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CIRCUITS and SYSTEMS – part I

CIRCUITS and SYSTEMS – part I. Prof. dr hab. Stanisław Osowski. Electrical Engineering (B.Sc.) Projekt współfinansowany przez Unię Europejską w ramach Europejskiego Funduszu Społecznego . Publikacja dystrybuowana jest bezpłatnie. Lecture 2.

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CIRCUITS and SYSTEMS – part I

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  1. CIRCUITS and SYSTEMS – part I Prof. dr hab. Stanisław Osowski Electrical Engineering (B.Sc.) Projekt współfinansowany przez Unię Europejską w ramach Europejskiego Funduszu Społecznego. Publikacja dystrybuowana jest bezpłatnie

  2. Lecture 2 Analysis of circuits in steady state at sinusoidal excitation

  3. Sinusoidal signal u(t) - instantaneous value of signal Um - maximum value (magnitude) of signal -initial phase (phase corresponding to t=0)  t+-phase angle at time t f=1/T - frequency in Hz T - period of sinusoidal signal -angular frequency measured in radians per second

  4. RMS value of signal For sinusoidal signal • voltage • current

  5. Steady state of the circuit Steady state of the circuit is the state in which the character of the circuit response is the same as the excitation. It means that at sinusidal excitation the response is also sinusidal of the same frequency. For the need of steady state analysis we introduce the so symbolic method of complex numbers. This method converts all differential and integral equations into algebraic equations of complex character.

  6. Symbolic method for RLC circuit The RLC circuit under analysis The circuit equation in time domain

  7. General solution of circuit The general solution of the circuit in time domain is composed of two components: x(t)=xs(t)+xt(t) • Steady state component – part xs(t) of general solution for which the signal has the same character as excitation (at sinusidal excitation the response is also sinusidal of the same frequency). This state is theoretically achieved after intinite time (in practice this time is finite). • Transient component - part xt(t) of general solution for which the signal may take different form from excitation (for example at DC excitation it may be sinusoidal or exponential). The general solution is just the sum of these two parts x(t)=xs(t)+xt(t)

  8. Solution in steady state Symbolicrepresenation of voltageexcitation Symbolic represenation of current response Symbolicequation of circuit

  9. Solution in steady state (cont.) After performing the appropriate manipulations we get The complex RMS notations of current and voltage The complex RMS equation of the circuit

  10. Complex represenation of the RLC elements Resistor Inductor Capacitor

  11. Complex impedances • Reactance of inductor • Reactance of capacitor • Impedance of inductor • Impedance of capacitor

  12. Final solution of RLC circuit • Complex algebraic equation of RLC circuit • Complex current • Magnitude RMS value of current • Phase of current

  13. Kirchhoff’s laws for complex representation • KCL • KVL • Ohm’s complex law Y=1/Z - complex admittance

  14. Symbolic method - summary • Conversion: time-complex representation of sources • Complexrepresenattion of RLC elements • Kirchhoff’slaws for complexvalues • Solution of complexequations -> complexcurrents & voltages.

  15. Example Determine the currents in in steady state of the circuit at the following values of parameters: R=10Ω,C=0,0001F, L=5mH, i(t)=7.07sin(1000t) A. Circuit structure

  16. Solution Complex symbolic values of parameters: ω = 1000 I = 5ej0 = 5 ZL = jωL = j5 ZC = -j/(ωC) = -j10 Admittance and impedance of the circuit

  17. Solution (cont.) Voltage and currents

  18. Solution (cont.) Time representation of the signals

  19. Phasor diagram for resistor Equation

  20. Phasor diagram for inductor Equation

  21. Phasor diagram for capacitor Equation

  22. Phasor diagram for RLC circuit • The construction starts from the farest branch from the source. For series connected elements of this branch start from current; for parallel connected elements start from voltage. Next we draw alternatingly the currents and voltages for the succeeding branches, approaching in this way the source. • The relation of the input voltage towards the input current determines the reactive character of the circuit. • If the input voltage leads its current the character is inductive. • If (opposite) the input voltage lags its current the character of the circuit is capacitive. • When the voltage is in phase with current – the circuit is of resistive character.

  23. Example Draw the phasor diagram for the circuit RLC circuit structure

  24. Construction of phasor diagram

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