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A transformer is a device that changes or transforms
alternating current of one voltage to alternating current of another
voltage. Transformers used in building
work consist essentially o iron core on to which two coils: a primary coil
winding and a secondary winding coil. A
voltage impressed on the primary coil induces a voltage in the secondary coil
in proportion to the ratio of turns in the coils. Thus a step down transformer has a larger number of turns in its
primary winding than in its secondary coil.
Transformers are available in single phase or three phase
construction. Transformer power
capacity is rate kilo-volt amperes (kVA).
OHM's Law
The electrical relationship between voltage, current,
and impedance is called Ohm's law.
Ohm's law is a simple mathematical formula which says the voltage in a
circuit can be computed by multiplying the current flowing in the circuit times
the impedance of the circuit. The
"impedance" of a circuit is measured in ohms and is represented by
the letter Z. The term impedance is
used to include both inductive and capacitive reactance and resistance because
all three are forms of opposition to the flow of current. Ohm's Law is written as V = A x Z where V = voltage (volts)
The electrical impedance of
a circuit is made up of both the electrical resistance and the electrical
reactance of the elements in the circuit. Both the resistance and reactance
impede the current flow through a circuit. Resistance loads include incandescent light bulbs and
electric heating elements. Reactive loads include electric motors and other
devices where the magnetic field created around a coil of wire or in a
capacitor is utilized as electrical energy. Resistive Loads In AC and DC circuits containing
purely resistive loads, like lights and heaters, Ohm's Law can be used to
compute current, voltage and resistance in the circuit. In a resistive DC circuit, both
current and voltage are fixed, steady values. In an AC resistive circuit, the
current alternates exactly in step with the voltage. In either case, Ohms Law
can be applied. In a resistive circuit Ohm's Law
states that: voltage is equal to current times resistance. Ohm's Law V = A x R where: V = voltage (volts) For example, a current of 2 amps
flowing through a resistance of 3 ohms is said to produce a voltage
"across" that resistance of 6 volts. Reactive Loads
The
current and voltage in AC
circuits that contain inductors, capacitors, or both, behave much differently
than in a purely resistive circuit. We cannot measure resistance directly in
these circuits. We measure what's known as "reactance." Inductors and capacitors react to current flow in
ways that oppose, or impede, changes in the flow of current, but they do it in
a different fashion than a pure resistor. Each device has its own
characteristics, which creates an impeding force. Whenever there is an inductor, or coil, in a circuit,
we call its impeding force to current flow "inductive reactance." For
a circuit with a capacitor, the impeding force is called "capacitive
reactance." We treat both resistance and reactance as impedance,
that is, any opposition to changes in the flow of current. Fuel
Cost Adjustment/Fuel Charge - How To You can calculate the cost of energy use for various
electrical appliances and equipment if you have these three pieces of
information:
The rated power, in kilowatts, is
multiplied by the operating hours to determine the energy use in kilowatt-hours.
The energy use is then multiplied by the electric rate to determine the cost of
electricity to run the appliance. For example, let's determine the cost to run a
1,500-watt heater for six hours based on an electricity cost of eight cents per
kilowatt-hour. The power rating of the heater is 1,500 watts. To convert this to kilowatts, divide 1,500
watts by 1,000. This yields 1.5
kilowatts. Now multiply the power use, 1.5 kilowatts, by the operating time of
6 hours; which equals 9 kilowatt-hours of energy use. Finally, multiply the 9-kilowatt
hours of energy use times the electric rate of eight cents per kilowatt-hour. You
now know it costs 72 cents to operate the heater for 6 hours. On larger appliances, like refrigerators and water
heaters, there is a yellow label on the side which states how much energy costs
to run the appliance for a year. If you look closely you can see that the
manufacturer has used "averages," family size, hours per day, etc, to
determine the annual operating costs for this appliance. Energy Use
= Power x Time KWh = kW x Hours Then: Operating
Cost ($) = Energy Use (kWh) x Electric Rate While electrical power measured in kilowatts is
important to the people who design and size the electrical system, most
consumers are more interested in the amount of electrical energy that is used
in their house, or by a single appliance. The term used in measuring electrical
energy is the "kilowatt-hour." This is a measure of the amount of
electrical power used in one hour. Kilowatt-hours are what the electric meter
on the side of your house measures. This is the basis for calculating your
monthly electric bill. Once
you know the energy use of any appliance, you can figure the cost to run that
appliance. There are three methods of finding the amount of electrical energy
being used by a piece of equipment. They
are: 1. Estimating wattage and time; Determining
kW Use Estimating You can estimate the power use in kilowatts of an
appliance from the nameplate information. If the nameplate indicates the
appliance's rated wattage to be 1200 watts at 120 volts, then divide the
wattage by 1000 to convert to the kilowatt power usage. If you've been curious
what a 100-watt light bulb uses in kilowatts, divide 100 by 1000 and get 0.1
kilowatts.
Meter-Disk
Revolutions The power used by an appliance can be measured by observing the meter-disk revolutions on the regular kilowatt-hour meter at the service entrance. This method has the advantage of accurately measuring the watts or watt-hours used by equipment under actual loading and service conditions. This method can also be used to measure the power use of the service, or for any desired number of combinations of loads operating at one time.
Every regular kilowatt-hour meter has a flat aluminum
disk with a black mark along its edge. This disk turns when energy is being
used by the service. Meters also have a meter constant called the K sub h,
which is shown on the meter nameplate. This K sub h constant indicates how many
watt-hours are used for each revolution of the meter disk. A constant of
"Kh = 7.2" means that for each revolution of the disk, 7.2 watt-hours
of energy have been used. The actual
number associated with the constant will vary with different meters. Common
values are 3.6 and 7.2. To determine how much power is used by counting
meter-disk revolutions, proceed as follows: 1. Make sure the appliance to be
checked is the only operating device on the service. This may require finding
the circuit that supplies the appliance you wish to check in the breaker box
and shutting off all the other breakers. This assures that the appliance will
be the only device operating and using electricity. 2. Disconnect, or power off, any other equipment that
is on the same circuit with the appliance to be checked. 3. With a watch, determine the time it takes for the
meter-disk to make a certain number of complete revolutions. Choose any number
of disk revolutions you desire. Depending on the appliance you're measuring,
the number of revolutions will vary. If you are measuring a smaller, less power
consuming device, the time to move the disk may be quite lengthy. There is also
the issue of devices like refrigerators and air conditioner compressors that
cycle on and off. Make sure you measure these appliances while they are on, not
when they have cycled off. 4. To calculate the kilowatts of power used by the
appliance, divide the number of disk revolutions by the time in seconds.
Multiply this times 3.6 and multiply again times the K sub h factor from the
meter. This is the amount of power actually used by the appliance in kilowatts.
As an example, the meter was timed with everything
off in the house except the air conditioner. The meter disk made 10 complete
revolutions in 30 seconds and the k sub h on the meter was 7.2. The power use,
in kilowatts, of the air conditioner is found by dividing 10 revolutions by 30
seconds, multiplying this times 3.6 times 7.2, which equals 8.6 kilowatts. Remember, though, an air conditioner doesn't
run all of the time - it cycles on and off.
Electric Rates/Prices
Electric rates, the cost of energy per kilowatt-hour,
vary from utility to utility; by rate class within a utility, and sometimes
even by the time of day or the season of the year. Each power supplier has
established rate schedules that are usually approved by regulatory agencies for
a given customer type. The base electric rate is applied to the number of
kilowatt-hours consumed during a billing period, approximately a month long, to
determine the electric bill. T he billing period will usually not be the same
as the calendar month and the meter may run 30 to 32 day intervals - and not on
exactly the same day of each month. Other charges may be added to the base cost of
electricity. These may include a charge for minimum monthly service, demand
charges on larger customer accounts, fuel adjustment charges, other surcharges,
power factor penalties and taxes. Rates charged in the summer months may differ
from those charged during the winter. Special rates may be offered for
customers who agree to let the power supplier cycle equipment on and off such
as electric heaters or air conditioners during high use times of the day or
year. All of these charges and their
application are covered under "Electric Bill Components." |
