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|AC/DC and DC/AC Conversion and Voltage Control|
Types of power conversion
There are four fundamental types of power conversion:-
AC to DC conversion in which the power is stepped up or down to the correct voltage, rectified (so it always from in the same direction) and smoothed out so that the voltage is steady and does not go up and down at the AC frequency. This is the most widely used conversion and it is what happens inside mobile phone chargers, battery chargers generally and the power supply unit inside computers.
DC to AC conversion in which DC input (either steady or not) is converted to an AC output either with or without an AC synchronizing signal. This is used to convert output from solar voltaic panels or a stationary battery to the AC we use. When excess solar power is sold back to the grid, the grid acts as the synchronizing signal.
DC to DC conversion in which DC input (either steady or not) is stepped up or down to produce a different (and usually steady) DC voltage.
AC to AC conversion in which the voltage is stepped up or down (transformer) and or the frequency is changed. The latter would mostly be achieved by first an AC to DC conversion and then a DC to AC conversion. One scenario is in HVDC (high voltage DC) inter-grid transmission. The source and sink grids may or may not have a different frequency but even if they have the same, the two can be out of sync. HVDC is a more efficient transmission method than AC because with HVDC all of the conductor is fully used. In AC, the thick cables typically hung from pylons suffer a skin effect, even at 50Hz.
The Mechanical Converter
Sometimes designing a power conversion system can be too challenging. And when it is there is always the brute force approach. The mechanical converter is an electric motor taking the available power input to drive the dynamo or alternator to generate the required output. The arrangement may optionally include a flywheel. While such a contraption might be better associated with the very beginings of the electric age, like the electronic valve, it has a persistance that defies the dustbin of history. But such persistances don't entirely depend on our nostalgic instincts and however quaint it might be, the mechanical converter completly isolates the input and output curcuits and when used in conjunction with a flywheel, copes well with erratic surges from such power sources as a wind turbine. It isn't particularly inefficient either. A crude prototype can easily get close to 90&percent; while more refined designs can easily exceed that figure. One particular form of mechanical converter still in use today has a homopolar generator to provide large currents at low voltage for electro-chemical processes such as aluminum smelting. For such applications the mechanical converter compares well to its more modern rivals.
One form of mechanical converter several companies have developed specifically exploits the flywheel as an energy store. This is called a Flywheel Battery. This has been used in some data centers and we have investigated its potential use in off grid scenarios as a means of efficiently capturing power surges from wind turbines. Flywheels have been built with energy densities better than twice that of lead acid batteries. And as these devices have high rates of revolution, they also have very good electromagnetic coupling giving them very high charge/discharge rates. If magnetic levitation is applied, flywheel batteries exhibit high storage efficiency, low rates of self discharge and long lifespans.
In spite of these alluring properties, the flywheel battery has little prospect of moving beyond a handful of niche applications. High power flywheels are difficult to engineer to the nesseary precision and are thus inhently expensive. They are also dangerous, prone to catastrophic failure without warning. To counter this danger a strong (and thus heavy) jacket surrounds the flywheel rendering the device both bulky and heavy. The flywheels exhibit collosal gyroscopic force, manefest as an extreme reluctance to alter the axis of rotation. The flywheel battery is a very poor choice for mobile applications unless the application also requires gyroscopic stabilization. And in stationary applications the gyroscopic force causes the flywheel to precess with the Earth's rotation nessesitating the further expense of a gimble. To summerize, power conversion is best done by a power converter and power storage is best done by a battery!
The Switch Mode Converter
The mainstream method of course is the switch mode converter. One form of this is the switch mode power supply in your PC which converts AC mains to the 5v needed to energize the chips and 12v needed to drive the fan. The basic idea behind all switch mode conversions is to break up the input energy stream by 'spraying' it by rapid switching into various energy storing components (capacitors and inductors), and then asembling an output stream by switching in the stores as required.
This process can be surprisingly efficient although it should be noted few small scale implimentations are. Many have a low 'power factor' meaning they do not draw power all the time. To save on components they are cutting out part of the AC cycle where the input voltage is outside certain limits. This sort of behaviour would be a nuicance in situations where large numbers of houses were selling electricity to the grid during peak sunlight conditions. It also is not how you export power between grids over a HVDC line. Such applications require a power factor as near as posible to 100 percent.
The Rise of DC
Switch mode converter designs have benefitted considerably from recent advances in supercapacitors. It is easier than it used to be to control the speed of powerful motors such as that of an electric car, by varying the voltage from the battery pack. The much higher capacitances mean much lower switch speeds and with this comes better efficiency. The 20KHz switching, typical of small switch mode power supplies, make cicuits leaky by RF radiation. It is small but irritating. And in the electric car at least, the supercapacitors confer the advantage of aiding regenerative braking.