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 Vc is equal to the addition of back emf Eb of motor and break over voltage VBO of the diac. By varying the resistance R2 the charging time of capacitor (r= R2C) 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 ( VBO + EB) may increase or decrease as EB 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.