Choosing the right frequency inverter is crucial for the normal operation of a control system. When selecting a frequency inverter, it is necessary to fully understand the load characteristics driven by the frequency inverter. In practice, people often categorize production machinery into three types: constant torque load, constant power load, and fan/pump load.
Constant Torque Load
The load torque TL is independent of the speed n, and TL remains constant or essentially constant at any speed. Examples of constant torque loads include conveyors, mixers, extruders, as well as cranes and elevators, which are potential energy loads. When a frequency inverter drives a constant torque load, the torque at low speeds needs to be sufficient and have enough overload capacity. If stable operation at low speeds is required, the heat dissipation capacity of the standard asynchronous motor should be considered to avoid excessive temperature rise.
Constant Power Load
The torque required in the main spindle of a machine tool, rolling mill, paper machine, or in a winding or unwinding machine in a plastic film production line is generally inversely proportional to the speed. This is known as a constant power load. The constant power nature of the load should be considered within a certain range of speed variations. When the speed is very low, TL cannot increase indefinitely due to mechanical strength limitations and transitions to a constant torque nature at low speeds. If the range of constant torque and constant power adjustment of the motor matches that of the load, also known as “matching,” both the motor capacity and frequency inverter capacity are minimized.
Fan and Pump Loads
In various fans, pumps, and oil pumps, the resistance generated by air or liquid within a certain speed range is roughly proportional to the square of the speed n. As the speed decreases, the torque decreases according to the square of the speed. The power required for this type of load is proportional to the cube of the speed. When the required airflow or flow rate decreases, energy can be significantly saved by using a frequency inverter to adjust the airflow or flow rate through speed control. Because the required power at high speeds increases too quickly with the speed, proportional to the cube of the speed, it is generally not recommended to exceed the line frequency operation for fan and pump loads.
Post time: Jul-31-2023