CHARACTERISTICS OF DC MOTORS

1. Torque / Armature current characteristics

2. Speed / Armature current characteristics

3. Speed / Torque characteristics

 Characteristics of dc shunt motor

Torque/Armature current characteristics.  

• The torque developed by the dc motor T Ia

• In case of dc shunt motors the field excitation current is constant and supply voltage is kept constant. Therefore flux per pole will be constant.

Therefore torque developed in a dc shunt motor will be directly proportional to the armature current. The graph representing the variation of torque with armature current.


Speed/Armature current characteristics

•    The speeds of the dc shunt motor decreases with increase in armature current due to loading.

•    The graph representing variation of speed with armature current is drooping slightly.

•    The drop is speed from no load to full load will be about 3 to 6 percent.

•    But the armature reaction effect weakens the field on load and tends to oppose drop in speed so that the rarely drops by more than about 5 percent from no load to full load.

•    Therefore shunt motor is considered as constant speed motor.

Speed/Torque characteristics

•    From the above two characteristics of dc shunt motor, the torque developed and speed at various armature currents of dc shunt motor may be noted.

•    If these values are plotted, the graph representing the variation of speed with torque developed is obtained.


This curve resembles the speed Vs current characteristics as the torque is directly proportional to the armature current.