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Controlled-Rectifier Fed Drive Armature Field ECE 442 Power Electronics 1 Chopper-Fed Drive Armature Field ECE 442 Power Electronics 2 Equivalent Circuit of a Separately-Excited DC Motor ECE 442 Power Electronics 3 v f Rf i f Lf di f dt di va Ra ia La a eg dt eg K vi f Td K t i f ia d Td J B TL dt ECE 442 Power Electronics 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 ECE 442 Power Electronics 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 ECE 442 Power Electronics 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 ECE 442 Power Electronics 7 Magnetization Characteristic ECE 442 Power Electronics 8 Characteristics of Separately-Excited Motors Use armature voltage control Use field-current control Rated speed ECE 442 Power Electronics 9 Equivalent circuit of a DC Series Motor ECE 442 Power Electronics 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 ECE 442 Power Electronics 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 ECE 442 Power Electronics 12 Characteristics of DC Series motors ECE 442 Power Electronics 13 Operating Modes • In variable-speed applications, a dc motor may be operating in one or more of the following “Modes” • • • • Motoring Regenerative braking Dynamic braking Plugging ECE 442 Power Electronics 14 Motoring Mode “Back emf”, Eg is < supply voltage Va Both Ia and If are positive Developed torque meets load demand ECE 442 Power Electronics 15 Regenerative Braking Mode Motor acts as a Generator Eg becomes > supply Voltage Va Ia becomes negative Kinetic energy of the motor is returned to the supply ECE 442 Power Electronics 16 Dynamic Braking Replace the supply voltage with a resistor Power dissipated in the resistor rather than given back to the source ECE 442 Power Electronics 17 Plugging Reverse the armature terminals while running Va and Eg act in the same direction Ia is reversed, producing “braking” torque ECE 442 Power Electronics 18 Four-Quadrant Operation ECE 442 Power Electronics 19 Single-Phase DC Drive Change Va by changing firing angle Inductor Lm is a “smoothing” Inductor to prevent discontinuous current ECE 442 Power Electronics 20 Armature Reversal ECE 442 Power Electronics 21 Field Reversal ECE 442 Power Electronics 22 Single-Phase Half-Wave Converter Drive Vm (1 cos a ) 2 0 a Va Vf Vm (1 cos f ) 0 f ECE 442 Power Electronics 23 Quadrant of Operation ECE 442 Power Electronics 24 Waveform Summary ECE 442 Power Electronics 25 Single-Phase Semiconverter Drives Va Vm (1 cos a ) 0 a Vf Vm (1 cos f ) 0 f ECE 442 Power Electronics 26 Quadrant(s) of Operation Semiconverter – one quadrant converter One polarity of output voltage and current ECE 442 Power Electronics 27 Waveform Summary ECE 442 Power Electronics 28 Single-Phase Full-Converter Drives ECE 442 Power Electronics 29 Quadrant(s) of Operation Full converter – two quadrant converter Output voltage polarity can be positive or negative Output current has one polarity ECE 442 Power Electronics 30 Waveform Summary ECE 442 Power Electronics 31 Single-Phase Dual-Converter Drives Dual converter can operate in all four quadrants Output voltage and current can be positive or negative ECE 442 Power Electronics 32