Frequency converter selection - Solutions - Huaqiang Electronic Network

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The correct choice of frequency converter is critical to the proper operation of the control system. When selecting a frequency converter, you must fully understand the load characteristics driven by the inverter. In practice, people often divide production machinery into three types: constant torque load, constant power load, and fan and pump load.
Constant torque load:
The load torque TL is independent of the speed n, and the TL always remains constant or substantially constant at any speed. For example, friction type loads such as conveyor belts, mixers, and extruders, and load loads such as cranes and hoists are all constant torque loads.
When the inverter is dragging a load of constant torque nature, the torque at low speed is large enough and has sufficient overload capability. If you need to run at a low speed, you should consider the heat dissipation capability of the standard asynchronous motor to avoid excessive temperature rise of the motor.
Constant power load:
The torque required for machine spindles and coilers, paper machines, coilers and unwinders in plastic film production lines is generally inversely proportional to the rotational speed. This is called a constant power load. The constant power nature of the load should be in terms of a range of speed variations. When the speed is very low, due to the limitation of mechanical strength, TL cannot be infinitely increased, and it is converted to a constant torque property at a low speed. The constant power zone and constant torque zone of the load have a great influence on the choice of the transmission scheme. When the motor is in constant flux regulation, the maximum allowable output torque is constant, which belongs to constant torque speed regulation. In the case of weak magnetic speed regulation, the maximum allowable output torque is inversely proportional to the speed, which belongs to constant power speed regulation. If the range of constant torque and constant power speed regulation of the motor is consistent with the constant torque and constant power range of the load, the so-called "matching", the capacity of the motor and the capacity of the inverter are the smallest.
Fan, pump load:
In various fans, pumps, and oil pumps, as the impeller rotates, the resistance generated by air or liquid in a certain speed range is roughly proportional to the second power of the speed n. As the speed decreases, the speed decreases by the second power of the speed. The power required for this load is proportional to the third power of the speed. When the required air volume and flow rate are reduced, the frequency converter and the flow rate can be adjusted by the frequency converter through the speed regulation, which can greatly save power. Since the power required at high speed increases rapidly with the speed, which is proportional to the cube of the speed, the fan and pump loads should not be operated at a higher power frequency.
Siemens can provide different types of inverters, users can choose different types of inverters according to their actual process requirements and applications. Pay attention to the following points when selecting the inverter:
1. Select the inverter according to the load characteristics. If the load is a constant torque load, select the siemens MMV/MDV inverter. If the load is a fan or a pump type, select the siemens ECO inverter.
2. When selecting the inverter, the actual motor current value should be used as the basis for the inverter selection. 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 inverter contains higher harmonics, which will cause the power factor and efficiency of the motor to deteriorate. Therefore, compared with the power frequency grid power supply, the motor current is increased by 10% and the temperature rise is increased by about 20%. Therefore, when selecting the motor and the inverter, this situation should be taken into consideration, and a margin should be left to prevent the temperature rise from being too high and affect the service life of the motor.
3. If the inverter is to run with a long cable, measures should be taken to suppress the influence of the long cable to the ground coupling capacitor to avoid insufficient output of the inverter. Therefore, the inverter should be amplified by one gear or installed with an output reactor at the output of the inverter.
4. When the inverter is used to control several motors in parallel, it is necessary to consider the total length of the cable from the inverter to the motor within the allowable range of the inverter. If the specified value is exceeded, enlarge the first or second gear to select the inverter. In addition, in this case, the control mode of the inverter can only be V/F control mode, and the inverter cannot protect the overcurrent and overload protection of the motor. In this case, a fuse is added 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 inverter to derate, the inverter needs to be amplified by a gear.
6 When the inverter is used to control the high-speed motor, the high-order motor also has a small reactance, and the high-order harmonic also increases the output current value. Therefore, when selecting a frequency converter for a high speed motor, it should be slightly larger than the inverter of a normal motor.
7 When the inverter is used in a pole-changing motor, pay full attention to the capacity of the inverter so that its maximum rated current is below the rated output current of the inverter. In addition, when performing the pole number conversion during operation, the motor should be stopped first, otherwise the motor will be idling, which will cause damage to the inverter.
8 When the explosion-proof motor is driven, the inverter has no explosion-proof structure, and the inverter should be placed outside the hazardous area.
9 When using a frequency converter to drive a geared motor, the range of use is limited by the lubrication method of the rotating part of the gear. When the lubricating oil is lubricated, there is no limit in the low speed range; in the high speed range exceeding the rated speed, there is a danger that the lubricating oil will be used. Therefore, do not exceed the maximum speed tolerance.
10. When the inverter drives the wound rotor asynchronous motor, most of the existing motors are used.
The winding motor has a lower impedance of the winding motor winding than a conventional squirrel cage motor. Therefore, an overcurrent trip due to ripple current is prone to occur, so select a converter that is slightly larger than the normal capacity. Generally, the winding motor is mostly used in the case where the flywheel torque GD2 is large, and more attention should be paid when setting the acceleration/deceleration time.
11. When the inverter drives the synchronous motor, the output capacity is reduced by 10% to 20% compared with the commercial frequency power supply. The continuous output current of the inverter is greater than the product of the rated current of the synchronous motor and the value of the synchronous pull-in current.
12. For a load with a large torque ripple such as a compressor or a vibrating machine, and a peak load with a hydraulic pump, if the inverter is selected according to the rated current or power value of the motor, the overcurrent protection action may occur due to the peak current. . Therefore, you should understand the power frequency operation and select the inverter with a rated output current greater than its maximum current. When the inverter 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 the inverter is selected, its rated current is greater than the rated current of the submersible pump motor.
13. When the inverter controls the Roots blower, since the starting current is very large, it is necessary to pay attention to whether the capacity of the inverter is large enough when selecting the inverter.
14. When selecting a frequency converter, be sure to check whether its protection level matches the situation on site. Otherwise, dust and water vapor on the site will affect the long-term operation of the inverter.
15. Single-phase motors are not suitable for inverter drives.