Using frequency converters to drive motors has become an irreversible trend. However, in actual use, improper matching between the frequency converter and the motor often leads to problems. When selecting a frequency converter, it is essential to fully understand the load characteristics of the equipment it drives. We can categorize production machinery into three types: constant power loads, constant torque loads, and fan/pump loads. Different load types have different requirements for frequency converters, and we should match them appropriately according to the specific circumstances.
constant power load
The torque required by machine tool spindles and in rolling mills, paper machines, and winding/unwinding machines in plastic film production lines is generally inversely proportional to the rotational speed, classifying them as constant power loads. The constant power nature of a load refers to a certain range of speed variation. When the speed is very low, due to mechanical limitations, it will change to a constant torque load. When a motor adjusts its speed using constant flux, it is a constant torque speed regulation; while when it adjusts its speed using field weakening, it is a constant power speed regulation.
Fan and pump loads
As the impeller of equipment such as fans, water pumps, and oil pumps rotates, the torque decreases with the square of the rotational speed. The power required by the load is directly proportional to the cube of the speed. When the required air volume or flow rate decreases, adjusting the air volume and flow rate using a frequency converter can significantly save electricity. Because the power required increases too rapidly with the rotational speed at high speeds, fans and pumps should not be operated above their operating frequency.
Constant torque load
TL remains constant or nearly constant at any speed. When a frequency converter drives a load with constant torque, the torque at low speeds must be large enough, and it must have sufficient overload capacity. If stable operation at low speeds is required, the motor's heat dissipation performance should be considered to prevent the motor from burning out due to excessive temperature rise.
Issues to consider when selecting a frequency converter
When a power frequency motor is driven by a frequency converter, the motor 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, a significant amount of high-order harmonics are generated. These harmonics increase the output current of the frequency converter. Therefore, when selecting a frequency converter, it should be one size larger than that used for a conventional motor.
Compared to ordinary squirrel-cage motors, wound-rotor motors are more prone to overcurrent tripping problems, so a frequency converter with a slightly larger capacity should be selected.
When using a frequency converter to drive a geared motor, the applicable range is limited by the lubrication method of the rotating gear parts. There is a risk of running out of oil when exceeding the rated speed.
The motor current value is used as the basis for selecting the frequency converter, and the rated power of the motor is only for reference.
The output of a frequency converter contains abundant high-order harmonics, which will reduce the power factor and efficiency performance of the motor.
If a frequency converter is to operate with a long cable, the impact of the cable on performance should be considered, and a dedicated cable should be selected if necessary. To compensate for this issue, the frequency converter should be selected with a range one or two increments.
In special environments such as high temperature, frequent switching, and high altitude, the capacity of the frequency converter may decrease. It is recommended to select the frequency converter with a capacity one level higher.
Compared to mains frequency power supply, the output capacity of a frequency converter driving a synchronous motor will be reduced by 10-20%.
For loads with large torque fluctuations, such as compressors and vibratory machines, and for loads with peak loads, such as hydraulic pumps, it is essential to fully understand the operating conditions at the power frequency and select a higher-range inverter.
