Characteristics of DC motor

Characteristics of DC motor
The characteristics curve of a motor are those curves which show relation ships between the following quantities.
(1) Torque and armature current (Ta/Ia) characteristics
(2) Speed and armature current (N/Ia) characteristics
(3) Speed and torque (N/Ta)


Characteristics of series motor:
(1) Ta/Ia characteristics:
In series motor field winding also carry the armature current.Therefore Ia is proportional to flux upto the point of magnetic saturation.
At the decreasing load Ia and flux is small.But as Ia is increase.Ta decrease as the square of the current.Hence Ta/Ia curve is a parabola. After saturation flux is almost independ of Ia.Hence Ta is directly proportional to Ia only so tje characteristics becomes straight lines.The shaft torque (Tsh) is less than armature torque due to stray loss.After saturation flux is constant over the Ia is increases.

(2) N/Ia characteristics
When load is heavy Ia is large.Hence speed is low this decrease Eb.Hence increasing Ia decrease Eb.But small Ia speed varies inversely as armature current.DC series motor has higher starting torque and low speed so it is useful for traction purpose like trolly bus, electirc train, roapway, machine.

(3) N/Ta characteristics
When torque is increased,armature has the supply more,current and Ia=If and hence flux increases but Eb decrease resulting low value of speed.When the torque is decrease or no load or light load condition the armature or field draws very low value of current and flux will be very low.So the speed is dangerously high.Therefore it is said that dc series motor should never be start on without load.