21-12-2012, 04:54 PM
Three-Phase Circuits
1Three-Phase.pdf (Size: 108.69 KB / Downloads: 91)
objectives
After studying this unit, you should be able to:
Discuss the differences between three-phase and
single-phase voltages.
Discuss the characteristics of delta and wye connections.
Compute voltage and current values for delta
and wye circuits.
Connect delta and wye circuits and make measurements
with measuring instruments.
Most of the electrical power generated in the world today
is three-phase. Three-phase power was first conceived by
Nikola Tesla. In the early days of electric power generation,
Tesla not only led the battle concerning whether the nation
should be powered with low-voltage direct current or
high-voltage alternating current, but he also proved that
three-phase power was the most efficient way that electricity
could be produced, transmitted, and consumed.
WYE CONNECTION
The wye or star connection is made by connecting one end of each of
the three-phase windings together as shown in Figure 12-5. The voltage
measured across a single winding or phase is known as the phase voltage,
as shown in Figure 12-6. The voltage measured between the lines is known
as the line-to-line voltage or simply as the line voltage.
VOLTAGE RELATIONSHIPS IN A WYE CONNECTION
Many students of electricity have difficulty at first understanding why the
line voltage of the wye connection used in this illustration is 208 V instead
of 240 V. Since line voltage is measured across two phases that have a value
of 120 V each, it would appear that the sum of the two voltages should be
240 V. One cause of this misconception is that many students are familiar
with the 240/120 V connection supplied to most homes. If voltage is measured
across the two incoming lines, a voltage of 240 V will be seen. If
voltage is measured from either of the two lines to the neutral, a voltage of
120 V will be seen. The reason for this is that this is a single-phase connection
derived from the center tap of a transformer, Figure 12-8. If the center
tap is used as a common point, the two line voltages on either side of it will
be 180° apart and opposite in polarity, Figure 12-9. The vector sum of
these two voltages would be 240 V.
SUMMARY
1. The voltages of a three-phase system are 120° out of phase with each
other.
2. The two types of three-phase connections are wye and delta.
3. Wye connections are characterized by the fact that one terminal of
each device is connected together.
4. In a wye connection, the phase voltage is less than the line voltage
by a factor of 1.732. The phase current and line current are the same.
5. In a delta connection, the phase voltage is the same as the line voltage.
The phase current is less than the line current by a factor of 1.732.