The Need for Electrical Conversion
In the 1880s, two major inventions determined the course of electric power systems. George Stanley built a practical transformer, and so allowed convenient conversion among various voltage levels for ac. Nikola Tesla invented the polyphase ac system, and with it showed an easy method for converting electrical energy into mechanical energy. The advantages of low-frequency ac were compelling to designers as early as the 1890s. Such systems form the basis of power systems worldwide.
However, many applications require direct current for proper operation. Electrochemical processes and most electronic circuits are among these. It turns out that dc, at very high voltage levels (HVDC), is the most economical way to transmit electricity over long distances. Until recently, dc motors were the choice for motion control applications. All this means that rectification has been necessary ever since Thomas Edison’s original dc system of the early 1880s began to be supplanted by ac. Rectification has become a more acute issue with the rise of electronics and computers. A personal computer often uses dc power at five or more different voltages, ranging from the internal 3 V battery for the real-time clock to 15 kV or more for the accelerating voltage in the cathode-ray tube display. The next generation may well see a revolution in superconducting materials, which will bring new conversion requirements. Modern conversion needs reach well beyond rectifiers, and include:
- Voltage level conversion ( for dc as well as ac). A typical system not only needs multiple levels, bit it often requires them to be mutually isolated so that their loads remain separate.
- Frequency conversion. Rectification is one example (e.g., 60 Hz input, 0 Hz output). Another example is conversion between the 50 Hz system used in about half the world and the 60 Hz system used in the other half. Mobile systems such as an aircraft often use higher frequencies, with 400 Hz serving as one standard value. Much higher frequencies are used for inducting heating.
- Waveshape conversion (square, sine, triangle, others). Sinusoidal waveforms for power minimize interference with frequency multiplexed communication systems. The have other advantages in steady-state ac systems. However, sine waves are not always best for power conversion or motors. Square waves are better for rectification. Triangle or trapezoid waves are used in some motors.
- Polyphase conversions. Single-phase ac power is by far the most widely available form of electrical energy. However, polyphase sources are by far the best form when energy is to be converted and transported. This type of conversion is important for introducing speed control and motor efficiency improvements into household appliances, for instance.
How do we accomplish these conversions? Originally, the
straightforward way was to link a motor and generator on the same mechanical shaft, as illustrated to the right. For example, an ac motor powered with ac electricity could drive a dc generator and thus perform ac to dc conversion. This process converts electricity to mechanical from along the way. This method sometimes still applies when power levels are very high (beyond 1MW or so), provided that the desired frequencies match available motors and generators. Commercial machines are generally rated for dc, for 50 Hz, and for 60 Hz. There are a few electric railway systems rated for lower frequencies such as 16.7 Hz or 25 Hz.
Some of the difficulties with this process of electromechanical conversion include:
- Limited conversion ranges and functions.
- Bulky, heavy, noisy equipment.
- Substantial maintenance requirements.
- Slow response times and limited control capability.
In the past, the distinction was made between rotating converters based on machines, and static converters based on electronic circuits. The term static power conversion has given way to the more general term power electronics.
The general nature of the conversion issues at hand can be summarized as follows:
- Electricity must always be converted back and forth to the energy forms of interest to people.
- Electrical engineers are in the business of energy conversion. Whether the issue involves information processing, motors, communication systems, remote sensing, or device fabrication, electrical energy is a means to an end.
- Electricity is easy to control. A light switch, a volume know, or a cathode ray tube manipulate electrons with speed and precision.
- Electrical supplies are needed in a variety of subtly different forms: ac, dc, high or low voltage, high or low current, and so on.
- Some applications of electricity are not compatible. Voltage transformation with magnetic transformers requires ac. Many chemical processes require dc. Ac motors operate at speeds that depend on the source frequency. Dc motors have speeds that can be adjusted as a function of voltage or current.
- Conversion of electricity among its various forms is important for a wide range of applications.
- Compact electronic components with adequate ratings are available, so that electronic alternatives to motor-generator sets can be built.
While rectification has always been a key issue, there are many possible conversion objectives. The technological significance of many of these is growing as new applications become available.