Electrical Engineering.pdf (Size: 6.34 MB / Downloads: 407)
The working voltage for a capacitor is generally specified by the manufacturer, thereby giving
the maximum voltage that can safely be applied between the capacitor terminals. Exceeding this
limit may result in the breakdown of the insulation and then the formation of an electric arc between
the capacitor plates. Unintentional or parasitic capacitances that occur due to the proximity of
circuit elements may have serious effects on the circuit behavior.
Physical capacitors are often made of tightly rolled sheets of metal film, with a dielectric
(paper or nylon) sandwiched in between, in order to increase their capacitance values (or ability
to store energy) for a given size.Table 1.2.4 lists the range of general-purpose capacitances together
with the maximum voltages and frequencies for different types of dielectric materials. Practical
capacitors come in a wide range of values, shapes, sizes, voltage ratings, and constructions. Both
fixed and adjustable devices are available. Larger capacitors are of the electrolytic type, using
aluminum oxide as the dielectric.
An ideal inductor is also an energy-storage circuit element (with no loss associated with it) like a
capacitor, but representing the magnetic-field effect. The inductance in henrys (H) is defined by
where λ is the magnetic-flux linkage in weber-turns (Wb·t), N is the number of turns of the coil,
and Nψ is the magnetic flux in webers (Wb) produced by the current i in amperes (A). Figure
1.2.7(a) illustrates a single inductive coil or an inductor of N turns carrying a current i that is
linked by its own flux.
A transformer is basically a static device in which two or more stationary electric circuits are
coupled magnetically, the windings being linked by a common time-varying magnetic flux. All
that is really necessary for transformer action to take place is for the two coils to be so positioned
that some of the flux produced by a current in one coil links some of the turns of the other coil.
Some air-core transformers employed in communications equipment are no more elaborate than
this. However, the construction of transformers utilized in power-system networks is much more
elaborate to minimize energy loss, to produce a large flux in the ferromagnetic core by a current in
any one coil, and to see that as much of that flux as possible links as many of the turns as possible
of the other coils on the core.
An elementary model of a two-winding core-type transformer is shown in Figure 1.2.12.
Essentially it consists of two windings interlinked by a mutual magnetic field. The winding that
is excited or energized by connecting it to an input source is usually referred to as the primary
winding, whereas the other, to which the electric load is connected and from which the output
energy is taken, is known as the secondary winding. Depending on the voltage level at which
the winding is operated, the windings are classified as HV (high voltage) and LV (low voltage)
windings. The terminology of step-up or step-down transformer is also common if the main
purpose of the transformer is to raise or lower the voltage level. In a step-up transformer, the
primary is a low-voltage winding whereas the secondary is a high-voltage winding. The opposite
is true for a step-down transformer.
By considering a constant voltage source (with a given internal resistance RS) connected to a
variable-load resistance RL, as shown in Figure E1.2.6(a), for a value of RL equal to RS given
by Equation (1.2.13), the maximum power transfer to the load resistance would occur when the
load resistance is matched with the source resistance.