Characteristics of D.C. Series Motor

i) Torque - Armature current Characteristics

       In case of series motor the series field winding is carrying the entire armature current. So flux produced is proportional to the armature current.

.^..                   Φ  α  Ia 

       Hence      T_a  α Φ I_a  α I_a²  

       Thus torque in case of series motor is proportional to the square of the armature current. This relation is parabolic in nature as shown in the Fig. 1.

       As load increases, armature current increases and torque produced increases proportional to the square of the armature current upto a certain limitt.

       As the entire I_a  passes through the series field, there is a property of an electromagnet called saturation, may occur. Saturation means though the current through the winding increases, the flux produced remains constant. Hence after saturation the characteristics take the place of straight line as flux becomes constant, as shown. The difference between T_a  and T_sh is loss torque T_f which is also shown in the Fig. 2.

       At start as T_a    αI_a²   , these types of motors can produce high torque for small amount of armature current hence the series motors are suitable for the applications which demand high starting torque.

ii) Speed - Armature current characteristics

       From the speed equation we get,

        N  α  (E_b/Φ)   )  αV - I_a  R_a - I_a  R_se)/ I_a                  as Φ α I_a    in case of series motor

       Now the values of R_a and R_se are so small that the effect of change in  I_a on speed overrides the effect of change in V - I_a  R_a - I_a  R_se  on the speed.

       Hence in the speed equation, E_{b ≈} Vand can be assumed constant. So speed equation reduced to,

             N α  1/I_a

       So speed-armature current characteristics is rectangular hyperbola type as shown in the Fig. 2.

iii) Speed - Torque characteristics

       In case of series motors,            T α I_a²      and   N α  1/I_a

Hence we can write,      N α  1/√T

       Thus as torque increases when load increases, the speed decreases. On no load, torque is very less and hence speed increases to dangerously high value. Thus the nature of the speed-torque characteristics is similar to the nature of the speed-armature current characteristics.

       The speed-torque characteristics of a series motor is shown in the Fig. 3.

Fig. 3  N  Vs  T for series motor