basics about three phase power supply

1. 1.1 Three phase power
supply
P. Maria Sheeba
AP/ECE
MZCET

2. IMPORTANCE OF THREE PHASE SYSTEM
• Uniform power transmission and
less vibration of three phase
machines.
– The instantaneous power in a 3φ
system can be constant (not
pulsating).
– High power motors prefer a steady
torque especially one created by a
rotating magnetic field.

3. IMPORTANCE OF THREE PHASE SYSTEM
• Three phase system is more
economical than the single
phase.
– The amount of wire required for a
three phase system is less than
required for an equivalent single
phase system.
– Conductor: Copper, Aluminum, etc

4. Differences
Single Phase Power Supply
 230 V, 50 Hz
 Not sufficient for producing
large amount of power.
 With single-phase current, the
voltage rises to a peak in one
direction of flow, subsides to
zero, reverses, rises to a peak
in the opposite direction,
subsides to zero, and so on.
 Single-phase current requires
the use of one transformer.
Three Phase Power
Supply
 440 V, 50 Hz
 Sufficient for producing large
amount of power.
 There are three separate and
distinct single-phase currents,
which are combined so they
can be transmitted over three
or four wires and these rise to
a peak in one direction,
subside, reverse, and so on;
however they do not peak at
the same time.
 Three-phase current requires
two or three transformers.

5. Three Phase CirCuiTs

6. 1.1.1 Introduction:
• The generator , motor , transformer or rectifier have only one
winding is called a single phase system
• If the current or voltage follows a phase difference 900
in a two
windings, called two phase systems
• If the phase difference is 1200
between voltages or currents in a
three winding, called as Three phase systems
• In poly-phase systems , there are more than three windings
Advantages of three phase system:
• More efficient than single phase system
• Cost is less
• Size is small . Compared to single phase system

7. Advantages of Three Phase Circuits
• The amount of conductor material is required less for transmitting
same power, over the same distance , under same power loss
• Three phase motors produce uniform torque , where as torque
produced by single motor is pulsating
• Three phase generators not produce the harmonics when they are
connected in parallel
• Three phase motors are self starting whereas single phase motors are
not self starting

8. THREE PHASE GENERATION

9. FARADAYS LAW
• Three things must be present
in order to produce electrical
current:
a) Magnetic field
b) Conductor
c) Relative motion
• Conductor cuts lines of
magnetic flux, a voltage is
induced in the conductor
• Direction and Speed are

10. GENERATING A SINGLE PHASE
Motion is parallel to the flux.
No voltage is induced.
N
S

11. x
N
S
Motion is 45° to flux.
Induced voltage is 0.707 of maximum.
GENERATING A SINGLE PHASE

12. GENERATING A SINGLE PHASE
x
N
S
Motion is perpendicular to flux.
Induced voltage is maximum.

13. GENERATING A SINGLE PHASE
Motion is 45° to flux.
x
N
S
Induced voltage is 0.707 of maximum.

14. GENERATING A SINGLE PHASE
N
S
Motion is parallel to flux.
No voltage is induced.

15. GENERATING A SINGLE PHASE
x
N
S
Notice current in the
conductor has reversed.
Induced voltage is
0.707 of maximum.
Motion is 45° to flux.

16. GENERATING A SINGLE PHASE
N
S
x
Motion is perpendicular to flux.
Induced voltage is maximum.

17. GENERATING A SINGLE PHASE
N
S
x
Motion is 45° to flux.
Induced voltage is 0.707 of maximum.

18. GENERATING A SINGLE PHASE
Motion is parallel to flux.
N
S
No voltage is induced.
Ready to produce another cycle.

19. THREE PHASE GENERATOR

20. GENERATOR WORK
• The generator consists of a
rotating magnet (rotor)
surrounded by a stationary
winding (stator).
• Three separate windings or
coils with terminals a-a’, b-b’,
and c-c’ are physically placed
120° apart around the stator.

21. • As the rotor rotates, its
magnetic field cuts the flux from
the three coils and induces
voltages in the coils.
• The induced voltage have equal
magnitude but out of phase by
120°.

22. GENERATION OF THREE-PHASE AC
N
xx
S

23. 1.1.2 Generation of Three Phase Voltages

24. BALANCED 3φ VOLTAGES
• Balanced three phase voltages:
– same magnitude (VM )
– 120° phase shift
( )
( )
( ) ( )°+=°−=
°−=
=
120cos240cos)(
120cos)(
cos)(
tVtVtv
tVtv
tVtv
MMcn
Mbn
Man
ωω
ω
ω

25. • Vectorially r.m.s values of voltages
induced in three windings are represented
in the diagram
• ER = E ∟0o
v,
• EY = E ∟-120o
v
• EB= E ∟+120o
v

26.  Three voltages are not in phase, since there is a
phase difference of
 The order of voltage waveform sequences in a
polyphase system is called phase
rotation or phase sequence.
 This sequence of phase shifts has a definite
order. For clockwise rotation, the order is 1-2-3
(i.e) RYB (winding 1 peaks first, them winding 2,
then winding 3), which is known as a positive
phase sequence.
MZCET
1.1.3 PHASE SEQUENCE

120

27. PHASE SEQUENCE

28.  For anti-clockwise rotation,
the order is 3-2-1 (i.e) RBY
(winding 3 peaks first, them
winding 2, then winding 1),
which is known as a
negative phase sequence.
 If we’re using a polyphase
voltage source to power
resistive loads, phase
rotation will make no
difference at all.
 Whether 1-2-3 or 3-2-1, the
voltage and current
magnitudes will all be the
same.

29. PHASE SEQUENCE
( )
( )°+=
°−=
=
120cos)(
120cos)(
cos)(
tVtv
tVtv
tVtv
Mcn
Mbn
Man
ω
ω
ω
°+∠=
°−∠=
°∠=
120
120
0
Mcn
Mbn
Man
VV
VV
VV
POSITIVE
SEQUENCE
NEGATIVE
SEQUENCE
°−∠=
°+∠=
°∠=
120
120
0
Mcn
Mbn
Man
VV
VV
VV

30. Overview of Three phase power supply
 Both single phase and three phase power systems
refer to units using alternating current (AC) electric
power. With AC power, the flow of current is
constantly alternating directions.
 The primary difference between single phase and
three phase AC power is the constancy of delivery.