DC Shunt Motor Characteristics

Dc motor characteristics are useful for deciding which dc motor to be used for which application.

By looking at various dc motor characteristics, you can get rough idea about the choice of the particular type of dc motor for a given application. Though, exact shape of the characteristics will depend on motor construction.

DC Shunt Motor Characteristics

The following equations will be useful in determining the characterisitcs. In all the equations flux Φ is assumed constant since demagnetizing effect of armature reaction is negligible at smaller armature currents. 

Characteristics are drawn in black ink when armature reaction is neglected and in red ink when armature reaction is taken into account.

DC Shunt Motor Speed-Current (N-I_a) Characteristics 

Putting  (2)  in (1), we get

Compairing it with standard equation of a straight line,    Y = mx+C

We find that the characteristic is a straight line with intercept on speed axis equal to  V_t/K_{1 ,}  called no-load speed i.e when I_a is negligibly small just to provide torque to overcome friction and windage.     

             

The characteristic show that dc shunt motor speed remains almost constant from no-load ( I_a≃ 0) to full load ( I_a=rated value) and that is why dc shunt motor is used in applications where speed is required to remain almost constant throughout the operation e.g Lathes, spinning and weaving machines, drills etc.

If armature reaction is taken into account, its demagnetizing effect will be negligible at small  I_a but for larger I_a net flux per pole is reduced and therefore  K₁ decreases and hence speed drops slowly, represented by the red curve.

DC Shunt Motor Torque-Current (T_a-I_a) Characteristics 

From (3)

T_a = K₃ I_a       

Therefore, in a dc shunt motor, the torque increases linearly with armature current as shown by the black curve. But, for larger I_a , the net flux per pole decreases due to the demagnetizing effect of armature reaction and hence the curve deviates from straight line as shown by red curve.

Unlike dc series motor, it has very low starting torque.

DC Shunt Motor Speed-Torque (N-T_a) Characteristics

Putting  (3)  in (4), we get 

From this equation speed-torque characteristics of dc shunt motor can be drawn which turns out to be a straight line shown by black curve.

When armature reaction is also considered i.e flux is no longer constant , the speed-torque equation becomes

Obviously, due to armature reaction, as flux decreases the first term increases more than the second term , therefore the speed drops more rapidly with increase in torque.