Understanding Electric Power
The reliability of any electric power system depends upon
knowledge, preventive maintenance, monitoring and testing of that
system.
The threatened limitations of conventional electric power sources
have focused a great deal of attention on electric power, its
application, monitoring and correction. The electric power utility's
primary goal is to meet the electric power demand of its customers
at all times and under all conditions. But as the electric power
demand grows in size and complexity, modifications and additions to
existing electric power networks have become increasingly expensive.
The measuring and monitoring of electric power have become even more
critical because of downtime associated with equipment breakdown and
material failures.
Typical Voltage Configurations
Single-Phase Systems
Single-phase residential electric power loads are almost universally
supplied through 120/240V, 3-wire, single-phase services. In this
system the two "hot" or current carrying conductors are 180 degrees
out-of-phase with respect to the neutral.
Three-Phase, 3-Wire Systems
In this type of electric power system, commonly known as the "DELTA"
configuration, the voltage between each pair of line wires is the
actual transformer voltage. This system is frequently used for power
loads in commercial and industrial buildings. In such cases, service
to the premises is made at 208V, three-phase. Feeders carry the
power to panel supplying branch circuits for motor loads. Lighting
loads are usually handled by a separate single-phase service. The
480V distribution is often used in industrial buildings with
substantial motor loads.
Three-Phase, 4-Wire Systems
Known as the "WYE" connection, this is the electric power system
most commonly used in commercial and industrial buildings. In office
or other commercial buildings, the 480V three-phase, 4-wire feeders
are carried to each floor, where 480V three-phase is tapped to a
power panel or motors. General area fluorescent lighting that uses
277V ballasts is connected between each leg and neutral; 208/120V
three-phase, 4-wire circuits are derived from step-down transformers
for local lighting and receptacle outlets.
Typical voltage:
phase-to-phase = 208/480V
phase-to-neutral = 120/277V
Balanced vs. Unbalanced Loads
A balanced load is an AC electric power system using more than two
wires, where the current flow is equal in each of the current
carrying conductors. Many systems today represent an unbalanced
condition due to uneven loading on a particular phase. This often
occurs when electrical expansion is affected with little regard to
even distribution of loads between phases or several nonlinear loads
on the same system.
RMS vs. Average Sensing
The term RMS (root-mean-square) is used in relation to alternating
current waveforms and simply means "equivalent" or "effective,"
referring to the amount of work done by the equivalent value of
direct current (DC). RMS measurements provide a more accurate
representation of actual current or voltage values. This is very
important for nonlinear (distorted) waveforms. With expanding
markets of computers, uninterruptible power supplies, and variable
speed motor drives, resulting nonlinear waveforms are drastically
different. Measuring nonsinusoidal voltage and current waveforms
requires a True RMS meter. Conventional meters usually measure the
average value of amplitudes of a waveform. Some meters are
calibrated to read the equivalent RMS value (.707 x peak); this type
calibration is a true representation only when the waveform is a
pure sine wave (i.e., no distortion). When distortion occurs, the
relationship between average readings and True RMS values changes
drastically. Only a meter which measures True RMS values gives
accurate readings for a nonsinusoidal waveform. RMS measuring
circuits sample the input signal at a high rate of speed. The
meter's internal circuitry digitizes and squares each sample, adds
it to the previous samples squared, and takes the square root of the
total. This is the True RMS value.
Power Factor
Power factor is the ratio of ACTUAL POWER used in a circuit to the
APPARENT POWER delivered by a utility. Actual power is expressed in
watts (W) or kilowatts (kW); apparent power in voltamperes (VA) or (kVA).
Apparent power is calculated simply by multiplying the current by
the voltage.
Power Factor = Actual Power = kW/Apparent Power kVA
Certain loads (e.g., inductive type motors) create a phase shift or
delay between the current and voltage waveforms. An inductive type
load causes the current to lag the voltage by some angle, known as
the phase angle. On purely resistive loads, there is no phase
difference between the two waveforms; therefore the power factor on
such a load will be 0 degrees, or unity.
The following examples of a soldering iron and a single-phase motor
illustrate how power factor is consumed in different types of loads.
In a soldering iron, the apparent electric power supplied by the
utility is directly converted into heat, or actual power. In this
case, the actual power is equal to the apparent power, so that the
power factor is equal to "1" or 100 percent (unity).
In the case of a single-phase motor, the actual power is the sum of
several components:
the work performed by the system; such as lifting with a crane,
moving air with a fan, or moving material, as with a conveyer.
heat developed by the power lost in the motor winding resistance
heat developed in the iron through eddy currents and hysteresis
losses
frictional losses in the motor bearings
air friction losses in turning the motor rotor.
Reactive Compensation Power
Reactive compensation power refers to the capacitive values required
to correct low electric power factor to as close to unity (1.0) as
possible. Most industrial loads are inductive, so the load current
lags the line voltage by some degree. In order to bring the value
closer to unity, something must be added to the load to draw a
leading current. This is done by connecting a capacitor in parallel
with the load. Since a capacitor will not dissipate any real power,
the charge for real power will be the same.
Many meters today have the ability to accurately work with these non
linear signals and display accurate results. A typical clamp-on
meter can measure power, power factor as well as volts, amps, watts
and VARs.
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