1. Teknologi Elektrik (BBT 3623) Bab 3: Sistem Tiga Fasa

2. At the end of this chapter you should be able to:
• Describe a single-phase supply
• Describe a three-phase supply
• Understand a star connection, and recognise that IL =Ip and VL = √3Vp
• Draw a complete phasor diagram for a balanced, star connected load
• Understand a delta connection, and recognise that VL =Vp and IL =3Ip
• Draw a phasor diagram for a balanced, delta connected load
• Calculate power in three-phase systems using P=√3 VLIL cos Ø
• Appreciate how power is measured in a three-phase system, by the one two and three-wattmeter methods
• Compare star and delta connections
• Appreciate the advantages of three-phase systems

3. 3.1 Introduction
Generation, transmission and distribution of electricity via the National Grid system is accomplished by three phase alternating currents.
The voltage induced by a single coil when rotated in a uniform magnetic field is shown in Figure and is known as a single-phase voltage.

4. 3.1 Introduction
Most consumers are fed by means of a single-phase a.c. supply.
Two wires are used, one called the live conductor (usually coloured red) and the other is called the neutral conductor (usually coloured black).
The neutral is usually connected via protective gear to earth, the earth wire being coloured green.
The standard voltage for a single phase a.c. supply is 240V.
The majority of single-phase supplies are obtained by connection to a three-phase supply

5. 3.2 Three-Phase Supply
A three-phase supply is generated when three coils are placed 120◦ apart and the whole rotated in a uniform magnetic field

6. 3.2 Three-Phase Supply
The convention adopted to identify each of the phase voltages is: R-red,Y-yellow, and B-blue,.
The phase-sequence is given by the sequence in which the conductors pass the point initially taken by the red conductor.
The national standard phase sequence is R,Y, B.

7. 3.3 Three Phase Connection
If the three-phase windings are kept independent then six wires are needed to connect a supply source (such as a generator) to a load (such as motor).
To reduce the number of wires it is usual to interconnect the three phases.
There are two ways in which this can be done, these being:
a star connection
a delta connection

8. 3.3 Three Phase Connection
Star connection (3 phase - 3 Wires)

9. 3.3 Three Phase Connection
Star connection (3 phase - 4 Wires)

10. 3.3 Three Phase Connection
Delta connection

11. 3.4 Star Connection
VR, VY and VB are called phase voltages or line to neutral voltages. ( generally denoted by VP).
VRY, VYB and VBR are called line voltages

12. 3.4 Star Connection IR, IY and IB  line current (denoted by IL)=
Phase current denoted by IP
For a star connection IL=IP

13. 3.4 Star Connection
For a balanced system

14. 3.4 Star Connection VRY =VR −VY VRY =VR +(−VY))
By trigonometry, or by measurement,

15. 3.4 Star Connection
The star connection of the three phases of a supply, together with a neutral conductor, allows the use of two voltages — the phase voltage and the line voltage.
A 4-wire system is also used when the load is not balanced. The standard electricity supply to consumers in Malaysia is 415/240V, 50 Hz, 3-phase, 4-wire alternating current

16. 3.4 Star Connection

17. 3.4 Star Connection

18. 3.4 Star Connection

19. 3.4 Star Connection
VRY=415V
(i) Draw VR =VY =VB =240V and spaced 120◦ apart
VR=240V
15˚
30˚
(ii) Power factor= cos φ=0.966 lagging phase angle is given by cos−1(0.966)=15◦
IR =IY =IB =5A, lagging VR, VY and VB respectively by 15◦.
IR=5A
IB=5A
VYB=415V
(iii) VRY =VR −VY
VB=240V
IY=5A
VY=240V
VYB =VY −VB
VBR =VB −VR
VBR=415V

20. 3.4 Star Connection

21. 3.4 Star Connection

22. 3.5 Delta Connection VRY, VYB and VBR  phase voltages
For delta connection VP=VL

23. 3.5 Delta Connection
IR =IRY −IBR =IRY +(−IBR)

24. IR =IRY −IBR =IRY +(−IBR)
3.5 Delta Connection
IR =IRY −IBR =IRY +(−IBR)
 By measurement

25. 3.5 Delta Connection

26. 3.5 Delta Connection 1

27. 3.5 Delta Connection

28. 3.5 Delta Connection

29. 3.5 Delta Connection
Thus when the load is connected in delta, three times the line current is taken from the supply than is taken if connected in star.

30. 3.6 Power in three-phase systems
The power dissipated in a three-phase load is given by the sum of the power dissipated in each phase
If a load is balanced then the total power P is given by:
P=3×power consumed by one phase.
The power consumed in one phase= or
VpIp cos Ø
(where Ø is the phase angle between Vp and Ip).

31. 3.6 Power in three-phase systems
For a star connection,

32. 3.6 Power in three-phase systems
For a delta connection,

33. 3.6 Power in three-phase systems
Hence for either a star or a delta balanced connection
the total power P is given by:
Total volt-amperes

34. 3.6 Power in three-phase systems

35. 3.6 Power in three-phase systems

36. 3.6 Power in three-phase systems

37. 3.6 Power in three-phase systems

38. 3.6 Power in three-phase systems

39. 3.6 Power in three-phase systems
Hence loads connected in delta dissipate three times the power than when connected in star, and also take a line current three times greater

40. 3.7 Measurement of power in three-phase systems
(i) One-wattmeter method for a balanced load

41. 3.7 Measurement of power in three-phase systems
(ii) Two-wattmeter method for balanced or unbalanced loads

42. 3.7 Measurement of power in three-phase systems
(iii) Three-wattmeter method for a three-phase, 4-
wire system for balanced and unbalanced loads

44. SEKIAN
TERIMA KASIH