PGT 105 ELECTRICAL ENGINEERING TECHNOLOGY CHAPTER 1 c

PGT 105 ELECTRICAL ENGINEERING TECHNOLOGY CHAPTER 1 (c) INTRODUCTION TO ELECTRICITY AND SUPPLY SYSTEM ~AC Three Phases Electric System ~ Lecturers: Mr. Khairul Affendi

Contents 1. Definition & comparison of AC Single Phase & Three Phases System. 2. Sine wave parameters: - Vrms, Vp-p, Vavg. 3. Typical connection between three phases generator and load: - Star-Wye (Y) & Delta system. 4. Comparison & Advantages of Single-Phase & Three. Phase Supply System Chapter 1(c)- AC Three Phases 2

Interest Approach • Does anyone have 3 phase current at home? • What would be an indication of 3 phase current? Chapter 1(c)- AC Three Phases 3

LESSON 1 Definition & comparison of AC Single Phase & Three Phases System. Chapter 1(c)- AC Three Phases 4

Definition & Comparison Let’s review the AC single phase system… Chapter 1(c)- AC Three Phases 5

Definition & Comparison And now, time for the AC three phase system… Chapter 1(c)- AC Three Phases 6

Definition & Comparison AC three phase system… • Three conductors carry the same single-phase sine wave power but reach their peak values at different times. • Seen that one of the other conductors is 1/3 of a cycle ahead of the reference and the other is 1/3 of a cycle behind the reference. • Got more constant power transfer over a cycle as compared to single-phase power & possible to produce a rotating magnetic field in a three-phase motor. Chapter 1(c)- AC Three Phases 7

Chapter 1(c)- AC Three Phases 8

Chapter 1(c)- AC Three Phases 9

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LESSON 2 Sine wave parameters: Vp, Vp-p, Vrms, Chapter 1(c)- AC Three Phases 11

Sine Wave Parameters Let’s review the AC single phase system… Chapter 1(c)- AC Three Phases 12

Chapter 1(c)- AC Three Phases 13

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LESSON 3 Comparison & Advantages of Single-Phase & Three-Phase Supply System Chapter 1(c)- AC Three Phases 15

Comparison & Advantages of Single -Phase & Three-Phase Supply System Advantage no. 1 • Three-phase current offers a steadier source of power. • Magnetic force which, causes motor rotation is strongest when current flow is at its peak in the cycle. Advantage no. 2 • The power delivered by a single-phase system pulsates and falls to zero. The three-phase power never falls to zero. The power delivered to the load in a three-phase system is the same at any instant. This produces superior operating characteristics for three-phase motors. Chapter 1(c)- AC Three Phases 16

Advantage no. 3 • Single-phase current peaks twice during one cycle, whereas, three-phase current peaks six times during one cycle. Advantage no. 4 • Three-phase motors are simpler, cheaper to buy and maintain, and safer to use around combustible materials since there is no sparking when they start. • This is because three-phase current allows a motor to be self-starting since it produces a rotating field of magnetism in the motor. • This eliminates the need for a separate starting winding, centrifugal switch, starting capacitor, or a system of brushes. • Chapter 1(c)- AC Three Phases 17

Advantage no. 5 • Three-phase motors are available in larger horsepower sizes than single-phase motors. • Current is supplied to the motor with three conductors rather than two. • This allowed the power supplied to larger threephase motors to be on the same size conductor as that required for smaller single-phase motors. Advantage no. 6 • A balanced three-phase, three-wire circuit with equal voltages uses 75% of the copper required for conductors. They supply a single phase two-wire circuit of the same capacity, voltage rating, length of circuit, and transmission efficiency. Chapter 1(c)- AC Three Phases 18

Advantage no. 7 • Three-phase motors are less expensive and usually lighter and smaller than single-phase motors of the same horsepower rating. • There is also a wider choice of enclosures available than for single-phase motors. Disadvantages • Three-phase current service is much more expensive to have than single-phase current at a local resident. • It requires more lines to be brought in and three transformers versus one transformer required for single-phase current Chapter 1(c)- AC Three Phases 19

LESSON 4 Typical connection/topology between three phases sources and load: • Star@Wye(Y) • Delta system. Chapter 1(c)- AC Three Phases 20

How are the sources connected? A three-phase system may be arranged in delta (∆) or star (Y) or wye • (a) shows the sources (phases) connected in a wye (Y). o Notice the fourth terminal, known as Neutral. • (b) shows the sources (phases) connected in a delta (∆). o Three terminals Chapter 1(c)- AC Three Phases 21

• Allows the use of two different voltages from all three phases, such as a 230/400 V system which provides 230 V between the neutral (centre hub) and any one of the phases, and 400 V across any two phases. Chapter 1(c)- AC Three Phases 22

Delta Source Vab = | Vab | 0 Vbc = Vab -120 Vca = Vab -240 • Arrangement only provides one voltage magnitude, however it has a greater redundancy as it may continue to operate normally with one of the three supply windings offline, albeit at 57. 7% of total capacity. Chapter 1(c)- AC Three Phases 23

Wye – Wye System Chapter 1(c)- AC Three Phases 24

Delta – Delta System Chapter 1(c)- AC Three Phases 25

Delta – Wye System Chapter 1(c)- AC Three Phases 26

Wye - Delta System Chapter 1(c)- AC Three Phases 27

Phase voltages are the voltages across phases. • For a Y load, phase voltage is measured from line to neutral (n) • For the 1(c)-generator, Chapter AC Three Phases EAN, EBN and ECN are the phase voltages. 28

Line voltages are the voltages between the lines A, B and C Chapter 1(c)- AC Three Phases 29

Phase/Line Voltages Line and phase voltages of the Yconnected three-phase generator. Chapter 1(c)- AC Three Phases Determining a line voltage for a three-phase generator. 30

Line/Phase Voltages for a Wye Circuit • Relationship between line and phase voltages • For a balanced Y system, the magnitude of line-to-line voltage is 1. 732 times the magnitude of the phase voltage. • Line to line voltage leads phase voltage by 30. Chapter 1(c)- AC Three Phases 31

CURRENTS Line currents are the currents in the line conductors. Phase currents are the currents through phases. For Y loads, line current and phase current are the same. Chapter 1(c)- AC Three Phases 32

Currents for a Wye Circuit • For Y loads, line current and phase current are the same. Line current Chapter 1(c)- AC Three Phases Phase current 33

Diagram of 3 -phase system Chapter 1(c)- AC Three Phases 34

Chapter 1(c)- AC Three Phases 35

Chapter 1(c)- AC Three Phases 36

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Look at a Y-Y System Chapter 1(c)- AC Three Phases 38

Definitions • Zg represents the internal generator impedance per phase • Zl represents the impedance of the line connecting the generator to the load • ZA, B, C represents the load impedance per phase • Zo represents the impedance of the neutral conductor Chapter 1(c)- AC Three Phases 39

Look at the Line and Load Voltages Chapter 1(c)- AC Three Phases 40

Line Voltages Chapter 1(c)- AC Three Phases Phase Voltages 41

Vector addition to find VAB=VAN-VBN Chapter 1(c)- AC Three Phases 42

Using the Tip-to-Tail Method -VBN VΦ = Line-to-Neutral, or Phase Voltage VAB = VAN – VBN = √ 3 VΦ Chapter 1(c)- AC Three Phases 43

Notes for Wye(Y) connection • The amplitude of the line-to-line voltage is equal to √ 3 times the amplitude of the phase voltage. • The line-to-line voltages form a balanced set of 3 phase voltages. • The set of line-to-line voltages leads the set of line-to -neutral (phase) voltages by 30°. Chapter 1(c)- AC Three Phases 44

Look at the Delta-Connected Load Chapter 1(c)- AC Three Phases 45

Chapter 1(c)- AC Three Phases 46

Line Currents Chapter 1(c)- AC Three Phases Phase Currents 47

Vector Addition to find Ia. A=IAB-ICA Chapter 1(c)- AC Three Phases 48

Using the Tip-to-Tail Method -ICA Ia. A = √ 3 IΦ/-30° Chapter 1(c)- AC Three Phases 49

Notes for the Delta Connection • The amplitude of the line current is equal to √ 3 times the phase current. • The set of line currents lags the phase currents by 30°. Chapter 1(c)- AC Three Phases 50

• • • Wye Connections In a wye system, the line voltage is higher than the phase voltage by a factor of the square root of 3 (1. 732). ELine = EPhase x 1. 732 EPhase = ELine / 1. 732 In a wye system, the line current is equal to the phase current. ILine = IPhase Chapter 1(c)- AC Three Phases 51

Line and phase voltages in a wye connection. Chapter 1(c)- AC Three Phases 52

Line and phase currents in a wye connection. Chapter 1(c)- AC Three Phases 53

-connected Load For loads, phase voltage and line voltage are the same thing. Chapter 1(c)- AC Three Phases 54

load definitions Y-Y Line currents are the currents in the line conductors. Phase currents are the currents through phases. Chapter 1(c)- AC Three Phases 55

Line/Phase Currents for a Delta Circuit Line current Chapter 1(c)- AC Three Phases Phase current 56

• • • Delta Connections In a delta system, the line current is higher than the phase current by a factor of the square root of 3 (1. 732). ILine = IPhase x 1. 732 IPhase = ILine / 1. 732 In a delta system, the line current is equal to the phase current. ELine = EPhase Chapter 1(c)- AC Three Phases 57

Delta system voltage and current relationships. Chapter 1(c)- AC Three Phases 58

Delta system division of currents. Chapter 1(c)- AC Three Phases 59

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Example Problem 1 EAN = 7620 -18 V What are the phase voltages EBN, and ECN? Chapter 1(c)- AC Three Phases 61

Example Problem 2 EAN = 7620 -18 V What are the line voltages ? Chapter 1(c)- AC Three Phases 62

Example Problem 3 For the load depicted below, EAN = 277 -30 V. Find the phase voltages and line voltages and currents. Sketch their phasor values. Chapter 1(c)- AC Three Phases 63

Review Questions 1. Describe the primary difference between singlephase and three-phase current. 2. Describe advantages and disadvantages of three-phase current as compared to singlephase current. 3. Explain the differences between deltaconnected, open delta-connected, and wyeconnected transformers and their resulting voltages. Chapter 1(c)- AC Three Phases 64

END OF CHAPTER 1 Chapter 1(c)- AC Three Phases 65