Oct 15, 2020
General inverter selection specification
The correct selection of the inverter for the normal operation of the control system is very important. When selecting an inverter, you must fully understand the characteristics of the load driven by the inverter. People often divide production machinery into three types in practice: constant torque load, constant power load and fan and water pump load. Constant torque load:
The load torque TL has nothing to do with the speed n, and TL always remains constant or almost constant at any speed. For example, frictional loads such as conveyor belts, mixers, extruders, and potential loads such as cranes and hoists are all constant torque loads.
When the inverter drives a constant torque load, the torque at low speed must be large enough to have enough overload capacity. Need to run at a steady speed at a low speed, the heat dissipation capacity of standard asynchronous motors should be considered to avoid excessive motor temperature rise. Constant power load:
The torque required by the machine tool spindle, rolling mill, paper machine, and plastic film production line, such as the coiler and decoiler, is roughly inversely proportional to the speed. This is the so-called constant power load. The constant power nature of the load should be in terms of a certain speed range. When the speed is very low, limited by the mechanical strength, TL cannot increase indefinitely, and it becomes a constant torque at low speed. The load constant power zone and constant torque zone have a great influence on the choice of transmission scheme. When the motor is in constant flux speed regulation, the maximum allowable output torque is unchanged, which belongs to constant torque speed regulation; while in the field weakening speed regulation, the maximum allowable output torque is inversely proportional to the speed, which belongs to constant power speed regulation. When the motor constant torque and constant power speed regulation range is consistent with the load constant torque and constant power range, that is, in the so-called "matching" case, the motor capacity and the inverter capacity are both the smallest. Fan, pump load:
In all kinds of fans, water pumps, and oil pumps, as the impeller rotates, the resistance generated by the air or liquid within a certain speed range is roughly proportional to the speed n2. The speed decreases, and the speed decreases according to the second power of the speed. The power required by this load is proportional to the third power of speed. When the required air volume and flow are reduced, using the inverter speed control method to adjust the air volume and flow can greatly save electric energy. At high speed, the required power increases with the speed too fast, which is proportional to the third power of the speed. Generally, fans and pumps should not be operated at over power frequency. Siemens can provide different types of inverters, and users can choose different types of inverters for their actual process requirements and application occasions. When selecting the inverter, pay attention to the following points:
1. Select the inverter for load characteristics. If the load is a constant torque load, select a constant torque load inverter. If the load is a fan or pump load, select an adapted inverter.
2. When selecting the inverter, the actual motor current value should be used as the basis for selecting 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 frequency converter contains high-order harmonics, which will cause the power factor and efficiency of the motor to deteriorate. Compared with the power supply of the power frequency grid with the frequency converter, the motor current increases by 10% and the temperature rise increases by about 20%. When selecting a motor and a frequency converter, this situation should be considered, and a margin should be properly reserved to prevent excessive temperature rise and affect the service life of the motor.
3. If the inverter needs to run with a long cable, measures should be taken at this time to restrain the influence of the long cable on the coupling capacitor to avoid insufficient output of the inverter. The inverter should be selected by amplifying one gear or an output reactor should be installed at the output end of the inverter.
4. When the inverter is used to control several motors in parallel, it must be considered within the allowable range of the total cable length from the inverter to the motor and the inverter. If it exceeds the specified value, zoom in one or two gears to select the inverter. In addition, in this case, the inverter control mode can only be the V/F control mode, and the inverter cannot protect the motor from overcurrent and overload protection. At this time, each motor needs to be protected by a fuse.
5. Some special applications, such as high ambient temperature, high switching frequency, high altitude, etc., will cause the inverter to degrade at this time, and the inverter needs to be enlarged by one gear.
6. When using a frequency converter to control a high-speed motor, the high-speed motor reactance is small, and higher harmonics also increase the output current value. , When choosing a high-speed motor frequency converter, it should be slightly larger than an ordinary motor frequency converter.
7. When the inverter is used in a pole-changing motor, full attention should be paid to selecting the inverter capacity so that the maximum rated current of the inverter is below the rated output current of the inverter. In addition, when changing the number of poles during operation, the motor should be stopped first, otherwise it will cause the motor to run idly and damage the inverter in severe cases.
8. When driving an explosion-proof motor, the inverter does not have an explosion-proof structure, and the inverter should be installed outside the hazardous area.
9. When using a frequency converter to drive a geared motor, the use range is restricted by the lubrication method of the gear rotating part. When lubricating oil, there is no limit in the low-speed range; in the high-speed range above the rated speed, the risk of running out of lubricant may occur. , Do not exceed the allowable maximum speed.
10. When the frequency converter drives the wound rotor asynchronous motor, the existing motor is mostly used.
Compared with ordinary squirrel cage motors, the winding resistance of the winding motor is small. , Ripple current is likely to cause overcurrent tripping, so you should choose a slightly larger inverter than usual. Generally, winding motors are mostly used in occasions where the flywheel torque GD2 is large, so pay more attention 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 synchronous motor rated current and the synchronous pull-in current.
12. In the case of large torque fluctuation loads such as compressors and vibrating machines and peak loads such as hydraulic pumps, if the rated current or power value of the motor selects the inverter, the overcurrent protection may be activated due to the peak current. , Should understand the power frequency operation, choose the inverter with a rated output current greater than its maximum current. When the frequency converter drives the submersible pump motor, the rated current of the submersible pump motor is larger than the normal rated current of the 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, its starting current is very large. When choosing the frequency converter, you must pay attention to whether the capacity of the frequency converter is large enough.
14. When selecting the inverter, be sure to pay attention to whether its protection level matches the site conditions. Otherwise, dust and moisture on site will affect the long-term operation of the inverter.
15. Single-phase motors are not suitable for inverter drive.