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ELECTRICAL DATA
ELECTRICAL DISTRIBUTION SYSTEMS
GENERAL
Power distribution systems use alternating current (ac) where
the current and voltage reverse each cycle. Voltage and current
follow a sine wave curve (Fig. 1) and go through zero twice
each cycle. The two most common frequencies are 60 Hertz
(cycles per second) and 50 Hertz.
Fig. 1. Sine Wave Curve for Single-Phase
Alternating Current and Voltage.
Both a voltage and a current flow are required to provide
power. When power is supplied to a pure resistance load (electric
heating element or incandescent light bulb) the current is in
phase with the voltage. When power is supplied to a reactive
load, either capacitive (capacitors) or inductive (motor, solenoid,
or fluorescent light), the current is out of phase with the voltage
by some angle called . The actual power delivered to the device
equals the power times Cos . Since the angle
available or easy to use, manufacturers often provide a value
on the nameplate called power factor (PF). PF will be used in
this discussion.
Current lags voltage in inductive loads and leads voltage in
capacitive loads. For the same power, a load having a low PF
draws more current than a load having a PF of one, requires
heavier wires to connect the load, and may result in a higher
utility cost. For these reasons capacitors are often added to
balance the inductance of motor and fluorescent light loads
and bring the PF closer to one.
Power distributed to a load is either single-phase or three-phase.
The systems for small loads are usually single-phase. The systems
for heavy loads are usually three-phase. Three-phase systems are
three single-phase circuits arranged such that each phase reaches
its peak at a different time (Fig. 2). The resultant total power is
steadier than single-phase power. A three-phase distribution system
may be connected in a three- or four-wire wye configuration or a
three-wire delta configuration.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
M10490
is not readily
473
GENERAL ENGINEERING DATA
PHASE 1
PHASE 2
Fig. 2. Sine Wave Curves for Three-Phase
Alternating Current and Voltage.
Three-phase distribution is the standard method of power
distribution from the generating plant and requires
approximately one-fourth less copper than a single-phase
system to deliver the same power. Power leaves the generating
source on high voltage distribution lines. The voltage is then
reduced at the point of usage via transformers.
SINGLE-PHASE SYSTEM
Figure 3 shows a typical single-phase power source. The
primary side of the transformer (460V) is usually supplied from
one phase of a three-phase distribution system. The secondary
side of the transformer has a center tap so it can supply both
115 and 230 volts.
115V
460V
115V
TRANSFORMER
Fig. 3. Single-Phase Transformer.
When PF (power factor) = 1:
The power delivered to the load is:
Where:
P = power in watts
E = voltage in volts
I = current in amperes
When PF < 1 the power delivered to the load is:
PHASE 3
M10491
E 1 = 115V
N
E 3 = 230V
E 2 = 115V
C1810
P = EI
P = EI x PF
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