Characteristics Of D.C. Shunt Motor

i) Torque – Armature current characteristics
For a d.c. motor                 T α  Φ I_a

For a constant values of R_sh and supply voltage V, I_sh is also constant and hence flux is also constant.
.^..              T_a  α Φ I_a 

The equation represents a straight line, passing through the origin, as shown in the Fig. 1. Torque increases linearly with armature current. It is seen earlier that armature current is decided by the load. So as load increases, armature current increases, increasing the torque developed linearly.

Fig. 1  T Vs Ia for shunt motor

Now if shaft torque is plotted against armature current, it is known that shaft torque is less than the armature torque and the difference between the two is loss torque T_f  as shown. On no load T_sh = 0 but armature torque is present which is just enough to overcome stray losses shown as T_a0. The current required is I_a0 on no load to produce T_a0 and hence T_sh graph has an intercept of I_a0 on the current axis.
To generate high starting torque, this type of motor requires a large value of armature current at start. This may damage the motor hence d.c. shunt motors can develop moderate starting torque and hence suitable for such applications where starting torque requirement is moderate.

ii) Speed – Armature current characteristics  

From the speed equation we get,
N α (V – I_a  Ra)/Φ
  αV – I_a  Ra     as Φ is constant

So as load increases, the armature current increases and hence drop I_a  Ra also increases.

Hence for constant supply voltage, V – I_a  Ra decreases and hence speed reduces. But as  Ra is very small, for change in I_a from no load to full load, drop I_a Ra is very small and hence drop in speed is also not significant from no load to full load.

So the characteristics is slightly droping as shown in the Fig. 2.

But for all practical purposes these type of motors are considered to be a constant speed motors.

iii) Speed – Torque characteristics

These characteristics can be derived from the above two characteristics. This graph is similar to speed-armature current characteristics as torque is proportional to the armature current. This curve shows that the speed almost remains constant through torque changes from no load to full load conditions. This is shown in the Fig. 3.