# Chapter 9 Alternating current circuits

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Chapter 9 Alternating current circuits
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Purpose This chapter provides detailed information about alternating current circuits It describes the effects of AC on inductors and capacitors including resonance It covers series and parallel AC circuits and power in AC circuits PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Introduction to alternating current circuits
Inductors and capacitors behave differently when they are connected to an AC supply compared to when they are connected to a DC supply Ohm’s Law still works for instantaneous values, but the effects of time and frequency mean that phasors are a necessary tool for AC circuit analysis PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Introduction to alternating current circuits (cont.) AC power is dependent on the phase angle which adds the complication of power factor and power factor correction Alternating current using sine waves is the basis of worldwide electricity distribution The convention is to use RMS values when discussing AC, unless stated otherwise PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Resistance in AC circuits
Figure 9.1 shows that the voltage and current are in phase for a resistive load connected to an AC supply PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Resistance in AC circuits (cont.)
Ohm’s Law is applicable to the resistive load connected to an AC supply—that is: IRMS = VRMS / R By plotting the instantaneous values of power, a power curve has been drawn PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Resistance in AC circuits (cont.)
Points to note regarding the power curve include: the power curve is sinusoidal in shape there are no negative power values the power curve completed two cycles for every cycle of current or voltage the area under the power curve is equal to half of the value obtained by multiplying the peak values of voltage and current PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Resistance in AC circuits (cont.)
The result is that, if RMS values of voltage and current are used, the power equations are: P = VRMS IRMS P = IRMS2 R P = VRMS2 / R PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Resistance in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Resistance in AC circuits (cont.)
At power-line frequencies resistors are assumed to be ‘pure’ or non-inductive resistance. Any inductance that does occur is negligible in most cases Examples of resistive loads include incandescent lamps, radiators and electric jug elements PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Resistance in AC circuits (cont.)
At higher frequencies inductance may impact on the circuit, if this is undesirable the resistor can be wound to negate any inductance (Figure 9.2) PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Resistance in AC circuits (cont.)
Capacitive effects must be dealt with by spacing the conductors far enough apart that the capacitance becomes negligible Since the current and voltage are in phase for resistive AC circuits, Ohm’s Law is applicable Kirchoff’s Current and Voltage Laws also apply For purely resistive AC circuits, the rules for DC circuits continue to work PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Inductance in AC circuits
A change in current flow in an inductive circuit results in an induced EMF that opposes the change in current flow In an inductive AC circuit the current is continually changing, thus an induced EMF is always opposing the change in current flow This opposition to current flow is called inductive reactance PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Inductance in AC circuits (cont.)
The current flow is always lagging behind the applied voltage by 90oE in a purely inductive circuit PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Inductance in AC circuits (cont.) The value of inductive reactance in a circuit depends on the inductance and the rate of change of current flow, which depends on the supply frequency. XL = 2πfL Where: XL = The inductive reactance, in ohms f = The frequency, in hertz L = The inductance, in henry PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Inductance in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Inductance in AC circuits (cont.)
Ohm’s Law applies in an inductive AC circuit but the opposition to current flow is the inductive reactance: I = V / XL Where: I = The current, in amperes V = The voltage, in volts XL = The inductive reactance, in ohms PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Inductance in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Inductance in AC circuits (cont.)
Exercise: A 230 V 50 Hz supply is applied to a choke coil of negligible resistance and the current through the coil is 2.5 A. Determine the inductance of the coil. PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Inductance in AC circuits (cont.)
Solution: PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Inductance in AC circuits (cont.)
When inductors are connected in series to an AC supply the total inductive reactance is the sum of the inductive reactance of each of the individual inductors. XLtotal = XL1 + XL2 + XL3 + ….. + XLn Similarly, the total inductance can be calculated: Ltotal = L1 + L2 + L3 + ….. + Ln PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Inductance in AC circuits (cont.)
Exercise: Two inductors, one with an inductive reactance of 11 Ω, and the second with an inductive reactance of 12 Ω are connected in series across a 230 V 50 Hz supply. What is the total inductive reactance? What is the total current? PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Inductance in AC circuits (cont.)
Solution: PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Inductance in AC circuits (cont.)
When inductors are connected in parallel to an AC supply the reciprocal of the total inductive reactance is the sum of the reciprocal of the inductive reactance of each of the individual inductors: 1/XLtotal = 1/XL1 + 1/XL2 + 1/XL3 + ….. + 1/XLn Similarly, the total inductance can be calculated: 1/Ltotal = 1/L1 + 1/L2 + 1/L3 + ….. + 1/Ln PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Inductance in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Inductance in AC circuits (cont.)
The average power consumed by a pure inductor is zero As the current begins to flow in the inductor the energy is used to produce the magnetic field When the current falls the magnetic field collapses and energy is returned to the supply This is shown in Figure 9.3 PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Inductance in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Inductance in AC circuits (cont.)
Notice that the power curve is sinusoidal in shape (when the voltage and current are sinusoidal), it has twice the frequency of the applied EMF and that the power returned to the supply is shown below the axis PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Capacitors in AC circuits
The charging and discharging of a purely capacitive circuit when it is connected to an AC supply causes the current to lead the supply voltage by 90oE PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Capacitors in AC circuits (cont.)
A capacitor reacts to the connected supply by either charging or discharging The current that flows is a displacement current, the value of which is affected by the supply voltage and frequency and the capacity of the capacitor The capacitor has the property of capacitive reactance PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Capacitors in AC circuits (cont.)
XC = 1/(2πfC) Where: XC = The capacitive reactance, in ohms f = The frequency, in hertz C = The capacitance, in farads PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Capacitors in AC circuits (cont.)
Ohm’s Law applies in a capacitive AC circuit but the opposition to current flow is the capacitive reactance: I = V / XC Where: I = The current, in amperes V = The voltage, in volts XC = The capacitive reactance, in ohms PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Capacitors in AC circuits (cont.)
Exercise: Calculate the current drawn by a 16 μF capacitor when connected to a 230 V 50 Hz supply. PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Capacitors in AC circuits (cont.)
Solution: PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Capacitors in AC circuits (cont.)
Connecting capacitors in series results in an effective increase in the plate separation This reduces the capacitance and therefore increases the capacitive reactance XCtotal = XC1 + XC2 + XC3 + ….. + XCn PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Capacitors in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Capacitors in AC circuits (cont.)
Connecting capacitors in parallel results in an effective increase in the plate area. This increases the capacitance and therefore decreases the capacitive reactance. 1/XCtotal = 1/XC1 + 1/XC2 + 1/XC3 + ….. + 1/XCn PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Capacitors in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Capacitors in AC circuits (cont.)
The average power consumed by a pure capacitor is zero As the current begins to flow in the capacitor the energy is stored in the capacitor When the current falls the capacitor returns stored energy to the supply This is shown in Figure 9.4 PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Capacitors in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Quick quiz What is the relationship between the voltage and current for a resistive load connected to an AC supply? What is the opposition to AC current due to inductance called? What determines the inductive reactance in a circuit? What determines the capacitive reactance in a circuit? How much power is consumed by a purely reactive AC circuit? PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Quick quiz answers The voltage and current are in phase for a resistive load connected to an AC supply Inductive reactance Supply frequency and inductance (XL = 2πfL) Supply frequency and capacitance (XC = 1/(2πfC)) Average power consumed by a purely reactive AC circuit is zero PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits on AC current
Practical AC circuits consist of combinations of resistance, inductive reactance and capacitive reactance The combined opposition to current flow in AC circuits is called impedance (Z) Ohm’s Law is still applicable: Z = VRMS / IRMS PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
In a series circuit the current is common to all parts of the circuit and the total voltage is the phasor sum of the individual voltage drops Figure 9.5 shows the phasor diagram for a series R-L circuit, VZ is the total circuit voltage and ϕ is the phase angle PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
A practical inductor has some resistance and some inductance, Figure 9.6 shows the phasor diagram for an example practical inductor Note that the phase angle is less than 90 degrees The total power loss in the inductor is made up of the copper losses and the iron losses PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
Commercially available capacitors are considered as pure capacitance for all practical purposes Figure 9.7 shows the phasor diagram for a series R-C circuit PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
If a circuit has resistance, inductive reactance and capacitive reactance then the total voltage is the phasor sum of the voltages across each of the individual circuit components PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell 9-50

Series R-L-C circuits (cont.)
Noting that each of the series R-L-C circuits consists of voltage phasors (and each of them is equal to the current times the opposition to current) and that the phasor addition produces a triangle, allows mathematical analysis of the circuit impedance PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
Z = √(R2 + (XC – XL)2) and ϕ = Tan-1(X/R) = Cos-1(R/Z) = Sin-1(X/Z) Where: Z = The impedance, in ohms R = The resistance, in ohms XL = The inductive reactance, in ohms XC = The capacitive reactance, in ohms X = (XL – XC) PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
If the inductive reactance is greater than the capacitive reactance the total current lags the applied voltage If the capacitive reactance is greater than the inductive reactance the total current leads the applied voltage If the inductive reactance equals the capacitive reactance the circuit is said to be resonant PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
Exercise: A circuit that has 20 Ω in series with 0.25 H and 80 μF is connected to a 230 V 50 Hz supply. Determine the impedance of the circuit, the current and its phase angle. PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
Solution: PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
In parallel circuits the voltage is common and is therefore used as the reference in phasor diagrams A pure inductor in parallel with a resistance will give a phasor diagram like Figure 9.10 PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
Since a practical inductor has a phase angle less than 90 degrees the phasor diagram for a practical inductor connected in parallel with a resistance will look like Figure 9.11 PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
Figure 9.12 shows the phasor diagram for a capacitor connected in parallel with a resistance PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
The total current in a parallel R-L-C circuit can be determined by phasor addition of the branch currents PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
The impedance triangle method cannot be used for parallel circuits PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Series R-L-C circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell 9-62

Slides prepared by Andrew O’Connell
Quick quiz What is the combined opposition to current flow in an AC circuit called? Of what is the total power loss in an inductor made up ? How does the amount of inductive and capacitive reactance affect the series circuit current? PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Quick quiz (cont.) How can the total impedance of a series R-L-C circuit be determined? How can the total impedance of a parallel R-L-C circuit be determined? PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Quick quiz answers The combined opposition to current flow in AC circuits is called impedance (Z) The total power loss in the inductor is made up of the copper losses and the iron losses PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

If the inductive reactance is greater than the capacitive reactance the total current lags the applied voltage. If the capacitive reactance is greater than the inductive reactance the total current leads the applied voltage. If the inductive reactance equals the capacitive reactance the circuit is said to be resonant. PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Using the impedance triangle The total current in a parallel R-L-C circuit can be determined by phasor addition of the branch currents—the total impedance can be calculated using Ohm’s Law PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Power in AC circuits A practical inductor consists of resistance and inductance The resistance consumes power and the inductance does not consume power PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
The power consumed by the resistance can be calculated from P = VRI From the phasor diagram VR = Vcosϕ Combining these two equations gives the equation for True Power in AC circuits P = VIcosϕ PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
Since VI is multiplied by cosϕ, cosϕ is known as the power factor (λ) If the power factor is not included in the equation the result is the Apparent Power (S) That is, S = VI Apparent power is measured in volt-amperes PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
Many AC machines are rated in VA to provide information about the current rating of the windings The true power and the reactive power can be included as two sides of a triangle known as the power triangle PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
The third side of the power triangle represents the power associated with the reactive components in the circuit. It is known as reactive power (Q) Q = VIsinϕ Since cosϕ varies between zero and one, the power factor also varies between zero and one PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
Power factor can be calculated from the ratio of true power to apparent power or from the ratio of resistance to reactance in the circuit Power factor (λ) = cosϕ = R/Z = P/S In general the lower the power factor the greater the current required to supply a given amount of power PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
The implications of the extra current include: larger CSA conductors larger transformers higher rated switchgear fuses of a higher current rating higher voltage drop across conductors extra copper losses decreased efficiency higher costs to supply electricity PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
The following example demonstrates the effect of power factor on the current required to supply a load with a given amount of power PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell 9-77

Power in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
Low power factor is usually caused by inductive loads such as fluorescent lights, electric motors and transformers The supply authority requires power factor to be controlled within an installation PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
The power factor of motors and transformers is better when they are operating close to full load Fluorescent lights have an improved power factor when a capacitor is connected across the terminals of the light PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
The power factor of a circuit can be obtained using a voltmeter, ammeter and wattmeter and then dividing the true power by the apparent power PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
Power factor may be measured directly with a power factor meter Power factor can also be determined using a phasor diagram or, if it is a series circuit, by using the impedance triangle PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
Exercise: An inductor draws a current of 20 A on 230 V DC and 10 A on 230 V AC. Calculate the angle of lag when on AC. PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
Solution: PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
Power factor correction is required to minimise the current drawn from the supply This is achieved by connecting a capacitor across the terminals of the load PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
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Power in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell 9-88

Power in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
The connection of the capacitor improves the power factor, reduces the current drawn from the supply and maintains the true power consumed by the load Rather than guessing the appropriate capacitor size, it can be calculated PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
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Power in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell 9-92

Power in AC circuits (cont.)
Another method of choosing power factor correction is to consider the reactive power required to improve the power factor to the desired level PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power in AC circuits (cont.)
PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell 9-94

Power in AC circuits (cont.)
Power factor correction can also be achieved using a synchronous motor The synchronous motor operates with a leading power factor and provides power factor correction and mechanical effort at the same time PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Resonance When the power factor is corrected to unity the circuit resistance becomes the only opposition to current flow This situation requires the inductive reactance and the capacitive reactance to be exactly equal (XL = XC) This condition is called resonance PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Resonance (cont.) At resonance, energy is transferred between the electromagnetic field of the inductor and the electrostatic field of the capacitor Because reactance is frequency dependent there is a specific frequency at which resonance will occur, it is called the resonant frequency PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Resonance (cont.) A series circuit operating at resonance may have voltages higher than the supply voltage across the reactive components PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Resonance (cont.) PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell 9-99

Slides prepared by Andrew O’Connell
Resonance (cont.) Generally series resonant circuits are avoided but they can be used to select specific frequencies Frequencies other than the resonant frequency are attenuated The resonant frequency can be calculated using fo = 1/(2π√(LC)) Where: fo = The resonant frequency, in hertz L = The inductance, in henry C = The capacitance, in farads PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Resonance (cont.) PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Resonance (cont.) Parallel circuit resonance causes a large circulating current to oscillate between the capacitive and inductive branches of the circuit Providing an adjustable inductor or capacitor allows the selection of a particular frequency as the resonant frequency. This circuit was traditionally used to tune radio frequencies PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Resonance (cont.) Parallel resonance is also used in some test equipment for measuring capacitance Frequency dependent transducers also use this phenomenon as a means of monitoring and regulating frequency Metal detectors also utilise resonant circuits PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Quick quiz What is true power? What is apparent power? What is reactive power? What is power factor? What causes resonance? PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Quick quiz answers True power is the power associated with the resistive components of a circuit, P = VIcosϕ Apparent power is the supply voltage multiplied by the supply current, S = VI Reactive power is the power associated with the reactive components in a circuit, Q = VIsinϕ PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Power factor is the ratio of true power to apparent power, λ = cosϕ = P/S Resonance occurs when the inductive reactance in a circuit equals the capacitive reactance in the circuit PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Summary For resistive AC circuits the current and voltage are in phase For purely inductive AC circuits the current lags the voltage by 90 oE For purely capacitive AC circuits the current leads the voltage by 90 oE PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Summary (cont.) The phase angle (ϕ) of an AC R-L-C circuit depends on the relative proportions of each component in the circuit Opposition to current flow in an inductor is called inductive reactance (XL) XL = 2πfL Opposition to current flow in a capacitor is called capacitive reactance (XC) XC = 1/(2πfC) PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Summary (cont.) The total opposition to current flow in an AC circuit is called impedance (Z) In a series circuit, Z = √(R2 + (XL-XC)2) In a parallel circuit, IZ = √(IR2 + (IL-IC)2) The power factor of a circuit is the ratio of the true power to the apparent power, λ = cosϕ PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Summary (cont.) The power triangle shows the relationship between true power, apparent power and reactive power The power factor of a circuit can be determined by measurement and calculation Low power factor results in an increased current for delivery of a given amount of power PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Summary (cont.) Electricity suppliers require control of an installation’s power factor Typical installations have a lagging power factor due to the connected inductive loads Power factor correction may be achieved using capacitors or synchronous machines both of which have a leading power factor PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

Slides prepared by Andrew O’Connell
Summary (cont.) Resonance occurs when the inductive reactance in a circuit is exactly equal to the capacitive reactance in the circuit The resonant frequency can be calculated using fo = 1/(2π√(LC)) Resonance can cause dangerous voltages and currents in circuits PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades 6e Slides prepared by Andrew O’Connell

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