Speed Control of DC Shunt Motor

V_dc  the output of rectifying circuit which is  applied to the motor field  as shown  in figure  for armature voltage V_dc  is controlled by SCR  which  is series connected to armature winding of dc motor. FD  is freewheeling diode connected across the armature winding . Daic is  used  as firing  circuit  of SCR  which is  trigged  through  capacitor C.

Initially this  discharging capacitor C state charging  through  armature winding diode  D6 and variable  resistance R₁  for each half cycle. When  this  capacitor voltage  is more  than the forward  break over  voltage  of diac VBO it gives  triggering  pulse  to the SCR  and SCR  gets  turned on which  completes  the armature  circuit.

At the end of the half  cycle capacitor C discharges through  diode D₅ resistance R₂  and field windings. At  this time  SCR  turns  off and FD provides a discharge path for  the energy stored in the inductance of the are armature winding. The  output  waveform  across armature of dc  shunt motor is  shown in figure  where  α is firing  angle and β is conducting angle.

When load is  fluctuating  the speed of  motor  fluctuates accordingly. For example  if the load  is increased motor  speed  goes  slow  down  which  will be  responsible to reduce the back emf E_b. this reduced back  emf increase the  voltage  across SCR  hence capacitor  charges more rapidly to turn on the SCR on somewhat earlier than the increase in load takes place. As a result the average dc voltage across the armature will change and speed of motor can control  accordingly. The controlled output  voltage is given by.