Constant torque characteristic of industrial drive

Most of the working machines that have mechanical nature of work like shaping, cutting, grinding or shearing, require constant torque irrespective of speed. Similarly, cranes during hoisting and conveyers handling constant weight of material per unit time also exhibit this type of characteristic. The speed-torque characteristic of this type of load is given by T=K and is shown in the Figure.

Industrial Drive > Torque proportional to speed (Generator type load):

Separately excited dc generators connected to a constant resistance load, eddy current brakes, and calendering machines have a speed-torque characteristic given by T=Kω. This type of characteristic is shown in Figure.

Industrial Drive > Torque proportional to square of the speed (Fan type load):

Another type of load met in practice is the one in which load torque is proportional to the square, of the speed. The typical examples are : fans, rotary pumps, compressors and ship propellers. The speed-torque characteristic of this type of load is given by T=Kω² and is shown in Figure.

Industrial Drive > Torque inversely proportional to speed (constant power type load):

Certain types of lathes, boring machines, milling machines, steel mill coiler, and electric traction load exhibit hyperbolic speed- torque characteristic. In such loads the torque is inversely proportional to speed or the load power remains constant. This type of characteristic is given by T=K/ω and shown in Figure.



Most of the loads require extra effort at the time of starting to overcome static friction. In power application it is known as breakaway torque and the control engineers call it 'stiction'. Because of stiction, the speed-torque characteristics of the loads are modified near the zero speed. For a constant torque type of load, the characteristic as modified due to stiction is shown in Figure.