Download - Eq4 Conver Ac-Ac
AC-AC Converters • converters convert AC electrical power of one
frequency into AC electrical power of another
frequency.
• This kind of converter also has the capability to
control the load voltage amplitude.
Classification
Phase control: AC voltage controller
Integral cycle control : AC power controller
AC Controllers PWM control: AC chopper
On/off swiitch: electronic AC switch
Phase control: thyristor
Frequency converter cycloconverter.
(Cycloconverter) PWM control: matrix
converter.
AC voltage controllers
Is an elelctronic module based on either thyristors,
TRAIACs, SCRs, or IGBTs.
Converts a fixed voltage, fixed frequency alternating
current (AC) electrical input supply to obtain variable
voltage in output delivered to a resistive load.
AC Voltage controllers • On-and-off control
In an on-and-off controller, thyristors are used to
switch on the circuits for a few cycles of voltage and
off for certain cycles, thus altering the
total RMS voltage value of the output and acting as a
high speed AC switch. The rapid switching results in
high frequency distortion artifacts which can cause a
rise in temperature, and may lead to interference in
nearby electronics. Such designs are not practical
except in low power applications.
AC Voltage controllers • Phase angle control
In phase angle control, thyristors are used to halve the voltage cycle during input. By controlling the phase angle or trigger angle, the output RMS voltage of the load can be varied. The thyristor is turned on for every half-cycle and switched off for each remaining half-cycle. The phase angle is the position at which the thyristor is switched on. TRIACs are often used instead of thyristors to perform the same function for better efficiency. If the load is a combination of resistance and inductance, the current cycle lags the voltage cycle, decreasing overall power output.
AC Voltage controllers
Applications:
• Lighting control
• Varying heating temperatures in
homes or industry
• Speed control of fans and winding
machines
Frequency Converter Is an electronic or electromechanical device that
converts alternating current (AC) of one frequency
to alternating current of another frequency. The
device may also change the voltage, but if it does,
that is incidental to its principal purpose.
Frequency convereter
• Thyristor cycloconverter
converts a constant voltage, constant frequency AC waveform to another AC waveform of a lower frequency by synthesizing the output waveform from segments of the AC supply without an intermediate DC link
The amplitude and frequency of converters' output voltage are both variable. The output to input frequency ratio of a three-phase CCV must be less than about one-third for circulating current mode CCVs or one-half for blocking mode CCVs
Frequency converter Matrix Converter
Is an AC/AC converter which offers a reduced
number of components, a low-complexity modulation
scheme, and low realization effort.
The matrix converter consists of 9 bi-directional
switches that allow any output phase to be
connected to any input phase.
Transformer
A transformer consists of two
windings of wire that are
wound around a common
core to induce tight
electromagnetic coupling bet
ween the windings. The core
material is often a
laminated iron core. The coil
that receives the electrical
input energy is referred to as
the primary winding, while the
output coil is called the
secondary winding.
• If an alternating electric current flows through the primary winding (coil) of a transformer, an electromagnetic field is generated that develops into a varying magnetic flux in the core of the transformer. Through electromagnetic induction, this magnetic flux generates a varying electromotive force in the secondary winding, which induces a voltage across the output terminals. If a load impedance is connected across the secondary winding, a current flows through the secondary winding drawing power from the primary winding and its power source.
Ideal transformer
Ideal transformer with a source and a load.
NP and NS are the number of turns in the primary
and secondary windings respectively.
The circuit diagram left shows the conventions
used for an ideal, i.e. lossless and perfectly-
coupled transformer having primary and
secondary windings with NP and NS turns,
respectively.
The ideal transformer induces secondary
voltage VS as a proportion of the primary
voltage VP and respective winding turns as given
by the equation:
Core form and shell form transformers
Core form = core type; shell form = shell
type
Closed-core transformers are constructed
in 'core form' or 'shell form'. When windings
surround the core, the transformer is core
form; when windings are surrounded by the
core, the transformer is shell form. Shell form
design may be more prevalent than core
form design for distribution transformer
applications due to the relative ease in
stacking the core around winding
coils. Core form design tends to, as a
general rule, be more economical, and
therefore more prevalent, than shell form
design for high voltage power transformer
applications at the lower end of their
voltage and power rating ranges (less than
or equal to, nominally, 230 kV or 75 MVA).
APPLICATIONS
• Transformers are used to increase voltage before transmitting electrical energy over long distances through wires. Wires have resistance which loses energy through joule heating at a rate corresponding to square of the current. By transforming power to a higher voltage transformers enable economical transmission of power and distribution. Consequently, transformers have shaped the electricity supply industry, permitting generation to be located remotely from points of demand. All but a tiny fraction of the world's electrical power has passed through a series of transformers by the time it reaches the consumer.
• Transformers are also used extensively in electronic products to step-down the supply voltage to a level suitable for the low voltage circuits they contain. The transformer also electrically isolates the end user from contact with the supply voltage.