•Reactive power (kVAR): virtual power that determines load/demand
•Utility pays for total power (kVA)
•Active
power, measured in kilowatt (kW), is the real power (shaft power, true power)
used by a load to perform a certain task. However, there are certain loads like
motors, which require another form of power called reactive power (kVAR) to
establish the magnetic field. Although reactive power is virtual, it actually
determines the load (demand) on an electrical system. The utility has to pay
for total power (or demand)
•The
vector sum of the active power and reactive power is the total (or apparent)
power, measured in kVA (kilo Volts-Amperes). This is the power sent by the
power company to customers.
•Here
the different powers are represented of a power triangle where the vector sum
of the active power and reactive power make up the total power used. This is
the power sent by the power utility companies for the user to perform a given
amount of work. Total power, also known as apparent power is measured in kilo
Volts-Amperes.
•You
can see from the figure that the active power, and the reactive power required
are 90 degrees apart vectorically in a pure inductive circuit. In other words
reactive power kVAr lagging the active kW. The apparent power, kVA, is the
vector sum of active and reactive power. Mathematically it may be represented
with the following formula
An atom consists of a positively charged
nucleus surrounded by a swarm of negatively charged electrons. The charge
associated with one electron has been found to be 1.602 × 10−19 coulombs; or, stated the other way around,
one coulomb can be defined as the charge on 6.242
× 1018 electrons. While most of the electrons associated
with an atom are tightly bound to the nucleus, good conductors, like copper,
have free electrons that are sufficiently distant from their
nuclei that their attraction to any particular nucleus is easily overcome.
These conduction electrons are free to wander from atom to atom, and their
movement constitutes an electric current.
Current
In a wire, when one coulomb’s worth of charge
passes a given spot in one second, the current is defined to be one ampere (abbreviated A), named after the nineteenth-century
physicist Andr´e Marie Amp`ere. That is, current i is
the net rate of flow of charge q past a point, or through an area:
In general, charges can be negative or
positive. For example, in a neon light, positive ions move in one direction and
negative electrons move in the other. Each contributes to current, and the
total current is their sum. By convention, the direction of current flow is
taken to be the direction that positive charges would move, whether or not
positive charges happen to be in the picture. Thus, in a wire, electrons moving
to the right constitute a current that flows to the left, When charge flows at
a steady rate in one direction only, the current is said to be direct current, or dc. A
battery, for example, supplies direct current. When charge flows back and forth
sinusoidally, it is said to be alternating current, or
ac.
Voltage
Electrons won’t flow through a circuit unless
they are given some energy to help send them on their way. That “push” is measured
in volts, where voltage is defined to be the amount of energy given to a unit
of charge, A 12-V battery therefore gives 12 joules of energy to each coulomb
of charge that it stores. Note that the charge does not actually have to move for
voltage to have meaning. Voltage describes the potential for charge to do work.
While currents are measured through a circuit component, voltages are measured across components. Thus, for example, it is correct
to say that current through a battery is 10 A, while the voltage across that
battery is 12 V. Other ways to describe the voltage across a component include
whether the voltage rises across the component or drops.
Power
Power and energy are
two terms that are often misused. Energy can be thought of as the ability to do
work, and it has units such as joules or Btu. Power, on the other hand, is the rate at which energy is generated or used, and
therefore it has rate units such as joules/s or Btu/h. There is often confusion
about the units for electrical power and energy. Electrical power is measured
in watts, which is a rate (1 J/s = 1 watt), so electrical energy is watts multiplied
by time—for example, watt-hours. Be careful not to say “watts per hour,” which
is incorrect (even though you will see this all too often in newspapers or
magazines).
Energy
Since power is the rate at which work is being
done, and energy is the total amount of work done, energy is just the integral
of power,
In an electrical circuit, energy can be
expressed in terms of joules (J), where 1 watt-second = 1 joule. In the electric power industry the
units of electrical energy are more often given in watt-hours, or for larger quantities
kilowatt-hours (kWh) or megawatt-hours (MWh). Thus, for example, a 100-W
computer that is operated for 10 hours will consume 1000 Wh, or 1 kWh of energy.
