Both high-voltage motors and low-voltage motors have their own advantages and disadvantages. What are their respective advantages and disadvantages?
High voltage motor:
The voltage of the high voltage motor is above 1000V. Commonly used are 6300V and 10000V. The motor power is proportional to the product of voltage and current. Therefore, if the power of the low-voltage motor is increased to a certain extent (such as 300KW/380V), the current is limited by the allowable capacity of the wire, or the cost is too high. A high power output needs to be achieved by increasing the voltage.
The advantages are: high power, strong impact resistance; small current.
The disadvantage is that the cost of the winding is relatively high, and the cost of the related insulating material will also become higher; the requirements for the use environment are far higher than the requirements of the low-voltage motor for the environment; the insulation treatment process is difficult and the labor cost is high. The motor has a long manufacturing cycle; the inertia is large, and it is difficult to start and brake.
The control device is determined according to the actual method: the motor capacity is much smaller than the power supply capacity and can be directly started under 1000 KW, and the inrush current is 3-6 times of the rated value. In order to prevent the inrush current from being too large, the starting mode for reducing the starting current must be considered for large motors: a series of reactance starting, variable frequency starting, and hydraulic coupling starting. There are complex and simple, and the price varies greatly. Due to the high voltage and large current surge, the motor must meet the overvoltage requirements and the insulation level must be high enough.
Hydraulic Coupler
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 kind of speed regulation method is essentially a slip power consumption type. The main disadvantage is that as the speed of the speed decreases, the efficiency of the motor and the load needs to be disconnected for installation and maintenance. Parts such as shaft seals and bearings are replaced. The site is generally dirty, which means that the equipment is of low grade and is a phase-out technology. Early manufacturers interested in speed control technology, or because there is no high-pressure speed control technology can choose, or take into account the cost factor, there are some applications for hydraulic couplers. Such as the water pump of the water company, the boiler feed pump and the induced draft fan of the power plant, and the dust removal fan of the steel mill. Now, some old equipment has been gradually replaced by high voltage frequency conversion in the transformation.
High and low high frequency inverter
The frequency converter is a low-voltage frequency converter, which uses an input step-down transformer and an output step-up transformer to realize the interface with the high-voltage power grid and the motor. This is a transitional technology when the high-voltage frequency conversion technology is not mature. Due to the low voltage of the low-voltage inverter, the current cannot rise unrestricted, which limits the capacity of the inverter. Due to the existence of the output transformer, the efficiency of the system is reduced and the floor space is increased. In addition, the magnetic coupling capability of the output transformer is weakened at low frequencies, so that the load capacity of the frequency converter is weakened at startup. The harmonics of the power grid are large. If 12-pulse rectification is used, the harmonics can be reduced, but the strict requirements for harmonics cannot be met. When the output transformer is boosted, the dv/dt generated by the inverter is equally amplified, and filtering must be added. The device can be applied to ordinary motors, otherwise corona discharge and insulation damage will occur. This can be avoided if a special variable frequency motor is used, but it is better to use a high and low frequency converter.
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 low-voltage inverters, 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 applications where there is no need to stop the machine. In addition, the motor and cable must be replaced, and the amount of work is relatively large. The cascade speed converter feeds back the rotor energy of the asynchronous motor to the grid, thereby changing the rotor slip to achieve speed regulation. This speed regulation adopts the thyristor technology and requires the use of a wound-type asynchronous motor, but now almost all industrial sites With a squirrel-cage asynchronous motor, it is very troublesome to replace the motor. The speed regulation range of this speed regulation mode is generally about 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 power factor 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 requires replacement of the motor, and other aspects of performance and cascade adjustment. 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 type of frequency converter, the input side is rectified by a thyristor, and the inductor is used for energy storage, and the inverter side uses SGCT as a switching element, which is a conventional two-level structure. Since the device's withstand voltage level is limited, multiple devices must be connected in series. Device series is a very complicated 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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