The basic mode of electric motor operation is constant-speed drive. For many applications where control precision requirements are not high or speed regulation is not necessary, constant-speed drive can generally meet production requirements. With the development of industrialization, higher demands have been placed on adjustable-speed drives. When using a variable frequency drive (VFD) to drive an electric motor to drive fans, pumps, and other machinery, the energy-saving effect is considerable compared to conventional non-adjustable motor drives. While some single energy-saving technologies applied in the industrial field over the past few decades have generally resulted in energy savings of a few percentage points, the adoption of VFD technology can produce 30% to 40% energy savings in pump and machinery operation, representing a significant leap in energy conservation. 1. Load Theory of VFD Systems In electric drive or electric transmission systems, the electric motor drives various machines, which are the loads of the motor. The nature of the load varies depending on the machine, and the load is also the controlled object. The content of VFD regulation differs for different loads and must be applied according to the characteristics of the controlled object. 1.1 Typical Load Torque Different driven loads require different torque characteristics from the motor. The relationship between torque and speed is often referred to as the load torque characteristic. a. Constant Torque Load: This type of load includes conveyor belts, extruders, etc. The load torque is shown as line ① in Figure 1. b. Square Torque Load: Typical examples of this type of load are pumps and fans. Curve ② shows the torque characteristic of this load. The load torque is proportional to the square of the speed. c. Constant Power Load: Hoists, machine tool spindles, etc., belong to this type of load. Curve ③ represents the torque characteristic of a constant power load. The speed and torque corresponding to point A can be used as the rated values. Point B corresponds to the rated value of the constant power load. [b][align=center]For more details, please click: A Brief Analysis of the Application of Frequency Converters in Thermal Power Plants[/align][/b]