Speed Control of DC Shunt Motor
Vdc the output of rectifying circuit which is applied to the motor field as shown in figure for armature voltage Vdc 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 Eb. 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.