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1
OPERATION
OF THE POWER-SAVE CONDITIONING UNIT
Ayhan A. Mutlu, Ph.D., and Mahmud Rahman, Ph.D.
Department of Electrical Engineering, Santa Clara University
500 El Camino Real, Santa Clara, CA 95053
December 12, 2004
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1.
Introduction
With the advent of electricity and technological breakthroughs,
electrical energy has been made available at a reasonable cost through an
elaborate and efficient distribution grid system to households and
businesses alike operating various kinds of appliances that run on
electricity.
The local distributors of electrical energy charges the consumers
based on the consumers’ rate of electrical energy consumption called
“power” expressed in numbers of thousands of watts, i.e., kilowatts
(kW). The electrical quantity kW that represents the rate of energy
consumed can be minimized if the circuitry is optimized in a way that
there is less “spurious” energy lost. The Power product, if
incorporated into the electrical circuit, is able to achieve such
minimization. In addition, the Power-Save 1200 brings about a number of
other benefits to the consumer without introducing any adverse effects, or
"side effects." A good understanding of various aspects of how
electrical power is consumed in a circuit is therefore essential to
understand how this product works. The principle of operation of the
Power-Save 1200, based on theoretical concepts which are substantiated by
measurement evidences, is presented in the following.
2.
Types of loads and their electrical behavior
Theoretically, there are three basic types of loads in an
electrical system, e.g., resistive, inductive, and capacitive. While
electrical energy is expended in pure resistive loads, electrical energy
is not expended but stored in ideally inductive and capacitive loads.
Although all practical loads and appliances at a consumer’s site
incorporate these three types of ideal loads, it is appropriate to
categorize them as mostly resistive, inductive or capacitive. The
following is an example of common practical loads that are used in a
household.
a.
Resistive: Oven, light bulbs, iron, electric heaters, etc.
b.
Inductive: Appliances with motors and transformers are examples of
inductive loads which include air-conditioners, washers, dryers,
refrigerators, induction motor, power transformer, lighting ballasts,
welder or induction furnace, etc.
c.
Capacitive: Rechargeable batteries, etc.
Since the currents flowing in inductive and capacitive loads are
half a cycle out of phase, it is possible to make their sum zero at any
particular time by adjusting their magnitudes, consequently reducing the
total current magnitude flowing through the Energy-meter (kW-hour meter)
installed by the local distributors to monitor energy consumed by a
subscriber. This is the essence of "power factor correction,"
where power factor refers to cosine of the phase angle between the voltage
and the total current. The phase angle θ = ωt, where t = time
and ω = 2π/T is the angular frequency of power supply and T =
1/f, where the principle frequency f of the power being delivered is
usually 60 Hz. For purely resistive load, θ = 0o,
hence power factor for resistive load = cosine 0o
= 1. For purely inductive and capacitive loads, power factor =
cosine (±90o) =
0. Power factor correction implies to the situation where the inductive
load current is balanced by capacitive load current thus reducing the
total current to a minimum and the phase angle between the voltage and the
total current representing the algebraic sum of the individual 3 load currents
approaches 0o, i.e., cosine 0o
= 1. At lower power factor, the total current is larger and vice
versa.
The current passing through the current coil of the Energy-meter installed
by the power distributor to monitor power consumption is the algebraic
aggregate of the individual resistive, inductive, and capacitive currents
flowing in different loads of the household. Power distributors require
the industrial consumers to keep the power factor of the household read at
the Energymeter above say, 0.8, since power factors below 0.8 would
require the distributors supply larger currents, therefore running larger
generators which in turn would cost them more. Also, smaller current
associated with higher power factor will minimize various resistive losses
in the distribution system. Therefore, industrial consumers are charged a
penalty at a predetermined rate based on their operating power factor.
The Power-Save 1200 is capable of correcting the power factor toward
various benefits of the consumer as explained below.
3.
The Role of the Capacitor in the Electrical Power System
Capacitor is a device that stores energy in the electric field
established between a pair of conductors on which equal but opposite
electric charges have been induced. Historically, capacitors have taken
the form of a pair of thin metal plates, whether flat or tightly coiled up
in a cylinder (like a sushi roll), but every multi-conductor geometry
exhibits the phenomenon of capacitance.
The capacitance of a traditional flat-plate capacitor--and,
resultantly, the amount of energy that can be stored in the capacitor--is
proportional to the surface area of the conducting plate and inversely
proportional to the distance between the plates. It is also proportional
to the permittivity of the dielectric substance that separates the plates,
whether vacuum, air, or specially engineered materials chosen for their
high electrical permittivity.
In a direct-current (DC) circuit, a capacitor acts like an open
circuit: no current flows through it, though the potential difference
initially induced between its conductors can serve as an exponentially
decaying energy source for the circuit. In an alternating-current (AC)
circuit, a capacitor cyclically stores and releases energy at twice the
frequency of the forcing source.
3.1. Power Factor Correction
Power-Save 1200 brings the advantages of the power factor correction
capacitor to household use.
Figure 1 shows a typical connection of the Power-Save 1200 unit in
a household. This unit is usually connected to the fuse panel where the
electricity is distributed to different locations in the house. As
mentioned above, the current passing through the current coil of the
Energy-meter installed
3.2 Motor Inrush Current
When the power switch is turned on, a stationary motor acts as
short circuit causing a much higher than normal current to flow. As time
passes the magnetic field builds up and the motor starts to rotate and
reaches steady state rpm and the current drops down to normal values. This
high current is called “inrush current” which has minimal impact on
the total power consumption of the motor but may adversely affect motor
lifetime by stressing out its wiring. The magnitude of this current is a
function of the motor horsepower and design characteristics.
Our experiments have shown that adding Power-Save 1200 has reduced
the peak inrush current of the tested motor by about 5%. Also, when the
motor was under full load, the unit has reduced the inrush current time
about 15%.
Fig. 5. Motor inrush current with and without Power-Save 1200 capacitor
unit at 1.6 lbf-in
dynamometer load.
3.3.
Voltage Sags
The current i, and the voltage V, in a capacitor C, is related to each other by the following equation, i
CdV dt =
where dV dt is the time rate of change of the voltage across the capacitor.
Therefore, the voltage 8
across the capacitor cannot change instantaneously (i.e., dt
= 0) because it will
then need an infinite amount of current to do so. As a consequence, whenever the
capacitor is subjected to a sudden voltage surge or sag, the capacitor
tends to reduce it to certain extent depending on its size. This behavior
of capacitors have lead to applications where these are connected in
parallel with the power circuits of most electronic devices and larger
systems (such as factories) to shunt away and conceal voltage and current
fluctuations from the primary power source to provide a "clean"
power supply for signal or control circuits. Such effects in capacitors
can also be interpreted to act as a local reserve for the DC power source,
to smooth out fluctuations by charging and discharging each cycle. Figure
6 presents the effects of Power-Save 1200 capacitor on voltage sag due to
an induction motor switching in a household.
116
117 118 119 120 121 122 123 0 50 100 150 200 250 300 Time
(sec.) Voltage (V) Line
Voltage (With Capacitor) Line Voltage (No Capacitor) >
1 V
Fig. 6. The effects of Power-Save 1200 capacitor on voltage sag due
to an induction motor switching
in a household.
3.4.
Harmonics and Temperature
Harmonic distortion is the deviation in the waveform of the supply
voltage from its ideal sinusoidal waveform due to inclusion of higher
frequency components in addition to the fundamental frequency. The major
adverse effect of the harmonics is heating of induction motors and
transformers in the household leading to reduced lifetime of the motor. It
has been known that a reduction of 10 oC in the operating temperature of a motor essentially doubles its
lifetime.
The Power-Save 1200 in conjunction with the resistance of the
household wiring forms a low-pass filter which prohibits higher frequency
components from the incoming supply into household loads. Consequently,
motors are subjected to less heating as illustrated in Fig. 7.
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Motor
Temperature 0
10 20 30 40 50 60 0 20 40 60 80 Time
(min.) Temperature (C) With
Capacitor (Tmax=42 C) No Capacitor (Tmax = 48 C)
Fig. 7. The effect of Power-Save 1200 capacitor on the temperature
of a motor running under dynamometer load.
4.
The power distribution panel installed by distributor
The distributor of electrical energy installs a power distribution panel
outside a household so that their personnel can monitor the total energy
consumed by the household by reading the Energymeter installed in the
panel. Two wires with 220 V across them are brought in from the locality
supply grid into the distribution panel. All loads in the house which run
on 220 V are connected across these two wires. Loads which run on 120 V
are connected across one of these two wires and the ground terminal which
is fabricated in each household by inserting a solid copper rod deep into
the soil. All 120 V loads are divided into two circuits, each consisting
of one the above mentioned two wires and the ground terminal. Figure 8
shows these two different configurations.
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Fig. 8. Distribution panel configurations in a household
5.
Conclusion
An attempt has been made to elucidate the principle of operation of the
Power-Save 1200 Power Conditioning Unit in offering significant benefits
in the areas of reduced power usage, decreased motor temperature, improved
power factor, reduced inrush current and suppression of voltage sags.
Investigations are being continued to understand and explain other effects
that may have implications on the Power-Save 1200 such as i) Harmonics
generated in the household loads, ii) Impact of the unit to the
distribution system during non-peak hours (i.e., nights).
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