronous motor is fed back to the power grid, so as to change the rotor slip to achieve speed regulation, this speed regulation method adopts thyristor technology, which requires the use of wound asynchronous motors, and now almost all industrial sites use squirrel cage asynchronous motors, and it is very troublesome to replace the motor. The speed regulation range of this speed regulation method is generally about 70%-95%, and the speed regulation range is narrow. Thyristor technology is prone to harmonic pollution to the power grid, and as the speed decreases, the power factor on the grid side also decreases, and measures need to be taken to compensate for it. Its advantage is that the capacity of the frequency conversion part is small, and the cost is slightly lower than that of other high-voltage AC frequency conversion speed regulation technologies.
There is a variation of this speed regulation mode, that is, the internal feedback speed regulation system, which omits the transformer of the inverter part, and makes the feedback winding directly in the stator winding, which needs to replace the motor, and the performance in other aspects is close to the cascade speed regulation.