Single Phase AC Voltage Controllers
A single-phase AC controlleris used to vary the value of the alternating voltage after it has been applied to a load circuit. A thyristor is also placed between the load and the constant source of AC voltage.
The root mean square alternating voltage is regulated by changing the thyristor triggering angle. In the case of phase control, the thyristors are employed as switches to establish a connection from the AC input supply to the load circuit during each input cycle. For every positive input voltage, chopping occurs and voltage is reduced.
Circuit Diagram with a Resistive Load
During half part of the cycle, the thyristor switch is turned ON to enable the voltage input to appear across the load. This is followed by the OFF state during the last half cycle so as to disconnect the load from the source voltage.
When the triggering angle α is controlled, the RMS value of the voltage on the load is also controlled. The triggering angle α is therefore, defined as the value of ωt at which the thyristor switches ON.
Single Phase AC Voltage Controllers – Multistage Sequence Control of AC Converter
When two or more sequence control stages are connected, it is possible to have an improvement in power factor and further reduction in THD. An n-stage sequence control converter has n windings in the transformer secondary part with each rated es/n .
When two AC converters are placed parallel to each other, the zero sequence way is created. A little difference between the two converters causes a great zero sequence in circulating current. The diagram below shows the parallel system of a converter. The direction of the current is anti-clockwise with respect to that of the voltage system.
A cycloconverter refers to a frequency changer that can to change AC power from one frequency to AC power at another frequency. This process is known as AC-AC conversion. It is mainly used in electric traction, AC motors having variable speed and induction heating.
A cycloconverter can achieve frequency conversion in one stage and ensures that voltage and the frequencies are controllable. In addition, the need to use commutation circuits is not necessary because it utilizes natural commutation. Power transfer within a cycloconverter occurs in two directions.
A major problem with cycloconverters is that when it is operating at small currents, there are inefficiencies created with firing delay. Furthermore, operations are only smooth at frequencies that are not equal half frequency input values. This is true because a cycloconverter is an AC- AC converter that is phase controlled.
Therefore, for it to give the required AC output voltage, it has to do a selection of the voltage input segments by applying linecommutation. This explains why the output frequency is lower than the frequency input.
Harmonics in a cycloconverter are mainly affected by methods of control, overlap effect, the number of pulses in a given cycle, operation mode and mode of conduction.
There are two types of cycloconverters−
- Step Up cycloconverter − These types use natural commutation and give an output at higher frequency than that of the input.
- Step Down cycloconverter − This type uses forced commutation and results in an output with a frequency lower than that of the input.
Cycloconverters are further classified into three categories −
- Single phase to single-phase − This type of cycloconverter has two full wave converters connected back to back. If one converter is operating the other one is disabled, no current passes through it.
- Three-phase to single-phase − This cycloconverter operates in four quadrants that is and being the rectification modes and and being the inversion modes.
- Three-phase to three-phase − This type of cycloconverter is majorly used in AC machine systems that are operating on three phase induction and synchronous machines.