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.









Download PDF version of these pages.





Block Diagram



Ramp Circuit


Speed Amplifier


Current Amplifier


 Firing Circuit


Power Circuit


Types of DC drives 



 Checking Thyristor


Voltage feedback Vs Tacho feedback


Current Limit


Constant HP & constant Torque


Field Weakening


Using CRO 



Motor runs at full speed


Fuses Blow


Hunting in speed


Motor gives jerks


Belts vibrate


Speed control not satisfactory 



Speed drops on load


Motor overheats


Sparking on commutator


DC drives and Power Factor


Your comments and suggestions 


Drive stops all of a sudden 


Motor not able to drive the load


DC to DC isolation


 Motor or Drive?