The power control equipment that controls the AC motor by changing the frequency of the motor's working power supply is called a frequency converter. The correct selection of the frequency converter also plays an important role in the normal operation of the control system. When selecting an inverter, it is necessary to 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. So how to choose a frequency converter? Here, MICNO will share with you the professional knowledge of electrical equipment selection.
The load torque TL has nothing to do with the speed n, and TL keeps stable at any speed. For example, conveyor belts, mixers, kneaders and other interference loads, cranes, trekking machines and other potential energy loads are all classified as constant torque loads. When the frequency converter drives a load with constant torque, the torque at low speed must be large enough, and there is enough overload before adjustment. Assuming that it is necessary to work at a steady speed at a low speed, we should consider standardizing the heat dissipation performance of the motor to prevent the temperature rise of the motor from being too high.
The torque required by the machine tool spindle, rolling mill, paper machine, coiler, and decoiler in the plastic film production line is roughly inversely proportional to the speed, which is called constant power load. The constant power characteristic of the load should be determined according to a certain speed change range. When the speed is very low, TL cannot increase indefinitely due to the limitation of mechanical strength, and becomes constant torque at low speed. The constant power zone and constant torque zone of the load have a great influence on the selection of the transmission scheme. When the motor is in constant flux speed regulation, the maximum allowable output torque remains unchanged, which belongs to constant torque speed regulation; but in the field weakening speed regulation, the maximum allowable output torque is inversely proportional to the speed, which belongs to constant power speed regulation. If the motor's constant torque and constant power speed regulation range is consistent with the load's constant torque and constant power range, that is, in the so-called "matching" situation, the capacity of the motor and the frequency converter is the smallest.
In various fans, water pumps, and oil pumps, with the rotation of the impeller, the resistance of air or liquid within a certain speed range is roughly proportional to the second power of the speed n. As the speed decreases, the speed decreases according to the second power of the speed. The power required by this load is proportional to the third power of the speed. When the required air volume and flow are reduced, the frequency converter can be used to adjust the air volume and flow through speed regulation, which can greatly save electric energy. Since the required power grows too fast with the speed, it is proportional to the third power of the speed, so it is usually not allowed to make the fan and pump load exceed the power frequency.