Abstract: How to properly select frequency converters is a challenge faced by many enterprise technicians and sales personnel. In addition, it is unscientific to select only the frequency converter itself without considering whether the selection and capacity matching are appropriate.
The following article will provide a detailed explanation:
I. Selecting a frequency converter based on load characteristics
Second, when selecting a frequency converter, the actual motor current value should be used as the basis for selection; the motor's rated power should only be used as a reference. Furthermore, it should be fully considered that the frequency converter's output contains high-order harmonics, which will degrade the motor's power factor and efficiency.
3. If the frequency converter is to be operated with a long cable, the frequency converter should be selected with a larger range or an output reactor should be installed at the output end of the frequency converter.
4. When a frequency converter is used to control several motors connected in parallel, it is essential to ensure that the total length of the cables from the frequency converter to the motors is within the allowable range of the frequency converter.
5. For some special applications, such as high ambient temperature, high switching frequency, and high altitude, the inverter's capacity may be reduced, and the inverter needs to be selected with a larger capacity .
6. When selecting a frequency converter for a high-speed motor, it should be slightly larger than that for a regular motor.
7. When using a frequency converter for a pole-changing motor, careful attention should be paid to selecting the capacity of the frequency converter so that its maximum rated current is below the rated output current of the frequency converter.
8. When driving an explosion-proof motor, if the frequency converter does not have an explosion-proof structure, the frequency converter should be installed outside the hazardous area.
9. When using a frequency converter to drive a geared motor, the operating range is limited by the lubrication method of the rotating gear parts. Do not exceed the maximum permissible speed.
10. When a frequency converter drives a wound-rotor asynchronous motor, it often utilizes an existing motor. Overcurrent tripping due to ripple current is prone to occur, so a frequency converter with a slightly larger capacity than usual should be selected.
11. When a frequency converter drives a synchronous motor, the output capacity is reduced by 10% to 20% compared with a power frequency power supply.
12. For loads with large torque fluctuations, such as compressors and vibratory machines, and for loads with peak loads, such as hydraulic pumps, the operating frequency should be understood, and a frequency converter with a rated output current greater than its maximum current should be selected.
Thirteen, when a frequency converter controls a Roots blower, because its starting current is very large, it is important to ensure that the capacity of the frequency converter is large enough when selecting a frequency converter.
Fourteen, when selecting a frequency converter, be sure to pay attention to whether its protection level matches the site conditions.
15. Single-phase motors are not suitable for frequency converter drives.
If the inverter itself has high reliability, but the inverter selection and capacity are not properly matched, the resulting variable frequency speed control system cannot achieve high reliability or may even fail to operate. So how can we ensure the normal and efficient operation of the variable frequency control system?
We need to ensure that the inverter's capacity is matched. First, select the correct inverter type based on the load characteristics. The general principle is to match the inverter with the appropriate load characteristics.
(1) Constant torque production equipment - within the speed regulation range, the load torque remains basically constant. A frequency converter with constant torque performance should be selected. Its overload capacity is 150% of the rated current for 1 minute.
(2) Square torque production equipment – Within the speed regulation range, the load torque is proportional to the square of the speed, i.e., M∝n². Centrifugal fans and water pumps are typical examples. Frequency converters with the M∝n² characteristic have relatively low overload capacity; they can withstand 110%-120% of the rated current overload for 1 minute.
(3) Constant power load production equipment - within the speed regulation range, the torque is large at low speed and small at high speed, i.e., M? NC (constant). Typical equipment includes machine tools and winding mechanisms.
