# Three-Phase circuits - PowerPoint PPT Presentation

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## Three-Phase circuits

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### Three-Phase circuits Prof. Siripong Potisuk Three-phase Voltage Sources Balanced Y-connected Voltage Source Balanced phase voltages are equal in magnitude and are out ... – PowerPoint PPT presentation

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Title: Three-Phase circuits

1
Three-Phase circuits
• Prof. Siripong Potisuk

2
The EMF induced in a circuit is directly
proportional to the time rate of change of
magnetic flux through the circuit.
The EMF can be produced by changing B
(induced EMF) or by changing the area, e.g., by
moving the wire (motional EMF).
• It is the relative movement between the coil
and
• the magnet that matters

3
Magnet
Magnetic Flux lines
DC source
brush
Coil
Conductor loop
Slip ring
4
¼ Cycle
Start
½ Cycle
¾ Cycle
½ Cycle
¾ Cycle
¼ Cycle
5
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6
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7
Phase 1
Phase 2
Phase 3
Start
¾ Cycle
½ Cycle
¼ Cycle
8
Three-phase Voltage Sources
Y-connected Source
D-connected Source
9
Balanced Y-connected Voltage Source
• Balanced phase voltages are equal in magnitude
and are out of phase with one another by 120
degrees.
• Phase voltages sum up to zero.
• Two possible combinations

acb or (?) sequence
abc or () sequence
10
Balanced Y-connected Voltage Source
• Balanced line voltages are equal in magnitude and
are out of phase with one another by 120 degrees.
• Line voltages sum up to zero.
• The magnitude of line voltages is 1.732 times the
magnitude of the phase voltages
• Line Voltages lead their corresponding phase
voltages by 30 degrees

11
A balanced load is one in which the phase
impedances are equal in magnitude and in phase.
12
Balanced Y-Y Connection
ZY ZS Zl ZL
13
Example Calculate the line currents in the
three-wire Y-Y system as shown.
14
Balanced Y-D Connection
15
• Example A balanced abc-sequence Y-connected
source with V is connected to a
D-connected balanced load of 8j4 ? per phase.
Calculate the phase and the line currents.

16
Balanced D-D Connection
17
Example A balanced ?-connected load having an
impedance of 20-j15 ? is connected to a
?-connected, positive sequence generator
having
V. Calculate the phase
currents of the load and the line currents.
18
Balanced D-Y Connection
19
• Example A balanced Y-connected load with a phase
impedance of 40j 25 ? is connected to a
balanced, positive sequence D-connected source
with a line voltage of 210 V. Calculate the phase
currents. Use Vab as reference.