subject: Electrical Rectifiers [print this page] Rectification is the conversion from alternating to direct current. Over the last few years the development of inexpensive, rugged, variable speed a.c. drives over a wide range of power ratings has vastly reduced the need for d.c.machines, which were hitherto indispensable for many industrial processes where fine control of speed was necessary. Such applications include rollingmills, electric overhead cranes and traction drives. Direct current nowadays
has much more limited use for processes such as electroplating plants, gas production plants, and to supply chargers for standby battery systems. Added to these is the somewhat exotic but limited application for high voltage direct current transmission. For most of these applications the conversion device will nowadays be a piece of power electronics, consisting of diodes or thyristors, but the principles of operation of rectifier equipments remain much the same regardless of the device which actually performs the rectification.
Three basic types of metal rectifier remain in common use: namely, selenium, germanium and silicon. The last two are generally referred to as semiconductor rectifiers. During the past 30 years there has been a continuous development of the selenium rectifier so that the stability of the rectifier is such that it can be operated at relatively high temperatures, i.e. 120C if required. Alternatively it can be operated for a much longer time than hitherto under normal operating conditions. Selenium rectifiers have been used widely for all low power requirements where initial cost is important and the ability to withstand substantial and repeated overloads eliminates the need for special protective devices that may be required for silicon or germanium rectifiers. Although the efficiency and performance of selenium rectifiers may be slightly inferior to the other two and the size somewhat greater, these features are often of less importance for outputs below 25 kW. Applications include electroplating where oil-immersed units provide currents up to 200 kA. At high values of current, water-cooled germanium units are used due to their smaller weight and the limited space which they require.
Germanium rectifiers :
Germanium rectifiers are used extensively for low power and medium power industrial applications where the ambient temperature is not high and particularly at voltages below 100V where high efficiency at such voltages is of overriding importance. The size of the installation may be in the megawatt range. The germanium is extracted from coal and zinc deposits and refined to a high degree of purity. The resultant grey metallic material is pulled into a single crystal which is specially cut to form small wafers. By heat treatment, an indium button is welded on to and diffused into the germanium wafer. The bond between the germanium and the indium forms the rectifying junction which is mounted in a hermetically sealed housing. Both germanium and silicon rectifiers are used extensively in equipments providing power for the electrolytic production of chlorine and hydrogen. Outputs in excess of 27 kA at 120V have been provided from these equipments, germanium offering the slightly better efficiency at d.c. voltages below 100 V, but they may not be economical in countries where the ambient temperature is high. Industrial d.c. power supplies are frequently provided from germanium and silicon semiconductor equipments although as stated earlier for powers below 25kW the selenium rectifier still remains the most attractive plant. Both types have been used extensively for large telephone exchanges. Chargers for battery electric vehicles are generally of the germanium rectifier type. Welding is another area where both germanium and silicon rectifiers compete with each other.
Silicon rectifiers :
Silicon is a metal obtained from sand and is refined to a high degree of purity, drawn into large monocrystals and cut into wafers. A thin plate of aluminium is bonded to and diffused into the silicon wafer and the junction between the aluminium and the silicon forms the rectifying junction. Many of the applications for this type of rectifier have already been outlined but this type of rectifier was considered to be the best for railway traction supplies. Variable speed drives is another area that this type of rectifier has been widely employed. The facility of the grid control made it ideal for speed control of d.c. machines up to the largest ratings.
Comparison of three types :
The semiconductor germanium and silicon rectifiers are, for a given output, more compact than selenium. This is due in part to the low forward resistance per unit area of rectifiers, and in part to the high voltages which they withstand in the reverse direction. Silicon will withstand a higher reverse voltage than germanium and will also operate at a higher temperature. The forward resistance of germanium is lower than that of silicon.
