First, Constant Torque Load
In this load, the load torque (TL) is independent of the speed (n), and it remains constant or nearly constant at any speed. The load power, however, increases linearly with the increase in load speed. This includes conveyors, mixers, extruders, and feed mechanisms for machinery and equipment.
Frictional loads, as well as gravity loads from cranes, hoists, and elevators, are all constant-torque loads. When a variable-frequency power supply drives a constant-torque load, it requires a sufficiently high torque at low speeds and must have adequate overload capacity. If long-term stable operation at low speeds is necessary, consider the cooling capability of standard squirrel-cage induction motors to prevent excessive motor temperature rise.
Two, Constant Power Load
The characteristic of this type of load is that the required torque TL and the speed n are approximately inversely proportional, but their product, the power, remains nearly constant. Spindles of metal cutting machine tools, rolling mills, paper machines, winding machines on film production lines, and unwinders all belong to constant power loads. The constant power nature of the load should be considered within a certain speed range. When the speed is very low, limited by mechanical strength, TL cannot increase indefinitely, and it transitions to a constant torque nature at low speeds. The constant power and constant torque regions of the load greatly influence the selection of transmission schemes. During constant flux linkage speed regulation of the motor, the output torque remains constant, which is known as constant torque speed regulation; whereas, during weak flux linkage speed regulation, the output torque is inversely proportional to the speed, which is known as constant power speed regulation. If the constant torque and constant power speed regulation ranges of the motor are consistent with the constant torque and constant power ranges of the load, which is referred to as "matching," the capacity of the motor and the inverter's capacity are both reduced.
Section 3: Fluid-Type Loads
The torque of this type of load is proportional to the square of the speed, and the power is proportional to the cube of the speed. Various fans, pumps, and oil pumps all belong to typical fluid loads. Fluid loads can adjust air volume and flow rate through variable-frequency power supplies, leading to significant energy savings. Due to the rapid increase in required power for fluid loads at high speeds, which is proportional to the cube of the load speed, these loads should not be operated beyond the rated frequency.
