EET 251

Electricity and Electronics

 

 

Lesson Six

 

 

ALTERNATING-CURRENT ELECTRICITY

 

                               

            Much of the electrical energy used today is called alternating current (ac). Most of the electrical equipment and appliances used in homes operate from the alternating-current energy delivered by power lines. Alternating-current electricity has many applications in homes, industries, and commercial buildings. Electrical power plants in our country produce alternating current or ac electricity. Most power plants have huge steam turbines that rotate ac generators. These generators produce three-phase ac which is distributed by long-distance power transmission lines to the places where the electrical power is used. Industries and large commercial buildings use three-phase ac. Homes use single-phase ac power. Alternating current is the most common form of electrical energy in the Unites States.

 

6.1 Alternating-Current (AC) Voltage

Current direction with an ac source

Effective or measured ac value, converting ac values, rms value, average value

Sine wave, nonsinusoidal or complex waves, square wave

Harmonic

Spectrum analyser

Pulse

Amplitude

Leading edge, trailing edge, pulse width, pulse repetition time, pulse repetition frequency

 

6.2 Single-Phase and Three-Phase AC

Single-phase ac source construction

Definition of phase

Advantages of the three-phase ac systems over the single-phase ac system

 

6.3 Measuring AC Voltage

A multimeter and the method for measuring dc voltage can be used with these differences:

            (1) proper polarity does not have to be observed

            (2) ac voltage ranges and scales of the meter must be used

 

6.4 Using an oscilloscope

            Controls available on the oscilloscope

            Procedure used to adjust the oscilloscope controls to measure ac voltage

           

6.5 Ohm’s and Kirchhoff’s Laws for AC Circuits

Application of laws to ac circuits:

            (1) Ohm’s and Kirchhoff’s laws can be used with ac circuits containing resistance.

            (2) they are influenced by inductance and capacitance

 

6.6 Inductance

Definition and symbol for inductance

Impedance

 

6.7 CapacitanceDefinition of capacitance and capacitor

Measurement of the capacitor’s size in farads, microfarads and picofarads

 

6.8 Inductive Effects in Circuits

Definition of inductance (L), reactance (X)

Mutual inductance (M)

Formulas to find total inductance in series and paralle circuits

 

6.9 Capacitive Effects in Circuits

Definition of capacitance and its unit of measure

Factors that determine the capacitance of a capacitor

Formulas to find total capacitance for capacitors in series and capacitors in parallel

 

6.10 Leading and Lagging Currents in AC Circuits

Effect of using inductors and capacitors in ac circuits

 

6.11 Capacitor Charging and Discharging

What happens when a capacitor is charging and discharging

Safety precautions

 

6.12 Types of Capacitors

Fixed or Variable capacitors

Fixed:

            (1)  Paper capacitors

            (2)  Mica capacitors

            (3)  Oil-filled capacitors

            (4)  Ceramic capacitors

            (5)  Electrolytic capacitors

            (6)  Ultracapacitors (liquid and dry)

 

6.13 Capacitor Testing

Three types of capacitor problems: open capacitor, shorts or leaks

Equipment to test a capacitor

6.14 Alternating-Current Circuits

Resistive circuits and formulas to calculate V, I and R

Inductive circuits

Resistive-inductive (RL) circuits

Capacitive circuits

Resistive-capacitive (RC) circuits         

 

6.15 Vector Diagrams

Understanding the vector diagram and its use

 

6.16 Mathematics for AC Circuits

Right triangles and trigonometry: sine, cosine, tangent

Rectangular coordinates (or Cartesian coordinates)

Quadrants

Polar coordinates

Angular velocity

Complex numbers

Imaginary numbers

Rectangular form of complex numbers

Addition of complex numbers

Polar and trigonometric forms

 

6.17 Series AC Circuits

Finding impedance (Z) of a series ac circuit using an impedance triangle

Series RL circuits: finding impedance using formulas or an impedance triangle

Series RC circuits

Series RLC circuits: finding total reactance, reactive voltage

General procedure to solve series ac circuit problems

 

6.18 Parallel AC Circuits

Finding current using a current triangle

General procedure to solve parallel ac circuit problems

 

6.19 Power in AC Circuits

Finding power values using power triangles

Apparent power and true power

Power factor and formula

Unity power factor

Power in three-phase circuits

 

6.20 Filters and Resonant Circuits

Filter circuits

Resonant circuits

            (1) Series resonant circuits

            (2) Parallel resonant circuits

6.21 Transformers

Transformer operation

Types of transformer:

            (1) air-core, iron-core and powered metal-core

            (2) step-up and step-down transformers

            (3) multiple secondary transformers

            (4) autotransformers, variable autotransformers

            (5) current transformers

            (6) isolation transformers

Transformer efficiency

Transformer testing

 

 

REVIEW

1. What type of alternating current in used in homes?

2. What type of alternating current is used in industries?

3. What is a sine wave?

4. What are the formulas used to find the following ac values when effective value ac is known?

            a. Peak

            b. Peak-to-peak

5. What is the difference between “in phase” and “out of phase”?

6. What is the difference between measuring ac voltage and dc voltage?

7. What are some advantages of using oscilloscopes?

8. List and describe three controls used on oscilloscopes.

9. What is Ohm’s law for ac circuits?

10. What is inductance?

11. What is inductive reactance?

12. What is impedance?

13. What is capacitance?

14. What is capacitive reactance?

15. What is the phase relationship between voltage and current in an inductive circuit?

16. What is the phase relationship between voltage and current in a capacitive circuit?

17. What are three factors that affect capacitance?

18. How does a capacitor charge and discharge?

19. What are five types of capacitors?

20. Why are vector diagrams used?

21. Draw a voltage triangle and label each side with VR=10 V, VL= 20 V, and VC=15 V

22. Draw a power triangle for an ac circuit with VARL= 15, VARC= 32, and W=12

23. Draw a current triangle for a parallel ac circuit with IC= 8 A, IL=4A, and IR= 9A

24. What is the power factor of a circuit?

25. What is the phase angle of a circuit?

26. What are the three types of filter circuits and how do they differ?

27. What are two types of resonant circuits?

28. What is the difference between a filter circuit and a resonant circuit?

29. What is a time constant?

30. Discuss the operation of a transformer.

31. Why are isolation transformer used?

32. How is transformer efficiency determined?