NCCER Electrcal L2M/1 MIX

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Slide 2  Objectives
 When trainees have completed this lesson, they should be able to do the following:
 1. Calculate the peak and effective voltage or current values for an AC waveform.
 2. Calculate the phase relationship between two AC waveforms.
 3. Describe the voltage and current phase relationship in a resistive AC circuit.
 4. Describe the voltage and current transients that occur in an inductive circuit.
 5. Define inductive reactance and state how it is affected by frequency.
 6. Describe the voltage and current transients that occur in a capacitive circuit.
 7. Define capacitive reactance and state how it is affected by frequency.
 8. Explain the relationship between voltage and current in the following types of AC circuits:
 • RL circuit • LC circuit
 • RC circuit • RLC circuit
 9. Explain the following terms as they relate to AC circuits:
 • True power • Reactive power
 • Apparent power • Power factor
 10. Explain basic transformer action.
 This is a knowledgebased module; there are no Performance Tasks.
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Slide 3  1.0.0 – 2.0.0
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 Introduction; Sine Wave Generation
 • An electric current in a conductor creates a magnetic field surrounding that conductor.
 • Relative motion between the conductor and a magnetic field creates a voltage in the conductor.

Slide 4  1.0.0 – 2.0.0
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 Angle Versus Rate of Cutting Lines of Flux
 • Lines of flux run from north to south in a generator.
 • Voltage is produced when a conductor cuts through the lines of flux. It is at zero when parallel to the lines of flux and at its maximum when perpendicular to the lines of flux.

Slide 5  1.0.0 – 2.0.0
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 One Cycle of Alternating Voltage

Slide 6  3.0.0 – 3.5.0
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 Sine Wave Terminology
 • The frequency of a waveform is the number of times per second an identical pattern repeats itself. Frequency is measured in Hertz (Hz).
 • The period of a waveform is the time (t) required to complete one cycle.

Slide 7  3.0.0 – 3.5.0
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 Amplitude Values for a Sine Wave
 • A sine wave has a peak voltage of 170V.
 • The rootmeansquare or RMS value is 0.707 times the peak value of 170V (approx. 120V), while the average value is 0.637 times the peak (approx. 108V).
 Next Session…
 AC Phase Relationships

Slide 8  4.0.0 – 4.2.0
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 AC Phase Relationships
 • Phase relationships can be compared for voltages with the same frequency.
 • When voltages are in phase, they reach their maximum and minimum values at the same time.
 • When voltages are 90 degrees out of phase, one is at zero while the other is at its maximum value.

Slide 9  4.0.0 – 4.2.0
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 Waves in Phase
 • A vector diagram represents the phase angle between two waveforms.
 • If a vector diagram is flat, the voltages are in phase. If it shows a right angle, the voltages are 90 degrees out of phase.
 Next Session…
 Nonsinusoidal Waveforms

Slide 10  5.0.0
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 Nonsinusoidal Waveforms
 • Any waveform that is not a sine or cosine is known as a nonsinusoidal waveform.
 • The cycle is measured between two points having the same amplitude and varying in the same direction.

Slide 11  6.0.0
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 Resistance in AC Circuits
 • In an AC circuit with only resistance, the current variations are in phase with the applied voltage.
 • The circuit values can be found using Ohm’s law, where I (current) = E (voltage)/R (resistance).

Slide 12  6.0.0
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 Voltage and Current in a Resistive AC Circuit
 • In a resistive circuit, the cycles begin and end at the same time and their peaks occur at the same time.
 • The current value depends on the applied voltage and the resistance.

Slide 13  6.0.0
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 What’s wrong with this picture?

Slide 14  7.0.0 – 7.3.0
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 Inductance in AC Circuits
 • Inductance is the circuit characteristic that opposes the change of current flow and is measured in henrys (H).
 • The induced current opposes the current flow that generated it.

Slide 15  7.0.0 – 7.3.0
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 Inductor Voltage and Current Relationship
 • In an inductive circuit, the voltage leads the current by 90 degrees, which means that the voltage begins at its maximum positive value.
 • The opposing force to the flow of AC current presented by an inductor is known as inductive reactance.
 Next Session…
 Capacitance

Slide 16  8.0.0 – 8.5.0
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 Capacitance
 Capacitance is the ability to store a charge.

Slide 17  8.0.0 – 8.5.0
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 Charging and Discharging a Capacitor
 • The capacitor begins to charge when voltage is applied to the circuit and stops when the potential difference across the capacitor is equal to the applied voltage.
 • Capacitance is measured in farads (F).
 • DC is blocked by a capacitor.

Slide 18  8.0.0 – 8.5.0
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 Capacitors in Parallel
 • When connecting capacitors in parallel, the total capacitance is calculated by adding the individual values.
 • The voltage is the same across the parallel capacitors.

Slide 19  8.0.0 – 8.5.0
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 Capacitors in Series
 • Connecting capacitors in series is equivalent to increasing the thickness of the dielectric. Therefore, the total value is smaller than any of the individual values.
 • The voltage across each capacitor in series is inversely proportional to its capacitance.

Slide 20  8.0.0 – 8.5.0
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 Voltage and Current in a Capacitive AC Circuit
 • The voltage and current in a capacitive AC circuit follow a standard sine wave.
 • In a capacitive circuit, the current leads the voltage by 90 degrees.
 Next Session…
 LC and RLC Circuits

Slide 21  9.0.0 – 9.1.2
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 LC and RLC Circuits

Slide 22  9.0.0 – 9.1.2
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 Series RL Circuit and Vector Diagram

Slide 23  9.0.0 – 9.1.2
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 Series RL Circuit with Waveforms and Vector Diagram
 • In a series circuit, the higher the value of XL compared with R, the more inductive the circuit is.
 • The series current lags the applied generator voltage.

Slide 24  9.0.0 – 9.1.2
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 Series R and XL Combinations
 • A ratio of 10:1 (XL/R) indicates that the circuit is mostly inductive.
 • A ratio of 1:10 (XL/R) indicates that the circuit is mostly resistive.

Slide 25  9.0.0 – 9.1.2
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 Parallel RL Circuit with Waveforms and Vector Diagram
 • In a parallel circuit, the higher the value of XL compared with R, the more resistive the circuit is.
 • The resistive branch current has the same phase as the applied generator voltage. The inductive branch current lags the applied generator voltage by 90 degrees.

Slide 26  9.0.0 – 9.1.2
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 Parallel R and XL Combinations
 • A ratio of 10:1 (XL/R) indicates that the circuit is mostly resistive.
 • A ratio of 1:10 (XL/R) indicates that the circuit is mostly inductive.

Slide 27  9.2.0 – 9.2.2
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 RC Circuits
 • In a circuit with both resistance and capacitance, the current will lead the voltage by an angle of less than or equal to 90 degrees.
 • In a series RC circuit, the current is the same in both the resistance (R) and reactance (XC).

Slide 28  9.2.0 – 9.2.2
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 Series R and XC Combinations
 • A ratio of 10:1 (XC/R) indicates that the circuit is mostly capacitive.
 • A ratio of 1:10 (XC/R) indicates that the circuit is mostly resistive.

Slide 29  9.2.0 – 9.2.2
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 Parallel RC Circuit with Vector Diagrams
 • A parallel RC circuit has both a capacitive and a resistive branch connected across a voltage source.
 • The branch voltages are equal and in phase.
 • The current in each branch depends on the applied voltage and the resistance or capacitance contained in the branch.

Slide 30  9.2.0 – 9.2.2
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 Parallel R and XC Combinations
 • A ratio of 10:1 (XC/R) indicates that the circuit is mostly resistive.
 • A ratio of 1:10 (XC/R) indicates that the circuit is mostly capacitive.

Slide 31  9.3.0 – 9.3.2
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 LC Circuits
 • An LC circuit contains an inductance and a capacitance connected in series or in parallel with the voltage source.
 • The current in a series LC circuit is in phase and the same at all points. The voltage drop across the inductor leads the circuit current by 90 degrees.

Slide 32  9.3.0 – 9.3.2
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 Parallel LC Circuit with Vector Diagram
 • In a parallel LC circuit, the branch voltages are equal and in phase.
 • The current in a parallel LC circuit divides among the branches and is dependent on the voltage and the resistance or capacitance contained in the branch.

Slide 33  9.4.0 – 9.4.2
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 RLC Circuits
 • Circuits in which the inductance, capacitance, and resistance are all connected in series are known as series RLC circuits.
 • The current in a series RLC circuit is in phase and the same at all points. The voltage drops across the inductance and capacitance are 180 degrees out of phase.

Slide 34  9.4.0 – 9.4.2
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 Parallel RLC Circuit and Vector Diagram
 • In a parallel RLC circuit, the branch voltages are equal and in phase.
 • The current in a parallel RLC circuit divides among the branches and is dependent on the voltage and the resistance or reactance contained in the branch.

Slide 35  9.4.0 – 9.4.2
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 Think About It: AC Circuits
 This photo shows a simple series circuit composed of an On/Off switch, small lamp, motor, and capacitor. How would you classify this circuit? When energized, which components insert resistance, inductive reactance, and capacitive reactance into the circuit?

Slide 36  10.0.0 – 10.3.0
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 Power in AC Circuits
 • True power measures the power consumed by resistance and is measured in watts (W).
 • Apparent power is the product of the source voltage and current and is measured in voltamperes (VA).
 • Reactive power is that portion of the apparent power that is caused by inductors and capacitors. It is measured in voltamperesreactive (VARs).

Slide 37  10.4.0
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 Power Factor
 • Power factor is the ratio of true power to apparent power.
 • True power is the product of the resistor current squared and the resistance.
 • Apparent power is the product of the source voltage and the total current.

Slide 38  10.5.0
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 Power Triangle
 • The power triangle demonstrates the relationships among true power (W), apparent power (VA), and voltamperesreactive (VARs).
 • The Pythagorean theorem can be used to find unknown quantities in the power triangle.
 Next Session…
 Transformers

Slide 39  11.0.0 – 11.1.2
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 Transformers
 • A transformer transfers energy via induction.
 • A basic transformer contains a primary winding, a secondary winding, and a core.
 • Transformers are used to step voltage up or down.

Slide 40  11.0.0 – 11.1.2
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 Transformer Action
 • The primary winding is connected to an AC voltage source.
 • The expanding/contracting magnetic field around the primary winding induces a voltage into the secondary winding.

Slide 41  11.0.0 – 11.1.2
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 Steel Laminated Core
 • Common core materials include air, soft iron, and steel.
 • Laminated sheets of steel are used to reduce the power loss caused by eddy currents around the core.

Slide 42  11.0.0 – 11.1.2
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 Cutaway View of a Transformer Core
 • Transformers work on the principle of mutual induction. In order to operate, they require a conductor (the secondary winding), a magnetic field (generated in the primary winding), and relative motion (AC current).
 • The windings are electrically insulated using varnish and insulating paper or cloth.

Slide 43  11.2.0 – 11.2.2
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 Operating Characteristics
 • When voltage is applied to the primary but no load is connected to the secondary, a small amount of current flows in the primary, exciting the primary coil and creating a magnetic field.
 • The secondary voltage may be either in phase with the primary (likewound) or out of phase (unlikewound).

Slide 44  11.3.0
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 Turns and Voltage Ratios
 • A transformer turns ratio is the number of turns in the primary divided by the number of turns in the secondary.
 • To find the voltage induced in the secondary, use the equation EPNS = ESNP, where EP is the primary voltage, NS is the number of turns in the secondary, ES is the secondary voltage, and NP is the number of turns in the primary.

Slide 45  11.3.0
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 Think About It:Turns and Voltage Ratios
 What is the magnitude of the voltage and current supplied by the secondary of the transformer in the circuit shown below?

Slide 46  11.4.0 – 11.4.4
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 Types of Transformers
 • The primary and secondary coils of a transformer can be tapped to produce multiple input and output voltages.
 • A centertapped transformer can be used to convert an AC input into a DC output.

Slide 47  11.4.0 – 11.4.4
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 Importance of an Isolation Transformer
 • Isolation transformers have equal primary and secondary voltages, and are used to isolate a piece of equipment from the power distribution system.
 • Isolation transformers can be used to protect workers from electrical contact with an equipment chassis that has been wired into a circuit.

Slide 48  11.4.0 – 11.4.4
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 Autotransformer Schematic Diagram
 • An autotransformer uses a single coil of wire to produce both a primary and a secondary winding through a process of selfinduction.
 • Autotransformers are used as variable AC voltage supplies, in fluorescent ballasts, and in lowvoltage motor starters.

Slide 49  11.4.0 – 11.4.4
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 Current Transformer Schematic Diagram
 • In a current transformer, the primary is a conductor to the load and the secondary is a coil wrapped around the wire to the load.
 • Current transformers are connected in series and are used to power currentsensing meters and relays.

Slide 50  11.4.0 – 11.4.4
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 Potential Transformer
 • The primary of a potential transformer is connected across (in parallel with) the voltage to be measured.
 • Potential transformers are used to power voltagesensing meters and relays.
 Next Session…
 Wrap Up

Slide 51  Wrap Up
 321
 3 – Write 3 important things learned during class
 2 – Write 2 questions you have about the material
 1 – Write 1 thought you had about the material
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Slide 52  Next Session…
 MODULE EXAM
 Review the complete module to prepare for the module exam. Complete the Module Review as a study aid.
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