The correct selection of the frequency converter is very critical to the normal operation of the control system. When selecting a frequency converter, it is necessary to fully understand the characteristics of the load driven by the frequency converter. In practice, people often divide production machinery into three types: constant torque loads, constant power loads, and variable torque loads.
1. Constant torque loads
The load torque TL has nothing to do with the speed n, and TL always remains constant or basically constant at any speed. For example, friction loads such as conveyor belts, mixers, and extruders, as well as potential energy loads such as cranes and hoists, are all constant torque loads. When the inverter drives a load with constant torque, the torque at low speed should be large enough and have enough overload capacity. If it is necessary to run stably at low speed, the heat dissipation capacity of the standard asynchronous motor should be considered to avoid an excessive temperature rise of the motor.
2. Constant power loads
The torque required by the spindle of the machine tool and the coiler and uncoiler in the rolling mill, paper machine, and plastic film production line is generally inversely proportional to the speed, which is the so-called constant power load. The constant power nature of the load should be in terms of a certain range of speed changes. When the speed is very low, limited by mechanical strength, it is impossible for TL to increase infinitely, and it turns into a constant torque property at low speed. If the motor’s constant torque and constant power speed regulation range are consistent with the load’s constant torque and constant power range, that is, the so-called “matching” situation, the capacity of the motor and the capacity of the frequency converter are the smallest.
3. Variable torque loads
In various fans, water pumps, and oil pumps, with the rotation of the impeller, the resistance produced by air or liquid within a certain speed range is roughly proportional to the square of the speed n. As the speed decreases, the torque decreases according to the square of the speed. The power required for such a load is proportional to the third power of the speed. When the required air volume and flow rate decrease, the frequency converter can be used to adjust the air volume and flow rate through speed regulation, which can greatly save electric energy. Because the required power increases too fast with the speed at high speed, and is proportional to the cube of the speed, the fan and pump loads should not be operated at super frequency.
The principle of frequency conversion selection
1. When selecting a frequency converter, the actual motor current value should be used as the basis for selecting the frequency converter, and the rated power of the motor can only be used as a reference. In addition, it should be fully considered that the output of the frequency converter contains high-order harmonics, which will cause the power factor and efficiency of the motor to deteriorate. Therefore, compared with using a power frequency grid to supply power to the motor with a frequency converter, the current of the motor increases by 10% and the temperature rise increases by about 20%. Therefore, when selecting the motor and frequency converter, this situation should be taken into consideration, and an appropriate margin should be left to prevent the temperature rise from being too high and affecting the service life of the motor.
2. If the inverter needs to run with a long cable, measures should be taken to suppress the influence of the long cable on the ground coupling capacitance to avoid the insufficient output of the inverter, so the inverter should amplify the first gear selection or install an output reactance at the output end of the inverter device.
3. For some special applications, such as high ambient temperature, high switching frequency (especially when used in applications with high noise restrictions such as building automation), high altitude, etc., it will cause the frequency converter to malfunction. For derating, the frequency converter needs to be amplified for selection.
4. When the frequency converter is used to control several motors connected in parallel, it must be considered that the total length of the cables from the frequency converter to the motor is within the allowable range of the frequency converter. If it exceeds the specified value, it is necessary to amplify one or two gears to select the frequency converter. In addition, in this case, the control mode of the frequency converter can only be V/F control mode, and the frequency converter cannot protect the overcurrent and overload protection of the motor. At this time, it is necessary to add a fuse to each motor to achieve protection.
5. For some special applications, such as high ambient temperature, high switching frequency, high altitude, etc., this will cause the derating of the inverter, and the inverter needs to be enlarged by one gear.
6. When a frequency converter is used to control a high-speed motor since the reactance of the high-speed motor is small, the high-order harmonics also increase the output current value. Therefore, when selecting a frequency converter for a high-speed motor, it should be slightly larger than that of an ordinary motor.
7. When the frequency converter is used in a pole-changing motor, full attention should be paid to selecting the capacity of the frequency converter so that the maximum rated current is below the rated output current of the frequency converter. In addition, when converting the number of poles during operation, the motor should be stopped first, otherwise, it will cause the motor to run idly, and the inverter will be damaged in severe cases.
8. When driving an explosion-proof motor, the frequency converter does not have an explosion-proof structure, so the frequency converter should be installed outside the dangerous place.
9. When a frequency converter is used to drive the gear reduction motor, the application range is restricted by the lubrication method of the gear rotating part. When lubricating oil is used, there is no limit in the low-speed range; in the high-speed range exceeding the rated speed, there may be a danger of running out of lubricating oil. Therefore, do not exceed the maximum allowable rotational speed.
10. When the frequency converter drives the wound rotor asynchronous motor, most of them use the existing motor. Wound motors have a lower impedance in the windings of a wound motor than a normal squirrel cage motor. Therefore, it is easy to cause overcurrent tripping due to ripple current, so an inverter with a slightly larger capacity than usual should be selected. Generally, wound motors are mostly used in occasions where the flywheel torque GD2 is relatively large, and more attention should be paid when setting the acceleration and deceleration time.
11. When the frequency converter drives the synchronous motor, compared with the industrial frequency power supply, the output capacity is reduced by 10% to 20%. The continuous output current of the frequency converter is greater than the product of the rated current of the synchronous motor and the nominal value of the synchronous pull-in current.
12. For loads with large torque fluctuations such as compressors and vibrators and peak loads such as oil pressure pumps, if the inverter is selected according to the rated current or power value of the motor, the overcurrent protection may occur due to the peak current. action phenomenon. Therefore, you should understand the power frequency operation and choose a frequency converter with a rated output current greater than its maximum current. When the frequency converter drives the submersible pump motor, because the rated current of the submersible pump motor is larger than the rated current of the normal motor, when selecting the frequency converter, its rated current should be greater than the rated current of the submersible pump motor.
13. When the frequency converter controls the Roots blower, due to its large starting current, it is necessary to pay attention to whether the capacity of the frequency converter is large enough when selecting the frequency converter.
14. When selecting a frequency converter, you must pay attention to whether its protection level matches the situation on-site. Otherwise, the dust and water vapor on site will affect the long-term operation of the inverter.
15. Single-phase motors are not suitable for inverter drives.
16. When the load of the motor is very light, even if the load current of the motor is within the rated current of the inverter, the inverter whose capacity is much smaller than that of the motor cannot be used. This is because the reactance of the motor varies with the capacity of the motor. Even if the load of the motor is the same, the larger the capacity of the motor, the greater the pulsating current value, which may exceed the currently allowable value of the inverter.
17. If the power supply of the inverter is self-contained, it is better to add a line reactor.
