Constant HP and Constant Torque operation of DC drive....




Mechanical Load connected to the
motor requires certain horsepower at certain speed. Let us say, this is as
shown by the black graph in the sketch below. Point to be noted here is that
the horsepower requirement of the load does not go up after a certain speed ,
Nf in the graph. However, the selection of the motor must take care of the
maximum horsepower demanded by the load. Therefore, ordinarily, we would have
chosen the horsepower of the motor as that equal to K1. But considering the fact that
the load does not require more horsepower after the speed Nf, we can select a
lower horsepower motor ( K2 ) and do field weakening at speed Nf. Reducing
the field voltage of the dc motor after Nf, reduces the torque which can be
generated by the motor. But as the speed is more the product of speed and
torque remains constant. This is CONSTANT HORSEPOWER region of the operation
of the motor. The zone between zero speed and
speed Nf, the horsepower generated by the motor goes on increasing. However,
the "maximum torque available" is constant. This zone of operation
is called CONSTANT TORQUE region. It is to be noted that the torque generated
by the motor is not constant but the maximum torque available
from the motor is constant. 



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Speed control not satisfactory











