Presentation on theme: "Alternating Current Circuits"— Presentation transcript:
1 Alternating Current Circuits Chapter 4Alternating Current CircuitsFCI- F. Univ.
2 Chapter 4: 4.6 Power in an AC Circuit 4.7 Resonance in a Series RLC Circuit4.8 The Transformer and Power Transmission4.9 Rectifiers and Filters4-1 AC Sources 4.2 Resistors in an AC Circuit 4.3 Inductors in an AC Circuit 4.4 Capacitors in an AC Circuit 4.5 The RLC Series CircuitFCI- F. Univ.
3 Objecties: The students should be able to: Describe the sinusoidal variation in ac current and voltage, and calculate their effective values.Write and apply equations for calculating the inductive and capacitive reactance for inductors and capacitors in an ac circuit.Describe, with diagrams and equations, the phase relationships for circuits containing resistance, capacitance, and inductance.FCI- F. Univ.
4 Describe the basic operation of a step up and a step-down transformer. Write and apply equations for calculating the impedance, the phase angle, the effective current, the average power, and the resonant frequency for a series ac circuit.Describe the basic operation of a step up and a step-down transformer.Write and apply the transformer equation and determine the efficiency of a transformer.FCI- F. Univ.
5 4-1 AC CircuitsAn AC circuit consists of a combination of circuit elements and a power sourceThe power source provides an alternative voltage, DvNotation NoteLower case symbols will indicate instantaneous valuesCapital letters will indicate fixed valuesFCI- F. Univ.
6 - AC VoltageThe output of an AC power source is sinusoidal and varies with time according to the following equation:Δv = ΔVmax sin ωtΔv is the instantaneous voltageΔVmax is the maximum output voltage of the sourceAlso called the voltage amplitudeω is the angular frequency of the AC voltageFCI- F. Univ.
7 - AC Voltage, cont. The angular frequency is ƒ is the frequency of the sourceT is the period of the sourceThe voltage is positive during one half of the cycle and negative during the other halfFCI- F. Univ.
8 - AC Voltage, finalThe current in any circuit driven by an AC source is an alternating current that varies sinusoidally with timeCommercial electric power plants in the US use a frequency of 60 HzThis corresponds with an angular frequency of 377 rad/sFCI- F. Univ.
9 4-2 Resistors in an AC Circuit Consider a circuit consisting of an AC source and a resistorThe AC source is symbolized byΔvR = Vmax sin wtΔvR is the instantaneous voltage across the resistorFCI- F. Univ.
10 Resistors in an AC Circuit, 2 The instantaneous current in the resistor isThe instantaneous voltage across the resistor is also given asΔvR = Imax R sin ωtFCI- F. Univ.
11 Resistors in an AC Circuit, 3 The graph shows the current through and the voltage across the resistorThe current and the voltage reach their maximum values at the same timeThe current and the voltage are said to be in phaseFCI- F. Univ.
12 Resistors in an AC Circuit, 4 For a sinusoidal applied voltage, the current in a resistor is always in phase with the voltage across the resistorThe direction of the current has no effect on the behavior of the resistorResistors behave essentially the same way in both DC and AC circuitsFCI- F. Univ.
13 Phasor DiagramTo simplify the analysis of AC circuits, a graphical constructor called a phasor diagram can be usedA phasor is a vector whose length is proportional to the maximum value of the variable it representsFCI- F. Univ.
14 Phasors, cont.The vector rotates counterclockwise at an angular speed equal to the angular frequency associated with the variableThe projection of the phasor onto the vertical axis represents the instantaneous value of the quantity it representsFCI- F. Univ.
15 rms Current and Voltage The average current in one cycle is zeroThe rms current is the average of importance in an AC circuitrms stands for root mean squareAlternating voltages can also be discussed in terms of rms valuesFCI- F. Univ.
16 PowerThe rate at which electrical energy is dissipated in the circuit is given byP = i 2 Ri is the instantaneous currentThe heating effect produced by an AC current with a maximum value of Imax is not the same as that of a DC current of the same valueThe maximum current occurs for a small amount of timeFCI- F. Univ.
17 Power, cont.The average power delivered to a resistor that carries an alternating current isFCI- F. Univ.
18 Notes About rms Valuesrms values are used when discussing alternating currents and voltages because:AC ammeters and voltmeters are designed to read rms valuesMany of the equations that will be used have the same form as their DC counterpartsFCI- F. Univ.
19 Example 1:Solution:Comparing this expression for voltage output with the general form∆v = ∆ Vmax sinωt, we see that∆ Vmax = 200 V. Thus, the rms voltage isThe voltage output of an AC source is given by the expression∆v = (200 V) sin ωt.Find the rms current in the circuitwhen this source is connected to a 100 Ohm resistor.FCI- F. Univ.
20 4-3 Inductors in an AC Circuit Kirchhoff’s loop rule can be applied and gives:FCI- F. Univ.
21 Current in an InductorThe equation obtained from Kirchhoff's loop rule can be solved for the currentThis shows that the instantaneous current iL in the inductor and the instantaneous voltage ΔvL across the inductor are out of phase by (p/2) rad = 90oFCI- F. Univ.
22 Phase Relationship of Inductors in an AC Circuit The current is a maximum when the voltage across the inductor is zeroThe current is momentarily not changingFor a sinusoidal applied voltage, the current in an inductor always lags behind the voltage across the inductor by 90° (π/2)FCI- F. Univ.
23 Phasor Diagram for an Inductor The phasors are at 90o with respect to each otherThis represents the phase difference between the current and voltageSpecifically, the current lags behind the voltage by 90oFCI- F. Univ.
24 Inductive ReactanceThe factor ωL has the same units as resistance and is related to current and voltage in the same way as resistanceBecause ωL depends on the frequency, it reacts differently, in terms of offering resistance to current, for different frequenciesThe factor is the inductive reactance and is given by:XL = ωLFCI- F. Univ.
25 Inductive Reactance, cont. Current can be expressed in terms of the inductive reactanceAs the frequency increases, the inductive reactance increasesThis is consistent with Faraday’s Law:The larger the rate of change of the current in the inductor, the larger the back emf, giving an increase in the reactance and a decrease in the currentFCI- F. Univ.
26 Voltage Across the Inductor The instantaneous voltage across the inductor isFCI- F. Univ.
27 Example 33.2 A Purely Inductive AC Circuit In a purely inductive AC circuit, L = 25.0 mH and the rms voltage is 150 V. Calculate the inductive reactance and rms current in the circuit if the frequency is 60.0 Hz.FCI- F. Univ.
29 4-4 Capacitors in an AC Circuit The circuit contains a capacitor and an AC sourceKirchhoff’s loop rule gives:Δv + Δvc = 0 and soΔv = ΔvC = ΔVmax sin ωtΔvc is the instantaneous voltage across the capacitorFCI- F. Univ.
30 Capacitors in an AC Circuit, cont. The charge is q = CΔVmax sin ωtThe instantaneous current is given byThe current is p/2 rad = 90o out of phase with the voltageFCI- F. Univ.
31 More About Capacitors in an AC Circuit The current reaches its maximum value one quarter of a cycle sooner than the voltage reaches its maximum valueThe current leads the voltage by 90oFCI- F. Univ.
32 Phasor Diagram for Capacitor The phasor diagram shows that for a sinusoidally applied voltage, the current always leads the voltage across a capacitor by 90oFCI- F. Univ.
33 Capacitive ReactanceThe maximum current in the circuit occurs at cos ωt = 1 which givesThe impeding effect of a capacitor on the current in an AC circuit is called the capacitive reactance and is given byFCI- F. Univ.
34 Voltage Across a Capacitor The instantaneous voltage across the capacitor can be written as ΔvC = ΔVmax sin ωt = Imax XC sin ωtAs the frequency of the voltage source increases, the capacitive reactance decreases and the maximum current increasesAs the frequency approaches zero, XC approaches infinity and the current approaches zeroThis would act like a DC voltage and the capacitor would act as an open circuitFCI- F. Univ.
35 Example.3 A Purely Capacitive AC Circuit ω=2 πf =377 s-1FCI- F. Univ.