Speed Control of DC Series Motor

A half  wave  circuit  for the  speed  control of DC  series motor is  shown in figure and corresponding wave from  is shown  in figure  consists  diac  and diode  to give the  triggering pulses to the SCR. Here SCR  is used to  control the armature voltage  of the DC motor.  By varying R₂ firing angle α can be  controlled  of SCR. During the positive  half  cycle of input  ac voltage capacitor C will through R₁ and R₂.

SCR will get  turned  on when the  capacitor voltage V_c is equal  to the addition  of back emf E_b  of motor  and break over voltage VBO of the diac. By  varying  the resistance R₂ the charging  time of  capacitor (r= R_2C)  hence the speed of motor can be controlled. Basically it is  the armature voltage control method. In case  when SCR  is not conducting the  armature voltage will be there due to  the residual flux which  is approximate proportional  to the speed .

Due to  the fluctuation  in load will results the  change  in speed of motor and according  back emf of the motor will change. Since  the break over voltage of  diac is constant and back emf is varying  means the difference of Vc and ( V_BO + E_B) may increase or decrease as E_B is increased or decreased respectively. The  firing  of SCR  depends on this difference. As  a results  the average dc voltage  across  the armature will change  and speed  of motor  can control  accordingly.