AC-AC Converters

Control de Maquinas Eléctricas

Equipo 4

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 • Example of phase angle control

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.