Operator Generic Fundamentals Basic Electricity - Part 2

Operator Generic Fundamentals Basic Electricity - Part 2

Basic Electricity 2 - TLOs
At the completion of this training session, the trainee will demonstrate mastery of this topic by passing a written exam with a grade of ≥ 80 percent on the following topics (TLOs): Describe the theory of operation and operating characteristics of an AC generator. Describe the construction and theory of operation of inductors and capacitors, their effects on AC electrical circuits, and relationship to power factor. Describe the construction, operation, and applications of transformers. Describe basic industrial electrical distribution, including typical wiring schemes used and the advantages of three-phase systems. Given an electrical measuring device or piece of test equipment, describe the use of that equipment including the electrical parameter measured. This module will review methods of generating, transforming, and distributing AC power, including use of common electrical test equipment. INTRO

AC Generator TLO 1 - Describe the theory of operation and operating characteristics of an AC generator. 1.1 Describe the operation of a simple AC generator. 1.2 Describe the development of a sine-wave output in an AC generator. 1.3 Define common terms in relation to AC generation. 1.4 Describe the relationship between peak, average and RMS values of voltage in an AC power source. 1.5 Given a diagram of two sine waves, describe the phase relationship between the two waves. TLO 1

AC Generator Operation
ELO Describe the operation of a simple AC generator. A simple AC generator consists of a conductor or loop of wire in a magnetic field The two ends of the loop connect to slip rings, that are in contact with brushes When the loop rotates, it cuts magnetic lines of force As the conductor passes through the magnetic field, a voltage is induced in the conductor and transferred through the slip rings as voltage output Figure: Simple AC Generator ELO 1.1

Magnitude of Generated Voltage Dependent on field strength and speed of rotor Most generators at constant speed; generated voltage depends on field excitation, or field strength ELO 1.1

Knowledge Check An AC generator has all of the following except: A commutator A magnetic field Slip rings A conductor in relative motion with the magnetic field Correct answer is A. Answer: A – a commutator ELO 1.1

Generator Sine Wave Output
ELO Describe the development of a sine-wave output in an AC generator. As the generator windings are rotated through the magnetic field, a voltage is induced in the conductors The magnitude and polarity of the induced voltage varies with The strength of the magnetic field The location and direction of travel of the conductors ELO 1.2

When the loop is in the vertical position, at 0°, the coils are moving parallel to the magnetic field and do not cut magnetic lines of force At that instant, there is no voltage induced As the coil rotates in a clockwise direction, each side of coil cuts the magnetic lines of force in opposite directions The polarity of the induced voltages depends on the direction of movement of the coil The induced voltages are additive, making slip ring X positive (+) and slip ring Y negative (-) Figure: Developing an AC Sine Wave Voltage ELO 1.2

This current increases until it reaches a maximum value when the coil is 90° At that instant, the horizontal coil is cutting the greatest number of magnetic lines As the coil continues to turn, the induced voltage and current decrease until both reach zero When the coil is again in the vertical position (180°) The next half revolution produces an equal voltage, with reversed polarity (270° and 360°) Figure: Developing an AC Sine Wave Voltage ELO 1.2

C B A Three-phase AC, voltage induced from rotating magnetic field
Time 1 Phase B Phase A Phase C T2 T3 T4 T5 T6 T7 Graphic Courtesy of Duke Energy – Basic Electricity Each mouse click results in 60 degrees of rotation ELO 1.2

Knowledge Check In a simple AC generator, _______ causes the AC sine wave output. the changes in relative motion of the conductor and the magnetic field commutation changing speed of the rotating element the pre-programmed oscillation of the field Correct answer is A. The correct answer is: A ELO 1.2

AC Generator Common Terms
ELO Define common terms in relation to AC generation. Period and Frequency Period – the time required for the generator to complete one cycle Frequency – the number of cycles completed per second, measured in hertz One complete cycle is when the generator coil rotates 360° In one cycle, the voltage increases from zero to Emax in one direction, decreases to zero, increases to Emax in the opposite direction (negative Emax), and then decreases to zero again Peak Voltage and Current Peak means the maximum appearing on an AC sine wave Peak voltage – Ep or Emax, occurs at 90° Peak current – Ip One way to quantify AC voltage or current is by peak value ELO 1.3 Define common terms in relation to AC generation. ELO 1.3

Peak to Peak Voltage and Current Another commonly used term associated with AC is peak-to- peak value (Ep-p or Ip-p) Peak to peak refers to the magnitude of voltage, or current range, spanned by the sine wave Figure: AC Sine Wave Voltage ELO 1.3 Define common terms in relation to AC generation. ELO 1.3

Effective Value of AC Amount of AC that produces the same heating effect as an equal amount of DC Heating effect of an AC current is proportional to the square of the current Calculate the effective value of AC, by squaring all the amplitudes of the sine wave over one period, taking the average of these values, and then taking the square root of the average The effective value, because it is the root of the mean (average) square of the currents, is the root-mean-square, or RMS value Figure: AC Voltage Sine Wave ELO 1.3 Define common terms in relation to AC generation. ELO 1.3

Knowledge Check Match the terms with their appropriate definitions. Correct answers are: 1-A, 2-C, 3-B, 4-D. The time required for the generator to complete one cycle Period The number of cycles completed per second Peak-to-peak The magnitude of voltage or current range spanned by the sine wave Frequency The root of the mean (average) square of the currents or voltages RMS Correct answers are: 1-A, 2-C, 3-B,4-D ELO 1.3

Peak, Average and RMS Voltage
ELO Describe the relationship between peak, average and RMS values of voltage in an AC power source. Effective Value of AC Effective value (current or voltage) of an AC signal is equal to the (RMS) of the signal Calculated by squaring the average amplitudes of the sine wave over one period, and then taking the square root The upper curve shows a plot of the values of I over time and the effective value of I Lower curve shows a plot of the values of I2 over time, and the average current Figure: Effective Value of AC Current ELO 1.4

The dashed line is the average of the I2 values, and the square root of that value is the RMS, or effective value The average value is ½ Imax2 The RMS value is √2/2 Imax which is equal to Imax The effective value of voltage or current can be found using: 𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑣𝑎𝑙𝑢𝑒 (𝑅𝑀𝑆) = 𝑝𝑒𝑎𝑘 𝑣𝑎𝑙𝑢𝑒 × 0.707 Normal convention is that stated values of AC current and voltage are RMS values No subscript is normally used Figure: Effective Value of AC Current ELO 1.4

Knowledge Check The peak value of voltage in an AC circuit is 250 volts. Calculate the effective voltage. 120 volts 125 volts 177 volts 353 volts Correct answer is C. Correct answer is C. 250 x = 177 volts ELO 1.4

Phase Relationship ELO Given a diagram of two sine waves, describe the phase relationship between the two waves. Phase Angle Guidelines Phase angle is the fraction of a cycle that has gone by since a voltage or current has passed through a given value Phase difference is another common term for phase angle It describes two different voltages with the same frequency, which pass through zero at different times ELO 1.5

Phase Relationship Phase Angle Guidelines Example
Take point 1 on the sine wave as the starting point or zero phase. The phase angle at point 2 is 30°, point 3 is 60°, point 4 is 90°, and so on until point 13 where the phase angle is 360°, or zero once again. Figure: AC Voltage Sine Wave ELO 1.5

Phase Relationship Phase Example
In the figure below, the angles along the axis indicate the phases of voltages e1 and e2. At 120°, e1 passes through the zero value, which is 60° ahead of e2 (e2 equals zero at 180°) Voltage e1 leads e2 by 60 electrical degrees, or voltage e2 lags e1 by 60 electrical degrees Figure: Phase Relationship ELO 1.5

Phase Relationship Knowledge Check
When two AC voltages reach their peak voltage at the same time, the voltages are said to be __________________. leading lagging in phase out of phase Correct answer is C. Correct answer is C. ELO 1.5

Inductors and Capacitors
TLO 2 - Describe the construction and theory of operation of inductors and capacitors, their effects on AC electrical circuits, and relationship to power factor. 2.1 Describe how current flow, magnetic field, and stored energy in an inductor relate to one another, and how an inductor opposes a change in current flow. 2.2 Describe the construction of a capacitor, how it stores energy, and opposes a change in voltage. 2.3 Describe inductive reactance (XL), and the phase relationship between current and voltage in an inductive circuit. 2.4 Define capacitive reactance (XC), and the phase relationship between current and voltage in a capacitive circuit. 2.5 Define impedance (Z). 2.6 Define apparent, true, and reactive power using a power triangle. 2.7 Define power factor as it relates to true power and apparent power, and define leading and lagging power factors. TLO 2

Inductor Theory ELO Describe how current flow, magnetic field, and stored energy in an inductor relate to one another, and how an inductor opposes a change in current flow. An inductor is a circuit element that will store electrical energy in the form of a magnetic field It is usually a coil of wire wrapped around a core of permeable material Circuits containing inductors will behave differently from a purely resistive circuit ELO 2.1

Inductor Theory Induced and Counter- Electromotive Force (EMF)
In the figure, when current is flowing through Wire A It generates a magnetic field around Wire A There is no electromotive force (EMF) induced into Wire B because there is no relative motion between the magnetic field and Wire B (DC circuit) Figure: Induced EMF ELO 2.1

Inductor Theory Induced and Counter- Electromotive Force (EMF)
If we now open the switch Current stops flowing in Wire A Magnetic field collapses As the field collapses, it moves relative to Wire B and induces an EMF in Wire B Figure: Induced EMF ELO 2.1

Inductor Theory Inducing Electromotive Force
The three requirements for inducing an EMF are: a conductor, a magnetic field, and relative motion between the two The faster the movement between the two, or the faster the magnetic field collapses or expands, the greater the induced EMF Coiling the wire in either Circuit A or Circuit B, or both, as shown in the figure below, increases the induction The EMF induced in Wire B causes a current to flow whose magnetic field opposes the change in the magnetic field that produced it For this reason, an induced EMF is termed Counter-Electromotive Force or CEMF Figure: Induced EMF in Coils ELO 2.1

Inductor Theory Self-Induced Electromotive Force (EMF)
Self-induced EMF is another phenomenon of induction The circuit shown in the figure below contains a coil of wire called an inductor (L). As current flows through the circuit, a large magnetic field sets up around the coil Since the current is not changing, there is no EMF produced If we open the switch, current flow stops and the field around the inductor collapses Produces a voltage This is a self-induced EMF . Figure: Self-Induced EMF ELO 2.1

Inductor Theory Lenz’s Law gives the polarity of self-induced EMF
The polarity is in the direction that opposes the change in the magnetic field that induced the EMF The result is that the current caused by induced EMF tends to maintain the same amount of current that existed in the circuit before opening switch Inductor maintains current flow until magnetic field has collapsed entirely For this reason, an inductor tends to oppose a change in current flow This is an example of Lenz’s Law, which states that the induced EMF opposes the EMF that caused it . ELO 2.1

Inductor Theory Inductance
Measure of an inductor’s ability to induce CEMF Measured in henries (H) The induced or counter EMF, is proportional to the time rate of change of current. The proportionality constant is the inductance (L) 𝐶𝐸𝑀𝐹=−𝐿 ∆𝐼 ∆𝑡 Where: 𝐶𝐸𝑀𝐹 = Induced voltage (volts) 𝐿 = Inductance (henries) ∆𝐼 ∆𝑡 = Time rate of change of current (amp/sec) The minus sign shows that the CEMF is opposite in polarity to the applied voltage . ELO 2.1

Inductor Theory Inductors in Series Example
To calculate the equivalence of inductors in series, add the inductance values Equivalent inductance (Leq) is: 𝐿 𝑒𝑞 = 𝐿 1 + 𝐿 𝐿 𝑛 Inductors in Parallel Example To calculate the equivalence of inductors in parallel, combine the values like resistors in parallel as shown below 1 𝐿 𝑒𝑞 = 1 𝐿 𝐿 2 + … 1 𝐿 𝑁 Figure: Inductors in Series . Figure: Inductors in Parallel ELO 2.1

Inductor Theory Knowledge Check
Select all of the statements about inductors that are true. An inductor is a circuit element that will store electrical energy in the form of a magnetic field. An inductor stores energy as a stored charge between two plates. An inductor is usually a coil of wire wrapped around a core of permeable material. Inductors oppose a change in voltage. Correct answers are A and C. Correct answer is A and C ELO 2.1

Capacitor Theory ELO Describe the construction of a capacitor, and how it stores energy, and opposes a change in voltage. Capacitors Capacitors are electrical devices that are constructed of two metal plates separated by an insulating material, called a dielectric (shown below) The schematic symbols shown below apply to all capacitors Figure: Capacitor Construction and Symbols ELO 2.2

Capacitor Theory Charging a Capacitor
Two conductor plates of the capacitor are electrically neutral There are as many positive as negative charges on plates The capacitor has no charge When switch is closed Negative charges on Plate A are attracted to the positive side of battery Positive charges on Plate B are attracted to the negative side of battery Figure: Charging a Capacitor ELO 2.2

Capacitor Theory Charging a Capacitor
Movement of charges continues until the difference in charge between Plate A and Plate B is equal to voltage of the battery Capacitor remains charged after battery is disconnected opposite charges on opposing plates attract each other tend to oppose any changes in charge Figure: Charging a Capacitor ELO 2.2

Capacitor Theory Discharging a Capacitor
When a conductor is placed across the plates, electrons will find a path back to Plate A, and the charges will be neutralized This is now a discharged capacitor Types of Capacitors Dielectric material serves to classify all commercial capacitors Most common dielectrics are air, mica, paper, and ceramic capacitors, plus the electrolytic type Figure: Discharging a Capacitor ELO 2.2

Capacitor Theory Capacitance The ability to store an electrical charge
𝐶 = 𝑄 𝑉 Where: 𝐶 = Capacitance (F) 𝑄 = Amount of charge (C) 𝑉 = Voltage (V) Unit is farad (F), which is the capacitance that will store one coulomb of charge when one volt acts across the plates of the capacitor. The dielectric constant (K, unitless) describes the ability to store electrical energy The capacitance of a capacitor depends on three things: Area of conductor plates Separation between the plates Dielectric constant of insulation material Air is used as a reference with K=1. ELO 2.2

Capacitor Theory The equation below illustrates the formula to find the capacitance of a capacitor with two parallel plates 𝐶=𝐾 𝐴 𝑑 8.85× 10 −12 Where: 𝐶 = Capacitance 𝐾 = Dielectric constant 𝐴 = Area 𝑑 = Distance between the plates x = Constant of proportionality ELO 2.2

Capacitor Theory Capacitors in Series
Capacitors in series are combined like resistors in parallel 1 𝐶 𝑇 = 1 𝐶 𝐶 𝐶 3 + … 1 𝐶 𝑁 When only two capacitors are in series, the equation simplifies as shown below 𝐶 𝑇 = 𝐶 1 𝐶 2 𝐶 1 + 𝐶 2 When all the capacitors in series are the same value, compute the total capacitance by dividing the capacitor’s value by the number of capacitors in series as shown below 𝐶 𝑇 = 𝐶 𝑁 Where: 𝐶 = Value of any capacitor in series 𝑁 = The number of capacitors in series with the same value Figure: Capacitors Connected in Series ELO 2.2

Figure: Capacitors Connected in Parallel
Capacitor Theory Capacitors in Parallel Example Capacitors in parallel are combined like resistors in series When connected in parallel, the total capacitance, CT, is the sum of the individual capacitances as given below 𝐶 𝑇 = 𝐶 1 + 𝐶 2 + 𝐶 3 + …+ 𝐶 𝑁 Figure: Capacitors Connected in Parallel ELO 2.2

Capacitor Theory Knowledge Check
A capacitor is __________________________________. two metal plates separated by an insulating material that opposes a change in current flow two metal plates separated by an insulating material that opposes a change in voltage a coil of wire around a magnetic core that opposes a change in voltage a coil of wire around a magnetic core that opposes a change in current flow Correct answer is B. Correct answer is B. ELO 2.2

Inductive Reactance ELO Describe inductive reactance (XL) and the phase relationship between current and voltage in an inductive circuit. In AC circuits, inductors present a resistance to current flow that is termed inductive reactance Any device relying on magnetism or magnetic fields to operate is a form of inductor Motors, generators, transformers, and coils are all inductors In an inductive AC circuit, the current is continually changing and is continuously inducing an EMF Effect is measured in ohms This opposition of the inductance to the flow of an alternating current is inductive reactance (XL) ELO 2.3

Inductive Reactance Mathematical representation of the current flowing in a circuit that contains only inductive reactance is: 𝐼= 𝐸 𝑋 𝐿 Where: 𝐼 = Effective current (A) 𝑋 𝐿 = Inductive reactance (Ω) 𝐸 = Effective voltage across the reactance (V) ELO 2.3

Inductive Reactance The value of XL is dependent on
Inductance of the circuit Rate the current is changing in the circuit This rate of change depends on frequency of the applied voltage 𝑋𝐿=2𝜋𝑓𝐿 Where: 𝜋 ≈ 𝑓 = Frequency (Hertz) 𝐿 = Inductance (Henries) ELO 2.3

Inductive Reactance The magnitude of induced EMF depends on how fast flux is changing For self-induced EMF (such as in a coil), a CEMF is induced in coil This CEMF opposes any change in current Inductors in AC circuits expand and collapse magnetic fields attempting to keep current in circuit constant In a purely inductive circuit, the resistance is negligible in comparison to the inductive reactance ELO 2.3

Figure: Current, Self-Induced EMF, and Voltage in an Inductive Circuit
Inductive Reactance Voltage and Current Relationship in an Inductive Circuit A change in current in a coil causes a change of magnetic flux around the coil Changes at maximum rate when going through its zero value at 90° (point b on figure) 270° (point d) The flux change is also the greatest at those times The self-induced EMF in coil is at maximum value at these points Figure: Current, Self-Induced EMF, and Voltage in an Inductive Circuit ELO 2.3

Inductive Reactance Voltage and Current Relationship in an Inductive Circuit Current is not changing when it is going through its peak value at 0° (point a) 180° (point c) 360° (point e) Flux change is also zero The self-induced EMF in the coil is at zero value at these points When the current is at its maximum positive value, the induced EMF is at a zero value and rising Figure: Current, Self-Induced EMF, and Voltage in an Inductive Circuit ELO 2.3

Inductive Reactance Voltage and Current Relationship in an Inductive Circuit When current reaches a zero value, the induced EMF is at its maximum positive value When the current is increasing from zero to its maximum negative value at 360° (point d to point e) the induced voltage is of the opposite polarity as the current and tends to keep the current from increasing in the negative direction The current lags the applied voltage by 90° in a purely inductive AC circuit Figure: Current, Self-Induced EMF, and Voltage in an Inductive Circuit ELO 2.3

Inductive Reactance Knowledge Check
Inductive reactance is caused by__________________________. the induced EMF in inductors stored electrical charge in circuit components hysteresis losses resistance in the conductors Correct answer is A. Correct answer is A. ELO 2.3

Capacitive Reactance ELO Define capacitive reactance (XC) and the phase relationship between current and voltage in a capacitive circuit. Capacitors in a circuit present a resistance to current flow known as capacitive reactance There are many natural forms of capacitance in AC power circuits, such as transmission lines, fluorescent lighting, and computer monitors Normally, the inductors counteract the effects of capacitance in an electrical distribution system However, where capacitors outnumber inductive devices, capacitive reactance will affect the amount of current flowing in an AC electrical circuit ELO 2.4

Capacitive Reactance Capacitive reactance is the opposition by a capacitor (or a capacitive circuit) to the flow of AC current Capacitors charge and discharge in an attempt to keep voltage constant Frequency of the voltage supply determines the rate at which the applied voltage is changing If the supply voltage frequency or the capacitance of a given circuit is increased, the current flow will increase Capacitive reactance is inversely proportional to frequency and capacitance ELO 2.4

Capacitive Reactance The units of capacitive reactance XC are ohms, just like inductive reactance 𝑋 𝐶 = 1 2𝜋𝑓𝐶 Where: 𝑓 = Frequency (Hz) 𝜋 ≈ 3.14 𝐶 = Capacitance (farads) A mathematical representation for the current that flows in a circuit with only capacitive reactance is: 𝐼= 𝐸 𝑋 𝐶 𝐼 = Effective current (A) 𝐸 = Effective voltage across the capacitive reactance (V) 𝑋 𝐶 = Capacitive reactance (Ω) ELO 2.4

Capacitive Reactance Voltage and Current Relationships in a Capacitive Circuit Current flow is greatest at points a, c, and e Voltage is changing at maximum rate From point a and point b Voltage and charge are increasing Current flow is into the capacitor, but decreasing in value At point b, capacitor fully charged From b to point c Voltage and charge are decreasing as the capacitor discharges Current flows in direction opposite to the voltage Figure: Voltage, Charge, and Current in a Capacitive Circuit NOTE: current flow depends on the rate at which the voltage changes. ELO 2.4

Capacitive Reactance Voltage and Current Relationships in a Capacitive Circuit From c to d Capacitor begins to charge in the opposite direction Voltage and current are again in the same direction At d Capacitor is charged Current flow again zero From d to e Capacitor discharges Flow of current is opposite to voltage In any purely capacitive AC circuit, current leads applied voltage by 90° Figure: Voltage, Charge, and Current in a Capacitive Circuit NOTE: current flow depends on the rate at which the voltage changes. ELO 2.4

Capacitive Reactance Knowledge Check
Capacitive reactance is dependent on all of the following except ____________________. applied voltage frequency area of the conducting plates dielectric constant Correct answer is D. Correct answer is D. ELO 2.4

Impedance Theory ELO 2.5 - Define impedance (z).
Both resistive and reactive components in an AC circuit oppose current flow The total opposition to current flow in an AC circuit depends on its resistance, its reactance, and the phase relationships between them Impedance Impedance is the total opposition to current flow in an AC circuit The mathematical representation for the magnitude of impedance in an AC circuit is: 𝑍= 𝑅 2 + 𝑋 2 Where: 𝑍 = impedance (Ω) 𝑅 = resistance (Ω) 𝑋 = net reactance (Ω) ELO 2.5

Impedance Theory Resistance, Reactance and Impedance
The figure below shows the relationship between resistance, reactance, and impedance in an AC circuit The current through a resistance is always in phase with the applied voltage. Resistance plots on the zero axis The current through an inductor lags applied voltage by 90°; inductive reactance plots along the 90° axis Current through a capacitor leads applied voltage by 90°; capacitive reactance plots along the -90° axis Figure: Relationship Between Resistance, Reactance, and Impedance ELO 2.5

Impedance Theory Knowledge Check
Adding a capacitor to an inductive circuit will ____________________. reduce the impedance, because the capacitive reactance counteracts some of the inductive reactance increase the impedance, because all reactance adds to the impedance reduce the resistance of the circuit cause no change to the circuit at all, since it is primarily inductive Correct answer is B. Correct answer is B. ELO 2.5

Apparent, True & Reactive Power
ELO Define apparent, true, and reactive power using a power triangle. The power triangle equates AC power to DC power, relates Generator output (Apparent Power) Usable power (True Power) Wasted or stored power (Reactive Power) The phase angle (θ) represents the inefficiency of the AC circuit Corresponds to the total reactive impedance (Z) to current flow in the circuit Can be used to find the efficiency level of generated power to usable power Figure: Power Triangle ELO 2.6

Apparent Power (S) The power delivered to an electrical circuit: 𝑆= 𝐼 2 𝑍=𝐼𝐸 Where: 𝑆 = Apparent power (VA) 𝐼 = RMS current (A) 𝐸 = RMS voltage (V) 𝑍 = Impedance (Ω) True Power The power consumed by the resistive loads in an electrical circuit: 𝑃= 𝐼 2 𝑅=𝐸𝐼 cos 𝜃 𝑃 = True power (watts) 𝐼 = RMS current (A) 𝐸 = RMS voltage (V) 𝑅 = Resistance (Ω) 𝜃 = Angle between E and I sine waves ELO 2.6

The power component necessary for the expansion and collapse of magnetic (inductive) and electrostatic (capacitive) fields 𝑄= 𝐼 2 𝑋=𝐸𝐼 sin 𝜃 Where: 𝑄 = Reactive power (VAR) 𝐼 = RMS current (A) 𝑋 = Net reactance (Ω) 𝐸 = RMS voltage (V) 𝜃 = Angle between the E and I sine waves Unlike true power, reactive power is unusable because it is stored in the circuit By inductors as they expand/collapse their magnetic fields in an attempt to keep current constant By capacitors because they charge/discharge in an attempt to keep voltage constant ELO 2.6

Reactive Power is function of a system’s amperage The power delivered to the inductance is stored in the magnetic field during field expansion, and returned to the source when the field collapses The power delivered to the capacitance is stored in the electrostatic field when the capacitor is charging, and returned to the source when the capacitor discharges The circuit conserves reactive power, since none of the reactive power delivered to the circuit by the source is consumed, but it is all returned to the source These reactive loads consume no True Power in order to maintain their magnetic and electrostatic fields Alternating current constantly changes; thus, the cycle of expansion and collapse of the magnetic and electrostatic fields constantly occurs ELO 2.6

Circulating current is the term for the current that is constantly flowing between the source and the inductive and capacitive loads in an AC circuit in order to maintain magnetic fields Circulating currents account for no real work in the circuit Total Power Delivered by the source The same as apparent power Part of this apparent power, called true power, dissipates by the circuit resistance in the form of heat The rest of the apparent power returns to the source by the circuit inductance and capacitance (reactive power) ELO 2.6

61 Apparent, True & Reactive Power Knowledge Check Match the power terms with their appropriate location on the power triangle. Apparent Power Reactive Power True Power Correct answers are A-1, B-3, and C-2. 1 3 2 Correct answers are: A - 1, B – 3, C – 2 ELO 2.6

Power Factor ELO Define power factor as it relates to true power and apparent power, and define leading and lagging power factors. Power Factor Power factor (pf) is the ratio between True Power and Apparent Power True Power is the power consumed by an AC circuit Apparent Power is a representation of the total power delivered to an AC circuit Reactive Power accounts for a portion of the Apparent Power, which is power that is stored in an AC circuit and accomplishes no real work in the circuit ELO 2.7

Power Factor Power factor is represented by cos θ in an AC circuit
It is the ratio of True Power to Apparent Power where θ is the phase angle between the applied voltage and current sine waves and is the angle between P and S on a power triangle The equation for power factor is: cos 𝜃 = 𝑃 𝑆 Where: cos 𝜃 = Power factor (pf) 𝑃 = True Power (watts) 𝑆 = Apparent power (VA) Figure: Power Triangle ELO 2.7

Power Factor Lagging Power Factor
Power factor also determines what part of the Apparent Power is True Power It can vary from 1, when the phase angle is 0°, to 0, when the phase angle is 90° In an inductive circuit, the current lags the voltage. This type of circuit has a lagging power factor, as shown in the figure below The direction of the power triangle that represents “lagging” and “leading” is a function of the “rotation” of the graph that represents it. The graphic above is based on a clockwise direction. This means the “P” line (current) LAGS the “S” line (voltage) by the angle difference represented. The Generator Capability Curve discussed in – Motors and Generators is based on a counterclockwise direction, but the concept is exactly the same. Figure: Lagging Power Factor ELO 2.7

Power Factor Leading Power Factor
In a capacitive circuit, the current leads voltage An electrical circuit that powers loads such as motors, will exhibit a lagging power factor. An electrical circuit that powers loads such as fluorescent lighting will exhibit a leading power factor Most industrial electrical distribution systems exhibit a lagging power factor because inductive loads normally account for a larger percentage of the reactance seen in these types of circuits The direction of the power triangle that represents “lagging” and “leading” is a function of the “rotation” of the graph that represents it. The graphic above is based on a clockwise direction. This means the “P” line (current) LAGS the “S” line (voltage) by the angle difference represented. The Generator Capability Curve discussed in – Motors and Generators is based on a counterclockwise direction, but the concept is exactly the same. ELI the ICE Man ELI refers to an inductive circuit (L) where current (I) lags voltage (E). ICE refers to a capacitive circuit (C) where current (I) leads voltage (E) Figure: Leading Power Factor ELO 2.7

Power Factor Knowledge Check
Select all of the statements about power factor (pf) that are true. Power factor is the ratio of true power and reactive power. Power factor cannot be greater than one. Power factor is the sine of the power triangle. Power factor (pf) is the ratio between True Power and Apparent Power. Correct answers are B and D. Correct answer is B and D. ELO 2.7

Transformer Construction & Operation
TLO 3 – Describe the construction, operation and applications of transformers. 3.1 Define the common terms as they pertain to transformers: mutual induction, turns ratio, impedance ratio, and efficiency. 3.2 Describe the construction of the following components of a transformer: primary coil, secondary coil, and iron core. 3.3 Describe the voltage, current and power relationships between the primary and secondary windings of transformers. 3.4 State the applications of each of the types of transformers: distribution transformer, power transformers, control transformers, auto transformers, isolation transformers, instrument potential transformers, instrument current transformers. TLO 3

Transformer Terms ELO Define the following terms as they pertain to transformers: mutual induction, turns ratio, impedance ratio, and efficiency. A transformer is a device that transfers electrical energy from one circuit to another by electromagnetic induction This energy always transfers without a change in frequency, but usually with changes in current and voltage Mutual Induction If flux lines from the expanding and contracting magnetic field of one coil cut the windings of another nearby coil, a voltage will be induced in that coil Inducing an EMF in a coil by magnetic lines of flux generated in another coil is mutual induction The amount of electromotive force (EMF) induced by this method depends on the relative positions of the two coils ELO 3.1

Transformer Terms Turns Ratio
The turns ratio is the ratio of the number of turns of wire in the primary winding to the number of turns of wire in the secondary winding, represented as: 𝑇𝑢𝑟𝑛𝑠 𝑅𝑎𝑡𝑖𝑜 = 𝑁𝑃 𝑁𝑆 Where: 𝑁𝑃 = number of turns on the primary coil 𝑁𝑆 = number of turns on the secondary coil ELO 3.1

Transformer Terms Impedance Ratio
Maximum power transfers through a transformer when the impedances are equal, or matched A transformer winding is constructed with a specific turns ratio The turns ratio establishes the proper relationship between the primary and secondary winding impedances 𝑁 𝑃 𝑁 𝑆 2 = 𝑍 𝑝 𝑍 𝑆 Efficiency Efficiency of a transformer is the ratio of the power output to the power input, as illustrated by the equation below 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦= 𝑃𝑜𝑤𝑒𝑟 𝑂𝑢𝑡𝑝𝑢𝑡 𝑃𝑜𝑤𝑒𝑟 𝐼𝑛𝑝𝑢𝑡 = 𝑃 𝑆 𝑃 𝑃 ×100 Where: PS = power of secondary PP = power of primary ELO 3.1

Transformer Terms Knowledge Check
Match the terms with their appropriate definitions. Correct answers are 1-B, 2-D, 3-C, and 4-A. Inducing an EMF in a coil by magnetic lines of flux generated in another coil Efficiency The ratio of the number of turns of wire in the primary winding to the number of turns of wire in the secondary winding Mutual induction The ratio between the two impedances Impedance ratio The ratio of the power output to the power input Turns ratio Correct answers are: 1-B, 2-D, 3-C, 4-A ELO 3.1

Transformer Components
ELO Describe the construction of the following basic components of a transformer: primary coil, secondary coil, and iron core. Every transformer has a primary winding and one or more secondary windings Primary winding receives electrical power from an AC source and induces electrical energy into the secondary winding(s) The energy appears as an electromotive force (EMF) across the secondary winding, and if a load connects to the secondary, energy in the form of current transfers to load A transformer works on the principle that varying magnetic flux transfers energy by magnetic induction from one set of coils to another An AC source produces this varying magnetic flux ELO 3.2

Transformers provide means of transferring electrical energy from one circuit to another, with no direct electrical connections Used extensively for: AC power transmission Various control and indication functions Isolating electrical circuits An important application of a transformer is for raising (stepping-up) or lowering (stepping down) the source of voltage The coil of a transformer energized from an AC source is called the primary winding (coil) The coil that delivers the induced AC to the load is called the secondary winding (coil) Transformers convert power at a given current and voltage to an equivalent power at a different current and voltage. ELO 3.2

In actual construction: Half of the primary and secondary coils wind on each of the two legs Sufficient insulation between the two coils and the core to properly insulate the windings Reduces magnetic leakage Magnetic leakage is the magnetic flux that passes through either coil, but not through both As the distance between primary and secondary windings increases, the magnetic circuit lengthens, and leakage increases Figure: Basic Core Type Transformer ELO 3.2

When (AC) voltage is applied to primary winding, an alternating current flows through the primary winding that magnetizes the magnetic core, first in one direction and then in the other direction This alternating flux flowing around the entire length of the magnetic circuit induces a voltage in both the primary and secondary windings The induced voltage will be at the same frequency as that of the AC source Since the same flux links both windings, the voltage induced per turn of the primary and secondary windings must be the same value and same direction In the primary winding, this voltage opposes the voltage applied to the primary winding and is counter-electromotive force (CEMF) ELO 3.2

Knowledge Check Match the transformer-related terms to their appropriate definitions. Correct answers are: 1-B, 2-C, 3-D, and 4-A. The coil that is energized by the AC source Leakage The coil that is connected to the load Primary Magnetic material that coils are wound around Secondary Magnetic flux that passes though only one of the coils Core Correct answers are: 1-B, 2-C, 3-D, 4-A ELO 3.2

Transformer Windings ELO Describe the voltage, current and power relationships between the primary and secondary windings of transformers. One of the most important functions associated with transformers is their ability to step-up or step-down voltage Transmission lines are high voltages Limits current flow for a given power, limiting heating losses The voltage induced in the secondary windings of a transformer is dependent on the ratio of turns of the primary winding to turns of the secondary winding ELO 3.3

Transformer Windings Primary to Secondary Voltage Relationship
The voltages induced in the windings of a transformer are directly proportional to the number of turns of the coils in the transformer 𝑉 𝑃 𝑉 𝑆 = 𝑁 𝑃 𝑁 𝑆 Where: 𝑉 𝑃 = voltage on primary coil 𝑉 𝑆 = voltage on secondary coil 𝑁 𝑃 = number of turns on the primary coil 𝑁 𝑆 = number of turns on the secondary coil 𝑉𝑅=𝑇𝑅 A voltage ratio of 1:5 means that for each volt on the primary, there will be 5 volts on the secondary. Step-up transformer: Secondary voltage > primary voltage Step-down transformer: Secondary voltage < primary voltage ELO 3.3

Transformer Windings Primary to Secondary Current Relationship
The current in the windings of a transformer is inversely proportional to the voltage in the windings. The equation to express this relationship is: 𝑉 𝑃 𝑉 𝑆 = 𝐼 𝑆 𝐼 𝑃 Where: 𝐼 𝑃 = primary coil current 𝐼 𝑆 = secondary coil current Since the voltage ratio is equal to the turns ratio, we can express the current ratio in terms of the turns ratio as well by 𝑁 𝑃 𝑁 𝑆 = 𝐼 𝑆 𝐼 𝑃 , or we can use: CR = 1 𝑉𝑅 = 1 𝑇𝑅 ELO 3.3

Transformer Windings Primary to Secondary Power Relationship
Regardless transformer is a step-up or step-down device, the input power of the transformer remains equal to the output power of the transformer (minus any internal losses). The equation below demonstrates this relationship. 𝑃 𝑃𝑟𝑖𝑚𝑎𝑟𝑦 = 𝑃 𝑆𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 and 𝑉 𝑃 𝐼 𝑃 = 𝑉 𝑆 𝐼 𝑆 Where: 𝑃 𝑃𝑟𝑖𝑚𝑎𝑟𝑦 = Input power 𝑃 𝑆𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 = Output power 𝑉 𝑃 = Primary voltage 𝑉 𝑆 = Secondary voltage 𝐼 𝑃 = Primary current 𝐼 𝑆 = Secondary current ELO 3.3

Transformer Windings Knowledge Check
Match the terms to their appropriate definitions. Correct answers are:1-C, 2-D, 3-A, and 4-B. Voltage on the primary side divided by voltage on the secondary side Step-up transformer Ratio of primary turns to secondary turns Step-down transformer A transformer with higher secondary voltage than primary voltage Voltage ratio A transformer with lower secondary voltage than primary voltage Turns ratio Correct answers are: 1-C, 2-D, 3-A, and 4-B ELO 3.3

Transformer Application
ELO State the applications of each of the common types of transformers: distribution transformer, power transformers, control transformers, auto transformers, isolation transformers, instrument potential transformers, and instrument current transformers. Transformers construction matches a transformer’s characteristics to its intended application Differences in construction: May involve the size of the windings Relationship between the primary and secondary windings Transformer types are also designated by function the transformer serves in a circuit an isolation transformer ELO 3.4

Distribution Transformer Extensively used in AC electrical distribution Has the highest power, or volt-ampere ratings, and the highest continuous voltage rating Cooling method determines the power rating Some transformers use oil or other heat-conducting material to remove heat Others use forced air cooling (fans) Ampere rating is increased by increasing the size of the primary and secondary windings Voltage ratings are increased by increasing the voltage rating of the insulation ELO 3.4

Power Transformers Used in electronic circuits Generally have a rating of 300 volt-amperes and below Normally provide power to power supply circuit of an electronic device Power amplifier in an audio receiver is an example of this type of transformer Control Transformers Used on electronic circuits that require constant voltage or constant current with a low power or volt-amp rating Various filtering devices, such as capacitors, minimize variations in the output of these types of transformers, resulting in a more constant voltage or current ELO 3.4

Auto Transformers Used in low power applications where a variable voltage is required Special type of power transformer, because it consists of only one winding Tapping or connecting at differing points on the winding yield different voltages Figure: Auto Transformer Schematic ELO 3.4

Isolation Transformers Low power transformers used to isolate noise from or to ground Any DC voltage present in the circuit (such as noise) will not pass through; therefore, the transformer acts to isolate this noise Application in electrical circuits to ensure that a fault developed in one portion of the circuit will not affect another portion no direct electrical connection between the primary and secondary windings Instrument Potential Transformers (PT) PT steps down the voltage of an electrical circuit to a low value Can effectively and safely be used for the operation of instruments such as ammeters ELO 3.4

Instrument Current Transformers (CT) Steps down the current of a circuit to a lower value Same types of equipment as the potential transformer use this type of transformer Secondary winding is a coil consisting of many turns of wire, wound around the primary coil, which contains only a few turns of wire Allows measurements of high values of current ELO 3.4

Instrument Current Transformers (CT) Because of design, it is necessary to follow a special procedure when operated at no load Current transformer should always be short-circuited when not connected to an external load The magnetic circuit design of a current transformer is for low magnetizing current when under load A large increase in magnetizing current will result in the build up a large flux in the magnetic circuit This will cause the transformer to act as a step-up transformer, inducing an excessively high voltage in the secondary when under no load ELO 3.4

Knowledge Check Match the terms to their appropriate definitions. Correct answers are 1-C, 2-A, 3- B, and 4-D. Used in electrical power distribution and transmission systems Isolation transformer Sometimes used in electrical circuits to ensure that a fault developed in one portion of the circuit will not affect another portion of the circuit Auto transformer Consists of only one winding Distribution transformer Steps down the voltage of an electrical circuit to a low value that can be effectively and safely used for the operation of instruments Instrument potential transformer Correct answers are: 1-C, 2-A, 3- B, 4-D ELO 3.4

Electrical Distribution & 3-Phase Systems
TLO 4 - Describe basic industrial electrical distribution, including typical wiring schemes used and the advantages of three-phase systems. 4.1 Define common terms associated with electrical distribution systems and wiring schemes used in these systems. 4.2 Describe the design of a basic industrial electrical distribution system. 4.3 Describe the two methods of connecting single-phase loads to a three-phase power source and the advantages of three-phase systems. 4.4 Given a diagram of a wye or delta-connected three-phase system, describe the voltage/current relationships of the circuit. 4.5 State the indications of an unbalanced load in a three-phase power system. 4.6 Describe the purpose of common power distribution schemes. TLO 4

Electrical Distribution Terminology
ELO Define common terms associated with electrical distribution systems and wiring schemes used in these systems. Electrical Distribution Terminology Electrical distribution systems include numerous types of devices that perform a specific function within the distribution system The following is a list of common devices found in electrical distribution systems and their definitions Area Substation – receives power from the site distribution system for use in a particular facility. A substation typically contains a switch, a transformer and a circuit breaker Distribution Substation – a substation that receives high voltage transforms it down to a lower voltage and distributes it to electrical loads via circuit breakers ELO 4.1

Load Center – receives power from a substation and supplies power to facility motor control centers and larger electrical loads Motor Control Center – receives 480 VAC power and distributes it to individual process loads and other electrical panels via circuit breakers, controllers, fuses, etc. Contactor – an electro-mechanical device that controls power to a piece of equipment, normally associated with motor controllers Panel boards – small distribution panels that contain numerous molded-case circuit breakers, usually provides power to 208 VAC or 120 VAC loads Tie-Breaker – circuit breaker used to tie two electrical busses together Main Breaker – circuit breaker used to connect bus bars of switchgear assemblies to the output of transformers ELO 4.1

Feeder Breaker – circuit breaker used to receive power from a switchgear bus bar, and direct the power to downstream electrical loads Switchgear – an assembly of circuit breakers electrically connected to a system of electrical busses (solid copper conductors) Normal Power – power received from the normal supply source at an industrial facility; normal power may be generated on-site or purchased from a commercial utility Standby Power – power source that comes on line upon a loss of normal power; a diesel generator or by battery backup power via a UPS may supply standby power Vital/Essential Loads – loads requiring constant power to ensure that a facility can operate safely ELO 4.1

Wiring Scheme Terminology To understand wiring schemes used in power distribution systems, familiarity with the following terms is required Ampacity – the maximum sustained current (in amperes) that a conductor can carry while remaining within its temperature rating Bond – the permanent joining of metallic parts or circuits assuring electrical continuity, and safe current conductance for any expected current Conductor – any wire, cable, or substance that is capable of carrying an electrical current Ground – a conducting connection, whether intentional or accidental, between a circuit or piece of equipment and the earth, or a body serving as earth, which has zero electrical potential ELO 4.1

Wiring Scheme Terminology (continued) Leg – a current-carrying conductor intended to deliver power to or from a load normally at an electrical potential other than ground Neutral – a current-carrying conductor normally tied to ground so that the electrical potential is zero Phase voltage – the greatest root mean square (effective) difference of potential between any two legs of the circuit ELO 4.1

Knowledge Check Match the terms with their appropriate definition. Correct answers are: 1-A, 2-C, 3-D, and 4-B. An electro-mechanical device that controls power to a piece of equipment. Contactor Receives power from a substation and supplies power to facility motor control centers and larger electrical loads. Feed breaker Small distribution panels which contain numerous molded-case circuit breakers. Usually provide power to 208 VAC or 120 VAC loads. Load center Circuit breaker, which receives power from a switchgear buss bar and directs the power to downstream electrical loads. Panel Correct answers are: 1-A, 2-C, 3-D, 4-B ELO 4.1

Basic Electrical Distribution System
ELO Describe the design of a basic industrial electrical distribution system. Electrical Distribution System Provides the power for process loads such as pumps, fans compressors, lighting, instrumentation, heating ventilation and air conditioning, and operating and control circuits It also provides the means to connect and disconnect electrical power to these loads via cables, wires and various types of protective devices Sources of backup power such as emergency diesel generators, batteries, and uninterruptible power supplies (UPS) help ensure constant power to vital components ELO 4.2

A basic electrical distribution system consists of three parts: A generating system A transmission system A distribution system Figure: Typical Industrial Electrical Distribution System ELO 4.2

Generating System Typical electrical systems consist of sources of power, and associated step-up, and step-down transformers Sources of power include local commercial (purchased) power system or on-site electrical generation Power falls into two categories: normal and standby power Normal power is the power normally supplied to the facility’s electrical equipment Standby power sources are generally from an on-site diesel generator or from backup batteries ELO 4.2

Transmission System Includes high-voltage cables, power poles, and switching stations Connect the generating system to the distribution system Forms a grid that extends across the industrial facility Voltage can be very high (115 kVAC is common) to limit power losses Limits power lost to the resistance of the electrical cables Distribution System Consists of one or more electrical substations The incoming high voltage from the transmission system goes through transformers down to a lower voltage for use by the facility’s electrical loads 13.8 kVAC or 480 VAC ELO 4.2

Substations Transform incoming high voltage down to usable value for the facility Distribute it to electrical equipment throughout the facility Transformation takes two steps Substation distribution transformers transform voltage from 115 kVAC to an intermediate value of voltage such as 13.8 kVAC This intermediate voltage may power some large facility loads directly In order to obtain the lower voltage (480 VAC) required by most facility electrical loads, a second substation transforms the kVAC again, to 480 VAC From the 480 VAC substation, a switchgear lineup distributes electrical power to various facility load centers, motor control centers, and individual electrical loads ELO 4.2

Load Centers Switchgear is a term used to describe a group of circuit breakers tied to a common electrical bus Electrical buss is a copper bar (single phase) or set of copper bars (three-phase) used to connect the switchgear to the distribution transformer and to its associated breakers This type of switchgear is a load center typically equipped with a main supply breaker to receive power from a distribution transformer several feeder breakers to supply power to various motor control centers in the facility and larger electrical loads Electrical switchgear is often provided with meters and indicating devices which can be used by facility operators to monitor distribution system performance ELO 4.2

Motor Control Centers (MCCs) Act as centralized distribution and control points for various 480 VAC components and loads Receive power from a load center and distribute this power to individual electrical loads via breakers, fuses, motor controllers, etc. May also feed lighting and instrumentation loads via transformers (480 VAC to 208/120 VAC) Like load centers, MCCs are equipped with monitoring and instrumentation devices such as voltmeters, ammeters, etc. ELO 4.2

Basic Electrical Distribution
Knowledge Check Match the sub-system of the electrical distribution system with the appropriate definition. Correct answers are: 1-C, 2-D, 3-A, and 4-B. One or more electrical substations, where the incoming high voltage is transformed down to a lower voltage Substations Includes the high-voltage cables, power poles, and switching stations forming a “grid” which extends across the industrial facility Load centers Transform incoming high voltage down to a value usable by the facility and distribute it to electrical equipment throughout the facility Distribution system Equipped with a main supply breaker to receive power and several feeder breakers to supply power to various motor control centers in the facility and larger electrical loads Transmission system Correct answers are: 1-C, 2-D, 3-A, 4-B ELO 4.2

Single-Phase Connections
ELO Describe the two methods of connecting single-phase loads to a three-phase power source and the advantages of three-phase systems. Single-Phase Load Connections The source of single-phase power in all facilities is by generation from a single-phase generator or by utilization of one phase of a three-phase power source Each phase of the three-phase distribution system is a single-phase generator electrically spaced 120 degrees from the other two Therefore, a three-phase power source is convenient and practical to use as a source of single-phase power ELO 4.3

Single-phase loads can connect to three-phase systems utilizing two methods Phase-to-ground scheme (part A) provides connection of the load from a phase leg to a ground point Phase-to-phase scheme (part B) connects the single-phase load between any two legs of the three-phase source The choice of schemes, allows several voltage options depending on whether the source system is a three-phase delta or wye configuration Figure: Three-Phase to Single-Phase Connection ELO 4.3

Advantages of Three-Phase Systems The design is more efficient at producing and utilizing an AC voltage Combination of three single-phase systems Power comes from a three-phase AC generator that produces three separate and equal voltages Each voltage is 120° out of phase with the other two voltages Figure: Three-Phase AC ELO 4.3

Single-Phase Load Connections The source of single-phase power is Generation from a single-phase generator Utilization of one phase of a three-phase power source Each phase of the three-phase distribution system is a single-phase generator electrically spaced 120 degrees from the other two Therefore, is convenient and practical to use as a source of single-phase power ELO 4.3

Knowledge Check Which of the following are common three-phase to single-phase connection methods? (select all that are correct). Phase to line Phase to ground Phase to phase Neutral to ground Correct answers are B and C. Correct answer is B and C. ELO 4.3

Wye and Delta-Connected
ELO Given a diagram of a wye or delta-connected three-phase system, describe the voltage/current relationships of the circuit. Wye-Connected The three common ends of each phase connect at a common point (neutral) The other three ends connect to a three-phase line Three-phase systems can connect in two different ways Wye (Y) Delta (Δ) Figure: Wye-Connected ELO 4.4

The three phases connect in series to form a closed loop Figure: Delta Connected ELO 4.4

Balanced Three-Phase System A three-phase system, that has identical impedance in each secondary winding, has balanced loads The impedance of each winding in a delta load is shown as Z∆ The impedance in a wye load is shown as ZY (part b) For either the delta or the wye connection, the lines A, B, and C supply a three-phase system of voltages Figure: Three-Phase Balanced Loads ELO 4.4

Voltage and Current in Delta-Connected Systems The table below contains the formulas for calculating line and phase voltage and current for delta-connected systems Delta-Connected Systems Formula Line Current 𝐼 𝐿 = 3 𝐼 𝑝ℎ𝑎𝑠𝑒 Line Voltage 𝑉 𝐿 = 𝑉 𝑝ℎ𝑎𝑠𝑒 ELO 4.4

Voltage and Current in a Wye-Connected System The table below contains the formulas for calculating line and phase voltage and current in a Wye-connected system Phase Power Because the impedance of each phase of a balanced delta or wye system has equal current, phase power is one third of the Total Power 𝑃 Φ = 𝑉 Φ 𝐼 Φ 𝑐𝑜𝑠 𝜃 Delta-Connected Systems Formula Line Current 𝐼 𝐿 = 𝐼 𝑝ℎ𝑎𝑠𝑒 Line Voltage 𝑉 𝐿 = 3 𝑉 𝑝ℎ𝑎𝑠𝑒 ELO 4.4

Total Power Total Power (PT) is equal to three times the single-phase power PT = 3VΦIΦcosθ Delta-Connected System 𝑉 𝐿 = 𝑉 𝑝ℎ𝑎𝑠𝑒 and 𝐼 𝑝ℎ𝑎𝑠𝑒 = 3 𝐼 𝐿 3 In a delta-connected system, so: 𝑃 𝑇 = 3 𝑉 𝐿 𝐿 𝐿 cos 𝜃 Wye-Connected System 𝐼 𝐿 = 𝐼 𝑝ℎ𝑎𝑠𝑒 and 𝑉 𝑝ℎ𝑎𝑠𝑒 = 3 𝑉 𝐿 3 In a wye-connected load, so: 𝑃 𝑇 = 3 𝑉 𝐿 𝐿 𝐿 cos 𝜃 The above equations demonstrate that Total Power formulas for delta- and wye-connected systems are identical ELO 4.4

Apparent Power and Reactive Power Total Apparent Power (ST) in volt-amperes and total Reactive Power (QT) in volt-amperes- reactive are related to total True Power (PT) in watts A balanced three-phase system has True, Apparent, and Reactive powers given by the following equations: 𝑃 𝑇 = 3 𝑉 𝑇 𝐼 𝐿 cos 𝜃 𝑆 𝑇 = 3 𝑉 𝑇 𝐼 𝐿 𝑄 𝑇 = 3 𝑉 𝑇 𝐼 𝐿 sin 𝜃 Figure: Power Triangle ELO 4.4

Calculating Voltage and Current Demonstration In a Wye-connected power system, phase voltage is 150 volts, and phase current is 5 amps Calculate line voltage and line current Solution: 𝐿𝑖𝑛𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡=𝑃ℎ𝑎𝑠𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡=5 𝑎𝑚𝑝𝑠 𝐿𝑖𝑛𝑒 𝑣𝑜𝑙𝑡𝑎𝑔𝑒= 3 𝑃ℎ𝑎𝑠𝑒 𝑣𝑜𝑙𝑡𝑎𝑔𝑒=260 𝑣𝑜𝑙𝑡𝑠 ELO 4.4

Knowledge Check A three-phase power system has the following parameters. Line current 12 amps. Phase current 12 amps. This system is a _____________________. DC system Delta-connected system Wye-connected system X-connected system Correct answer is C. Correct answer is C. ELO 4.4

Unbalanced Loads 3-Phase
ELO State the indications of an unbalanced load in a three-phase power system. Unbalanced Loads An important property of a three-phase balanced system is that The phasor sum of the three line or phase voltages is zero The phasor sum of the three line or phase currents is zero When the three load impedances are not equal to one another, the phasor sums and the neutral current (In) are not zero, and the load is unbalanced An imbalance occurs when an open or short circuit appears at the load ELO 4.5

Unbalanced Loads 3-Phase
In a fault condition, the neutral connection in a wye-connected load will carry more current than the phase under a balanced load Abnormally high currents in one or more of the phases indicate an unbalanced three-phase circuits If allowed to continue, this may cause damage to equipment Figure: Three-Phase Systems ELO 4.5

Knowledge Check A three-phase Wye-connected system with an unbalanced load will have a neutral current of zero. False True Correct answer is A. Correct answer is A - False. ELO 4.5

Power Distribution Schemes
ELO Describe the purpose of common power distribution schemes. Three-Wire, Single-Phase Edison System 3-wire, single-phase Edison system – approved method of wiring single-phase power from a three-phase system The illustration depicts a center-tapped transformer providing half voltage (120 V) connections on either side or full voltage (240 V) across both sides Figure: Three-Wire Edison Scheme ELO 4.6

The physical connections to the secondary involve Two insulated conductors – current-carrying legs or neutral legs requiring insulation One bare conductor – serves as a safety ground, bonded to the ground point of the system Safety ground connects to each junction box, or device, in the system For (120 V) use, the intended path of the current is from the supply leg through the load and back to the source via the neutral leg Ground carries no current unless a fault occurred in the system Figure: Three-Wire Edison Scheme ELO 4.6

For (240 V), the insulated conductors are connected across the full winding of the transformer, and the un-insulated conductor is again bonded to the grounded center tap In a balanced system, all currents will flow on the insulated conductors In case of either an unbalanced load or a fault in the system, the bare conductor will carry current, but the potential will remain at zero volts because it connects to the ground point Figure: Three-Wire Edison Scheme ELO 4.6

Three-Phase Wiring Schemes The three-phase system needs neither a separate neutral nor a ground to operate safely To prevent an unsafe condition, all 3 and 4-wire, three-phase systems can include an effective ground path Three-Wire, Three-Phase Delta System The simplest three-phase system is the 3-wire Delta configuration Normally used for transmission of power in the intermediate voltage class from approximately 15,000 volts to 600 volts Figure: Three-Wire, Three-Phase Delta Scheme ELO 4.6

Electrical Distribution System
Upper diagram is an ungrounded Delta Lower diagram shows a ground point affixed to one corner of the Delta Lowers one phase’s reference to ground, but retains a phase-to- phase voltage Similar to the grounded neutral of the single-phase Edison system Has economy in wiring costs Grounded phase can physically protect the other two phases from accidental grounding or lightning strikes Rarely used for low voltage (under 600 V), however, because of the absence of a safety ground Figure: Three-Wire, Three-Phase Delta Scheme ELO 4.6

Four-Wire, Three-Phase Delta System Combines the ungrounded Delta discussed above for three-phase loads with the Edison system for single-phase loads One side of the Delta has a grounded-neutral conductor connected to a center tap winding on one phase Provides the same half-or full-voltage arrangement seen in the normal Edison scheme with a grounded neutral At any location in the system, either three-phase power at full voltage or single-phase power with half or full voltage is equally possible Figure: Four-Wire Delta System ELO 4.6

Four-Wire, Three-Phase Delta System Several strict procedures are required in system operation Carefully balance all loads on both the single-phase and three- phase legs Measurement between the neutral and the phase must be taken to identify the high leg, or abnormal voltage Voltage between one leg and grounded neutral is considerably higher than the rest of the system Never use the high leg as a single-phase source because no ground or grounded neutral exists for this circuit Figure: Four-Wire Delta System ELO 4.6

4-Wire, Three-Phase Wye System Completely different voltage characteristics from the Delta system In the Wye system, the ground voltage or voltage available from phase to ground is the phase voltage divided by 1.73 Depending on the selection of conductors, one of the following is available: Reduced-voltage single phase between a phase leg and neutral Full-voltage single-phase circuit between any two phase legs Full-voltage three-phase power Figure: Four-Wire, Three -Phase Wye System ELO 4.6

4-Wire, Three-Phase Wye System Size the full load ampacity of the neutral to 1.73 times the highest phase ampacity Avoids Over-current condition if a fault is present Operation of single-phase loads at reduced voltage if an accidental interruption causes severely unbalanced loads This system is the safest possible, multi-purpose distribution system for low voltage and is common in the 208/120-volt range. Figure: Four-Wire, Three -Phase Wye System ELO 4.6

Knowledge Check The _________________________ is normally confined to protected environments such as fully enclosed ducts or overhead transmission lines that cannot be reached by personnel without extraordinary means. 4-wire, three-phase delta system Ungrounded delta system Edison system 4-wire, three-phase Wye system Correct answer is B. Correct answer is B ELO 4.6

Electrical Test Equipment
TLO 5 - Given an electrical measuring device or piece of test equipment, describe the use of that equipment including the electrical parameter measured. 5.1 Describe the use of the following electrical test meters: ground detector, multimeter, megger, and synchroscope. 5.2 Describe the operation and electrical parameter measured of the following electrical test equipment: ground detector, multimeter, megger and synchroscope. TLO 5

Use of Common Test Equipment
ELO Describe the use of the following electrical test meters: ground detector, multimeter, megger, and synchroscope. Ground Detectors Instrument used to detect conductor insulation resistance to ground An ohmmeter, or a series of lights, can detect the insulation strength of an ungrounded distribution system The grounded varieties comprise most power distribution systems in use today; however, some ungrounded systems are still used Multimeter Portable, single instrument capable of measuring various electrical values including Voltage Resistance Current ELO 5.1

Use of Common Test Equipment
Meg-Ohm Meter (Megger) Portable instrument used to measure insulation resistance Hand-driven (DC generator and a direct reading ohmmeter) and newer digital versions Synchroscope Indicates when two AC generators are in the correct phase relation for connecting in parallel Shows whether the incoming generator is running faster or slower than the on-line generator Newer model Meg-Ohm meters are digital and do not require manual cranking to generate a voltage. ELO 5.1

Knowledge Check Parameters normally measured by installed meters include _________. (select all that are true) hysteresis power (watts) voltage Current (amps) Correct answers are B, C, and D. Correct answer is B, C, and D ELO 5.1

Operation of Common Test Equipment
ELO State the electrical parameters measured by a ground detector, multimeter, megger and synchroscope. Ground Detectors Measures insulation resistance to ground in ohms Ohmmeter Method of Ground Detection DC voltage is applied to the conductor, if a leakage path exists between the conductor insulator and ground, a current will flow through the ground to the ohmmeter proportional to the insulation resistance of the conductor Figure: Simple Ohmmeter Ground Detector ELO 5.2

Lamp-Type Ground Detector Set of three lamps connect through transformers to the system to check for grounds The switch is closed and the brilliance of the lamps observed Lamps are equally bright: no ground, all the lamps receive the same voltage One of the three lamps is dark, and the other two lamps are brighter: the darkened lamp has a short to ground Figure: Lamp-Type Ground Detector Circuit ELO 5.2

Multimeter Most commonly used is the volt-ohm-milliammeter (VOM) A single meter movement indicates current, AC and DC voltage, and resistance These instruments can be analog or digital devices Range switches provided for scale selection on analog-type Many digital multimeters are auto-ranging and will automatically respond to indicate the measured parameter on the correct scale Voltmeters By placing a resistor (RS), called a multiplier, in series with the ammeter movement, and marking the meter face to read voltage as shown in next slide Voltmeters are connected in parallel with the load (RL) being measured ELO 5.2

The voltmeter draws current from circuit causing a voltage drop across the resistance of the meter Meter subtracts this voltage drop from the voltage measured Loading effect – can have a serious effect on voltage measurement accuracy, especially for low current circuits Countered by constructing with an extremely high resistance to limit the current flow through the voltmeter The accuracy of a voltmeter is the ratio of measured voltage when the meter is in the circuit to the voltage measured with the meter out of the circuit Figure: Simple DC Voltmeter ELO 5.2

Ammeter Operation Measures electric current It may read in units of amperes, milliamperes, or microamperes The ammeter must be in series with the circuit to be tested, in order to measure current Figure: Ammeter ELO 5.2

When ammeter is in series with a circuit, it will increase the resistance of that circuit by an amount equal to the internal resistance of the meter Rm The equation without the meter installed is: 𝐼 𝑜 = 𝑉 𝑅 𝑜 The next equation is the mathematical representation of the current with the meter installed in the circuit: 𝐼 𝑤 = 𝑉 𝑅 𝑜 + 𝑅 𝑚 The accuracy of the ammeter KA is the ratio of the current when the meter is in the circuit, Iw, to the current with the meter out of the circuit, Io ELO 5.2

Ammeter Shunts An ammeter with a full scale current deflection (Im) can be shunted with a resistor (RSH) in order to measure currents in excess of full scale deflection current (Im) as shown below The reason for shunting an ammeter is to extend the range of the ammeter and, thereby, measure currents higher than the original full scale value Figure: Ammeter with Shunt Installed ELO 5.2

Ohmmeters When used as a test device, an ohmmeter aids the troubleshooter in determining if a ground or a short exists in a circuit A simple ohmmeter consists of a battery, a meter movement calibrated in ohms, and a variable resistor To obtain an accurate measurement of a component’s resistance, connect the ohmmeter to a component removed from the circuit If the component remains in the circuit, and a parallel path exists in the circuit, the current will flow in the path of least resistance and give an erroneous reading Ro is used to zero the ohmmeter and correct for battery aging It also helps to limit current along with Rm Zeroing the ohmmeter is accomplished by shorting the ohmmeter terminals a and b and adjusting Ro to give full-scale deflection Figure: Simple Ohmmeter Circuit ELO 5.2

Meg-Ohm Meter (Megger) Consists of two coils, A and B rigidly mounted to a pivoted central shaft, free to rotate over a C-shaped core Coils connect by means of flexible leads Current is provided by the hand-driven generator through coil B With the test terminals open (infinite R) no current flows in coil A Thereby, coil B governs the motion causing it to move to the extreme counter-clockwise position, which is marked as infinite resistance. Newer model Meg-Ohm meters do not require manual cranking. Figure: Simple Megger Circuit ELO 5.2

Meg-Ohm Meter (Megger) With the terminals marked line and earth shorted (zero R) the current flow through the coil A is sufficient to produce enough torque to overcome the torque of coil B The pointer then moves to the extreme clockwise position, which is marked as zero resistance Resistance (R2) will protect coil A from excessive current flow in this condition Newer model Meg-Ohm meters do not require manual cranking. Figure: Simple Megger Circuit ELO 5.2

Meg-Ohm Meter (Megger) When the test terminals, line and earth, connect across an unknown resistance, the opposing torques of coils A and B balance each other so that the instrument pointer comes to rest at some point on the scale The scale is calibrated such that the point indicates the value of resistance being measured, usually in MΩ Newer model Meg-Ohm meters do not require manual cranking. Figure: Simple Megger Circuit ELO 5.2

Synchroscope Consists of a two-phase stator Windings are at right angles to one another Current in one phase leads the current of the other phase by 90°, thereby generating a rotating magnetic field The stator windings connect to the incoming generator, and a polarizing coil connects to the running generator The rotating element is unrestrained and is free to rotate through 360° It consists of two iron vanes mounted in opposite directions on a shaft, one at the top, and one at the bottom, magnetized by the polarizing coil ELO 5.2

Synchroscope If the frequencies of the incoming and running generators are different, the synchroscope will rotate at a speed corresponding to the difference If incoming frequency is higher than running frequency, it will rotate in the clockwise direction If incoming frequency is less than running frequency, it will rotate in the counterclockwise direction When the synchroscope indicates 0° phase difference, the pointer is at the 12 o’clock position and the two AC generators are in phase ELO 5.2

Knowledge Check A ground detector measures __________________________. insulation resistance to ground difference in phase currents difference in phase voltage current on the neutral connection Correct answer is A. Correct answer A, insulation resistance to ground ELO 5.2

Operator Generic Fundamentals Basic Electricity - Part 2

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