Frequency Conversion Load Classification, Frequency Conversion Type Selection Principle

1、 Frequency conversion load classification

The correct selection of frequency converter is very important for the normal operation of the control system. When selecting a frequency converter, you must 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 water pump load.

1. Constant torque load

The load torque TL is independent of the speed n, and TL always remains constant or basically constant at any speed. For example, friction loads such as conveyor belt, mixer and extruder and potential energy loads such as crane and hoist belong to constant torque loads. When the frequency converter drives the load with constant torque, the torque at low speed should be large enough and have sufficient overload capacity. If it is necessary to operate at a steady speed at a low speed, the heat dissipation capacity of the standard asynchronous motor should be considered to avoid excessive temperature rise of the motor.

2. Constant power load

The required torque of machine tool spindle and coiler and uncoiler in 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 property of the load should be in terms of a certain speed variation range. When the speed is very low, TL cannot increase infinitely due to the limitation of mechanical strength, and changes to constant torque at low speed. If the range of constant torque and constant power speed regulation of the motor is consistent with the range of constant torque and constant power of the load, that is, in the case of so-called "matching", the capacity of the motor and the capacity of the frequency converter are the smallest.

3. Fan and pump loads

In various fans, water pumps and oil pumps, with the rotation of the impeller, the resistance generated by air or liquid within a certain speed range is roughly proportional to the power of speed n. As the speed decreases, the torque decreases to the second power of the speed. The power required for this load is proportional to the third power of speed. When the required air volume and flow are reduced, the air volume and flow can be adjusted by frequency converter through speed regulation, which can greatly save electric energy. Because the power required at high speed increases too fast with the speed and is directly proportional to the third power of the speed, the fan and pump load shall not exceed the power frequency.

2、 Selection principle of frequency conversion

1. When selecting the frequency converter, the actual motor current value shall 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 shall be fully considered that the output of the frequency converter contains high-order harmonics, which will deteriorate the power factor and efficiency of the motor. Therefore, the current of the motor increases by 10% and the temperature rise increases by about 20% compared with the power frequency power grid. Therefore, when selecting motor and frequency converter, this situation should be taken into account and appropriate allowance should be reserved to prevent excessive temperature rise from affecting the service life of motor.

2. If the frequency converter wants to run with a long cable, measures should be taken to suppress the influence of the long cable on the ground coupling capacitance and avoid insufficient output of the frequency converter. Therefore, the frequency converter should be selected by amplifying the first gear or install an output reactor at the output end of the frequency converter.

3. For some special applications, such as high ambient temperature, high switching frequency (especially in the application places with high noise restrictions such as building automation), high altitude, etc., the capacity of the frequency converter will be reduced, and the frequency converter needs to be amplified for one gear selection.

4. When the frequency converter is used to control several motors in parallel, it must be considered that the total length of the motor cable is within the allowable range of the frequency converter. If the specified value is exceeded, the frequency converter shall be selected by amplifying the first gear or the second gear. 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 on each motor to realize the protection.

5. For some special applications, such as high ambient temperature, high switching frequency, high altitude, etc., the capacity of the frequency converter will be reduced, and the frequency converter needs to enlarge the first gear selection.

6. When using frequency converter to control high-speed motor, due to the small reactance of high-speed motor, high-order harmonic also increases the output current value. Therefore, when selecting the frequency converter for high-speed motor, it should be slightly larger than that of ordinary motor.

7. When the frequency converter is used for pole changing motor, full attention shall be paid to selecting the capacity of the frequency converter to make its maximum rated current below the rated output current of the frequency converter. In addition, when pole number conversion is carried out during operation, the motor shall be stopped first, otherwise the motor will idle and the frequency converter will be damaged in bad conditions.

8. When driving explosion-proof motor, the frequency converter has no explosion-proof structure, and the frequency converter should be set outside the dangerous place.

9. When the frequency converter is used to drive the gear reduction motor, the application range is restricted by the lubrication mode of the gear rotating part. When the lubricating oil is lubricated, there is no limit in the low speed range; In the high speed range above the rated speed, there may be a risk of running out of lubricating oil. Therefore, do not exceed the allowable maximum speed.

10. When the frequency converter drives the wound rotor asynchronous motor, most of them use the existing motor. The winding impedance of wound motor is smaller than that of ordinary squirrel cage motor. Therefore, over-current tripping due to ripple current is easy to occur, so the frequency converter with slightly larger capacity should be selected. Generally, wound motors are mostly used in occasions with large flywheel torque gD2, and more attention should be paid to setting acceleration and deceleration time.

11. When the frequency converter 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 frequency converter is greater than the product of the rated current of the synchronous motor and the standard value of the synchronous pull in current.

12. For loads with large torque fluctuation such as compressors and vibrators and peak loads such as oil pumps, if the frequency converter 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, understand the power frequency operation and select the frequency converter with 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 greater than that of the normal motor, when selecting the frequency converter, its rated current shall be greater than that of the submersible pump motor.

13. When the frequency converter controls the roots blower, due to its large starting current, attention must be paid to whether the capacity of the frequency converter is large enough when selecting the frequency converter.

14. When selecting the frequency converter, be sure to pay attention to whether its protection grade matches the situation on site. Otherwise, the dust and water vapor on site will affect the long-term operation of the frequency converter.

15. Single phase motor is not suitable for frequency converter drive.

16. When the motor load is very light, even if the motor load current is within the rated current of the frequency converter, the frequency converter with much smaller capacity than the motor cannot be used. This is because the reactance of the motor varies with the capacity of the motor. Even if the motor load is the same, the larger the motor capacity is, the greater the pulsating current value is, so it may exceed the allowable current value of the frequency converter.

Frequency Conversion Load Classification, Frequency Conversion Type Selection Principle 1

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