A. Constant Torque and Variable Speed Loads Characteristic
The torque required by this type of load is constant regardless of the speed. In contrast, the power is linearly proportional to the speed. Equipment, such as screw compressors, conveyors and feeders, has this type of chacteristic. Fig. 2 shows constant torque and variable speed loads characteristic of induction motor.
B. Variable Torque and Variable Speed Loads Characteristic
This is most commonly found in the industry and sometimes is known as a quadratic torque load. The torque is the square of the speed, while the power is the cube of the speed. This is the typical torque-speed characteristic of a fan or a pump. Fig. 3 shows variable torque and variable speed loads characteristic of induction motor.
C. Constant Power Loads Characteristic
This type of load is rare but is sometimes found in the industry. The power remains constant while the torque varies. The torque is inversely proportional to the speed, which theoretically means infinite torque at zero speed and zero torque at infinite speed. In practice, there is always a finite value to the breakaway torque required.
This type of load is characteristic of the traction drives, which require high torque at low speeds for the initial acceleration and then a much reduced torque when at running speed. Fig. 4 shows constant power loads characteristic of induction motor.
D. Constant Power and Constant Torque Loads Characteristic
This is common in the paper industry. In this type of load, as speed increases, the torque is constant with the power linearly increasing.
When the torque starts to no constant torque loads character decrease, the power then remains constant. Fig. 5 shows constant power load characteristic of induction motor.