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What is the difference between Variable Torque and Constant Torque for starter ratings?

Many types of loads require reduced torque when driven at speeds less than the base speed of the load. Conversely, such loads may require increased torque when driven at speeds greater than the base speed of the load. These are classified as variable torque loads. Many variable torque loads decrease with the square of the speed. This is characteristic of centrifugal pumps, and certain types of fans and blowers. Typically, as the speed decreases, the torque decreases with the square of the speed and the horsepower decreases with the cube of the speed.

With constant torque loads, the torque loading is not a function of speed. Typical applications are: Traction drives, Conveyors, Positive displacement pumps and Hoists . As the speed changes, the load torque remains constant and the horsepower changes linearly with speed. Constant torque loads cause motors to draw relatively high current at low speeds when compared to variable torque applications. This is why the same size device may have a lower HP rating for Constant Torque applications.

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