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Controlled-Rectifier Fed Drive Armature Field 1 Chopper-Fed Drive Armature Field 2 Equivalent Circuit of a Separately-Excited DC Motor 3 v f Rf i f Lf di f dt dia va Ra ia La eg dt eg K vi f Td K t i f ia d Td J B TL dt 4 Steady state d 0 dt Vf Rf I f E g K v I f Va Ra I a Eg Va Ra I a K v I f Td K t I f I a Td B TL Pd Td 5 Solve for the motor speed Va Ra I a K v I f Va I a Ra Va I a Ra Kv I f Vf Kv R f 6 Control of the motor speed • Control armature voltage, Va – Voltage control • Control the field current, If – Current control • Control the armature current, Ia Va I a Ra Vf Kv R f 7 8 9 Equivalent circuit of a DC Series Motor 10 Eg K v I a Va ( Ra R f ) I a Eg Va ( Ra R f ) I a K v I f Td Kt I a I f Td B TL Va ( Ra R f ) I a Kv I f 11 a a f a g V ( R R ) I K I a a f a v Controlling the Motor Speed • Control the armature voltage, Va Td Kt I a I f Td B TL Va ( Ra R f ) I a Kv I f • Control the armature current, Ia 12 13 Motoring Mode “Back emf”, Eg is < supply voltage Va Both Ia and If are positive 14 Regenerative Braking Mode Field Terminals Reversed Motor acts as a Generator Eg becomes > supply Voltage Va Ia becomes negative 15 Dynamic Braking Replace the supply voltage with a resistor Power dissipated in the resistor rather than given back to the source 16 Plugging Reverse the armature terminals while running Va and Eg act in the same direction Ia is reversed, producing “braking” torque 17