People often divide production machinery into three types according to different load torque characteristics in practice: constant torque load, constant power load and fluid load.
1. Constant torque load
In this load, the load torque TL has nothing to do with the speed n, TL always remains constant or basically constant at any speed, and the load power increases linearly as the load speed increases. Feeding mechanisms for conveyor belts, mixers, extruders and mechanical equipment, etc.
Friction loads and gravity loads such as cranes, hoists, elevators, etc. are all constant torque loads. When the variable frequency power supply drives a load with constant torque, the output torque must be large enough at low speed, and there must be enough overload capacity. If you need to run at a low speed for a long time at a steady speed, you should consider the heat dissipation capacity of the standard cage asynchronous motor to avoid excessive temperature rise of the motor.
2. Constant power load
The characteristic of this type of load is that the required torque TL is roughly inversely proportional to the speed n, but its product, the power, remains approximately unchanged. The main shaft of the metal cutting machine tool, the rolling mill, the paper machine, and the coiler and decoiler in the film production line are all constant power loads. The constant power nature of the load should be based on a certain speed change range. When the speed is very low, limited by the mechanical strength, TL cannot increase indefinitely, and it becomes a constant torque property at low speed. The constant power area and constant torque area 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; while in field weakening speed regulation, the maximum allowable output torque is inversely proportional to the speed, which belongs to constant power speed regulation. If the constant torque and constant power speed regulation range of the motor is relatively consistent with the constant torque and constant power range of the load, that is, in the case of "matching", the capacity of the motor and the capacity of the variable frequency power supply are both the smallest.
3. Fluid load
The torque of this type of load is proportional to the second power of the speed, and the power is also proportional to the third power of the speed. All kinds of fans, water pumps and oil pumps are all typical fluid loads. The fluid load adjusts the air volume and flow rate through the variable frequency power supply speed regulation, which can greatly save electric energy. Since the demand power of fluid loads at high speeds increases too fast, which is proportional to the third power of the load speed, this type of load should not be operated at over power frequency.
