Megmeet inverter - how to choose the inverter?

1. Understand the type of variable frequency load

The correct selection of the frequency converter is very critical for the proper operation of the control system. When selecting a frequency converter, it is important to fully understand the load characteristics driven by the frequency converter. In practice, production machinery is often divided into three types: constant torque load, constant power load and fan and pump load.

1. Constant torque load

The load torque TL is independent of the rotational speed n, and the TL is always constant or substantially constant at any rotational 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 drags a load of constant torque nature, the torque at low speed should be large enough and have sufficient overload capacity. If you need to run at a steady speed at low speeds, you should consider the heat dissipation capacity of the standard asynchronous motor to avoid the temperature rise of the motor being too high.

2. Constant power load

The torque required for machine tool spindles, rolling mills, paper mills, coilers, uncoilers, etc., in plastic film production lines, is roughly 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 variations. When the speed is very low, due to the limitation of mechanical strength, TL cannot increase indefinitely, and it is transformed into a constant torque property at low speed. 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, that is, in the case of the so-called "matching", the capacity of the motor and the capacity of the inverter are the smallest.

3. Fan and pump loads

In various fans, water pumps, and oil pumps, the resistance generated by air or liquid in a certain speed range is roughly proportional to the 2nd power of the speed n with the rotation of the impeller. As the rotational speed decreases, the torque decreases to the 2nd power of the rotational speed. The power required for such a load is proportional to the 3rd power of speed. When the required air volume and flow rate are reduced, the inverter can be used to adjust the air volume and flow rate by adjusting the speed, which can greatly save electric energy. Because the required power at high speed increases too fast with the speed and is proportional to the cubic of the speed, the fan and pump load should not be operated beyond the power frequency.

2. Familiar with the principle of frequency conversion selection

1. Combined with the overall framework of the project, starting from the process characteristics and electrical control, the load type, use environment, communication architecture and interface type must be considered, such as serial port, DP or PN communication interface.

2. Select the inverter according to the load characteristics, if the load is constant torque load, you can choose Siemens G120 inverter, if the load is a fan, pump load can choose Siemens G120XA inverter.

3. When selecting the inverter, the actual motor current value should be used as the basis for the selection of the inverter, 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 inverter contains higher harmonics, which will cause the power factor and efficiency of the motor to deteriorate. Therefore, the current of the motor increases by 10% and the temperature rise increases by about 20% compared to the power supply to the motor with the frequency grid compared to the power supply from the power grid with the frequency of the motor. Therefore, when selecting motors and inverters, this situation should be taken into account and appropriate margins should be left to prevent the temperature rise from being too high and affecting the service life of the motor.

4. When the inverter wants 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 inverter's insufficient output, so the inverter should be enlarged by one gear or the output reactor should be installed at the output end of the inverter.

5. 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 derating of the inverter, and the inverter needs to be enlarged by one gear.

6. When the inverter is used to control several motors connected in parallel, it must be considered that the sum of the length of the cable from the inverter to the motor is within the allowable range of the inverter. If the specified value is exceeded, you need to zoom in one or two gears 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 can not protect the overcurrent and overload protection of the motor, so it is necessary to add a fuse to each motor to achieve protection.

7. For some special applications, such as high ambient temperature, high switching frequency, high altitude, etc., it will cause the derating of the inverter, and the inverter needs to be enlarged by one gear.

8. When using the inverter to control the high-speed motor, because the reactance of the high-speed motor is small, the higher harmonic also increases the output current value. Therefore, when choosing an inverter for a high-speed motor, it should be slightly larger than the inverter for a normal motor.

9. When the inverter is used in a pole-changing motor, full attention should be paid to selecting the capacity of the inverter, so that its maximum rated current is below the rated output current of the inverter. In addition, when the number of poles is converted in operation, the motor should be stopped first, otherwise it will cause the motor to run idle, and the inverter will be damaged when it is bad.

10. When driving the explosion-proof motor, the inverter does not have an explosion-proof structure, and the inverter should be set outside the dangerous place.

11. When the inverter is used to drive the gear reducer motor, the range of use is restricted by the lubrication method of the rotating part of the gear. When lubricating oil, there is no limit in the low-speed range; In the high-speed range above the rated speed, there is a risk that the lubricant will run out. Therefore, do not exceed the maximum speed tolerance.

12. When the inverter drives the wound rotor asynchronous motor, most of them use the existing motor. The winding motor has a small impedance in the winding of the winding motor compared to the ordinary squirrel cage motor. Therefore, it is easy to overcurrent tripping due to ripple current, so it is necessary to choose an inverter with a slightly larger capacity than usual. Generally, winding motors are mostly used in occasions where the flywheel torque GD2 is large, and more attention should be paid when setting the acceleration and deceleration time.

13. When the inverter drives the synchronous motor, compared with the power frequency power supply, the output capacity is reduced by 10%-20%, and the continuous output current of the inverter is greater than the product of the rated current of the synchronous motor and the standard value of the synchronous intake current.

14. In the case of loads with large torque fluctuations such as compressors and vibrating machines, and peak loads such as hydraulic pumps, if the inverter is selected according to the rated current or power value of the motor, the phenomenon of overcurrent protection due to peak current may occur. Therefore, it is necessary to understand the operation of the power frequency and choose an inverter with a rated output current that is larger 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 ordinary motor, the rated current of the inverter is greater than the rated current of the submersible pump motor.

15. When the inverter controls the roots blower, because its starting current is very large, we must pay attention to whether the capacity of the inverter is large enough when choosing the inverter.

16. When choosing an inverter, be sure to 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.

17. Single-phase motors are not suitable for inverter drive.

18. When the motor load is very light, even if the motor load current is within the rated current of the inverter, the inverter that is much smaller than the motor capacity cannot be used. This is because the reactance of the motor varies with the capacity of the motor, and even if the motor load is the same, the greater the capacity of the motor, the greater the pulsating current value, so it is possible to exceed the current tolerance of the inverter.

19. If the power supply of the inverter is its own power supply, it is best to add the incoming reactor.

Megmeet inverter - how to choose the inverter?

Megmeet inverter - how to choose the inverter?

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