CH 39 Three-Phase Electricity & Power
characteristics of three-phase delta circuit
1. AC generators and/or loads connected in delta shape. 2. delta connections = line-to-line 3. no neutral connection 4. has phase voltage, line voltage.
3 sinusoidal voltage spacing
120 degrees apart due to location of windings.
impedance required to fully load a 3-phase voltage source.
3
terminal voltages of 3-phase AC GENERATOR produces
3 sinusoidal voltages of equal amplitude but different phase angles.
three-phase power: complex power delta load
S
3-phase wye connection line voltage
VL = measured between the lines.
wye connected source made by
connecting one ed of each on 3 windings with equal amplitudes that are separated by phase angle of 120 degrees.
delta-wye connection: convenient to
convert delta system to equivalent wye system and vice versa.
since power transfer for 3 phases = constant & not pulsating as for a single or 2 phase machine, 3 phase ....
vibrations are reduced = bearing life is extended
3-phase wye connection phase current
I p = current in single winding or phase.
three-phase power: apparent power
Q.
three-phase power: aka
complex power
three-phase power: components of power
complex, real, imaginary
3-phase wye connection line current
current in the lines
unless otherwise clarified, 3-phase voltages & currents are
effective values = RMS VALUES.
in 3-phase delta networks, line current = IL is always
greater than phase current IP.
delta-wye connection: have relationship with
impedance between balanced 3-phase delta-connected load & equivalent wye-connected load.
a six-conductor transmission line could be used to transmit power generated by 3 voltages, it is more efficient to
interconnect the windings & reduce # of conductors to 3 / 4.
for balanced circuits, the equations are used to find
line current from phase current.
what are all identical in a balanced system?
magnitude of voltages & line currents. real, complex, & reactive powers.
3-phase wye connection phase voltage
measured across a single winding or phase.
three-phase power: use of complex conjugates
necessary to keep complex power in the first and fourth quadrants bc apparent power is arbitrarily assigned a positive sign with inductive loads.
three-phase power: real power is never
negative or imaginary.
balanced systems analyzed on a
per-phase basis = one-line analyses.
3-phase wye connection: line-to-line voltages expressed in terms of
phase voltages.
3-phase system can produce magnetic field that
rotates in a specified direction = simplifying the design of electric motors.
single-phase alternating voltage
rotation of magnetic field thru the conductors of stationary coil.
power factor for each phase
same
3-phase wye connection phase sequence
what line to neutral voltages go through their maxima in regular order. Va reaches its peak before Vb. Vb peaks before Vc
3-phase wye connection line-to-neutral voltages differ from each other by
a third of an electrical cycle or 120 degrees.
3-phase wye connection balanced system, net current in neutral line =
0.
3-wire wye circuits have
1. 3 energized wires 2. more economical = eliminate neutral wire. 3. high-voltage transmission circuits.
4-wire wye circuits have
1. 3 energized wires and one neutral wire. 2. more flexible. 3. common distribution voltage circuits
using 3-phase power results in...
1. 3-phase currents that tend to cancel each other. 2. 3 phases sum to 0 in linear balanced load.
2 types of 3-phase wye sources
1. 4 wire wye circuits 2. 3 wire wye circuits.
benefit of 3-phase currents canceling each other?
1. allows size of neutral conductor to be reduced or eliminated.
2 methods used to interconnect the windings & reduce # of conductors to 3 / 4.
1. delta (mesh) connections 2. wye (start) connections
benefits of 3 phasor power.
1. smaller conductors. 2. more efficient than multiple single-phase systems proving same power. 3. uniform torque. 4. induction motors do not require additional starting windings or associated switches. 5. when rectified, has smoother waveform & less ripple to be filtered.
characteristics of three-phase delta circuit: phase voltage
Vp = voltage measured across a single winding or phase.
phase voltage
Vp or coil voltage, generated voltage from terminal voltages.
impedances in 3-phase system are BALANCED...
when they are identical in magnitude and angles.
characteristics of three-phase delta circuit: line voltage
VL = voltage measured between the lines.
3-phase wye connection. balanced circuits line-to-neutral voltage
VLN. equation Vl = sq rt 3 * Vp = sq rt 3 * VLN.
3-phase wye connection : ABC sequence
Va reaches its peak before Vb. Vb peaks before Vc
3-phase wye connection: CBA
ACB = negative sequence. obtained by rotating the field magnet in opposite direction, the phase of generated sinusoids is reversed.
3-phase wye connection which is greater line or phase voltage
V L > V P.
three-phase power: complex conjugate in equation
Z* of actual impedance. with 180 degrees phase change.
3-phase wye connection: line-to-line voltage calculated
apply KVL to generator circuit, use equations.
3-phase wye connection line and phase currents
are equal.
describe induced voltages changes polarity in magnetic field
as alternate polarities of magnetic field lines intercept conductors of stationary coil, induced voltage changes its polarity at the same speed as the rotating magnetic field.
three-phase power: components of power combine
as vectors to form complex power triangle.
three-phase power: equations are used to calculate complex power for
balanced loads.
