It has become an irreversible trend to drive motors through frequency converters.
However, in actual usage, there are often problems due to the mismatch between the frequency converter and the motor.
When selecting a frequency converter, it is important to fully understand the load characteristics of the equipment being driven by the frequency converter.
We have compiled some relevant information to share with everyone, in the hope that it will be useful for practical work.
We can divide production machinery into three types: constant power load, constant torque load, and load types such as fans and water pumps. Different load types have different requirements for frequency converters, and we should match them reasonably according to specific situations.
Constant power load
The torque required for the spindle of machine tools, rolling mills, paper machines, and winding machines in plastic film production lines, unwinding machines, etc., is generally inversely proportional to the speed, and belongs to constant power load.
The constant power nature of the load should be considered within a certain range of speed variations. When the speed is very low, it is limited by mechanical strength and will become a constant torque load at low speeds.
When the motor adjusts the speed with constant magnetic flux, it is constant torque control; while in weak magnetic speed control, it is constant power control.
Fan and pump loads
Fans, water pumps, oil pumps, and other equipment rotate with the impeller. As the speed decreases, the torque decreases according to the square of the speed, and the power required by the load is directly proportional to the cube of the speed.
When the required air volume or flow rate decreases, using a frequency converter to adjust the air volume or flow rate through speed regulation can greatly save energy.
Since the power required at high speeds increases too quickly with the speed, fans and pump loads should not exceed the rated frequency during operation.
Constant torque load
TL remains constant or basically constant at any speed.
When the inverter drives a load with constant torque characteristics, the torque at low speeds should be sufficient and have enough overload capacity.
If stable operation at low speeds is required, the motor's heat dissipation performance should be considered to avoid motor burnout due to excessive temperature rise.
Issues to consider when choosing a variable frequency drive
● When the AC motor is driven by a frequency converter, the motor's current will increase by 10-15%, and the temperature rise will increase by about 20-25%.
● When using a frequency converter to control a high-speed motor, it will generate a significant amount of high-order harmonics. These high-order harmonics will increase the output current value of the frequency converter.
Therefore, when selecting a frequency converter, it should be one level higher than a regular motor.
● Compared to ordinary squirrel cage motors, wound motors are more prone to overcurrent tripping issues. Therefore, it is recommended to select a slightly larger capacity inverter than usual.
● When using a frequency converter to drive a gear reduction motor, the range of use is limited by the lubrication method of the gear rotating part. There is a risk of running out of oil when exceeding the rated speed.
● The motor's rated power is only used as a reference when selecting the frequency converter based on the motor's current value.
● The output of the frequency converter contains abundant high-order harmonics, which will reduce the power factor and efficiency performance of the motor.
● If the frequency converter needs to operate with long cables, the impact of the cables on performance should be considered, and if necessary, special cables should be selected. To compensate for this issue, the frequency converter should be set to a higher level by one or two gears.
● Special occasions such as high temperature, frequent switching, and high altitude can cause a decrease in the capacity of the frequency converter. It is recommended to choose a frequency converter with one level higher capacity.
● Compared to the mains power supply, when driving a synchronous motor with a variable frequency drive, the output capacity will decrease by 10~20%.
● For loads with large torque fluctuations such as compressors, vibrators, and peak loads such as hydraulic pumps, it is necessary to fully understand the operating conditions of the power frequency and choose a larger gear frequency converter.
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