The automatic speed control of a series motor to a constant is done by automatic adjustment of triggering angle at various loads using feedback signal which is proportional to the actual speed of the motor. The feedback signal can be obtained using tachogenerator or from the back emf produced across the armature.
It can be run both DC and AC input, speed control of a universal motor can also possible using this circuit.
Capacitor ‘C’ charges through Resistance ‘R1' and variable resistance ‘Rv’ during +ve half cycle. When capacitor voltage become ‘Vc’ (equal to Break over voltage of diac ‘VBO + back emf ‘Eb’ of the armature),SCR gets triggered. Now the motor current flows through the series field, conducting SCR, and armature of the motor. The speed of the motor can be controlled by varying the charging time of the capacitor and which can be done by varying variable resistance ‘Rv’. When the SCR is not conducting the armature current is due to residual magnetism and is approximately proportional to the speed. Diode D is used to avoid –ve excursions of capacitor voltage.If the load increased, it will tends to slow down the motor. Reduced speed results reduction and lower value of back emf ‘Eb’.The increased difference between ‘Eb’ and ‘Vc’ will trigger the SCR earlier than before in next positive half cycle. ‘VBO’is constant and back emf ‘Eb’ decreased, the capacitor takes less time to reach ‘VBO+Eb’,there for SCR triggers earlier. As a result the voltage across the armature increased and motor retain the preset value speed. If the speed rises because of some decreasing of load,the triggering angle will automatically advance and bring down the speed to preset value.
In series motor drive, the current flows through the armature and field when the SCR is triggered at an angle ‘alpha’.The current continues beyond ‘omega*t=pai’until the stored energy of the armature is dissipated. The armature voltage is –ve for some time beyond ‘omega*t=pai’.
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