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A mercury arc valve (mercury vapor rectifier) is a type of electrical rectifier which converts alternating current into direct current. Rectifiers of this type were used in electric motor power supplies for industry, in electric railways, streetcars and diesel-electric locomotives, and in static inverter stations in electric power transmission. Mercury arc rectifiers were invented by Peter Cooper Hewitt in 1902 and further developed throughout the 1920s and 1930s by researchers in both Europe and North America. Before the advent of solid-state devices, mercury arc rectifiers were the most efficient form of conversion from alternating to direct current. By the 1970s, the development of high-voltage solid-state devices made the mercury arc rectifier obsolete even in high-voltage DC applications. AC, half-wave and full wave rectified signals A rectifier is an electrical device, comprising one or more semiconductive devices (such as diodes) or vacuum tubes arranged for converting alternating current to direct current. ...
Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. ...
Overhead wire in Coventry, England Overhead wire and its suspension system in Bridgeport, Connecticut, USA A railway electrification system is a way of supplying electric power to electric locomotives and multiple units. ...
A number of vehicles use a diesel-electric powerplant for providing locomotion. ...
A locomotive (from Latin loco motivus) is a railway vehicle that provides the motive power for a train, and has no payload capacity of its own; its sole purpose is to move the train along the tracks. ...
An inverter is a circuit for converting direct current (DC) to alternating current (AC). ...
Peter Cooper Hewitt (May 5, 1861 - August 25, 1921) was an American electrical engineer, who demonstrated the mercury-vapor lamp for which he deposited a patent. ...
1902 (MCMII) was a common year starting on Wednesday (see link for calendar). ...
It has been suggested that this article or section be merged with Social issues of the 1920s. ...
This article or section is missing references or citation of sources. ...
The 1970s decade refers to the years from 1971 to 1980, inclusive. ...
Solid state may refer to: In computing: Solid state devices are data storage device components that uses memory chips, such as SDRAMs, to store data. ...
Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. ...
 Mercury arc valve from the 1940s Image File history File links Download high resolution version (1743x1149, 1411 KB) Summary Mercury Arc Rectifier, Belsize Park deep level shelter. ...
Image File history File links Download high resolution version (1743x1149, 1411 KB) Summary Mercury Arc Rectifier, Belsize Park deep level shelter. ...
Use
Mercury arc valves were used until the 1960s for the production of high voltage direct current. Applications included power supply for streetcars and electric railways, variable-voltage power supplies for large radio transmitters, and static inverters. Small mercury arc rectifiers were used in the power supplies for vacuum tube (valve) electronic equiment such as power amplifiers and transmitters. Static inverter stations were used to provide DC power to legacy Edison style DC power grids in urban centers until the 1950s. The 1960s decade refers to the years from 1960 to 1969, inclusive. ...
Thomas Alva Edison (February 11, 1847 – October 18, 1931) was an American inventor and businessman who developed many devices which greatly influenced life in the 20th century. ...
In the War of Currents era in the late 1880s, Nikola Tesla and Thomas Edison became adversaries due to Edisons promotion of direct current (DC) for electric power distribution over the more efficient alternating current (AC) advocated by Tesla. ...
The 1950s were a decade that spanned the years 1950 through 1959, although some sources say from 1951 through 1960. ...
Since 1960 mercury arc valves were increasingly replaced by silicon rectifiers and thyristors. The last domain of the mercury arc valves was for high voltage direct current transmission until 1975. 1960 (MCMLX) was a leap year starting on Friday (the link is to a full 1960 calendar). ...
General Name, Symbol, Number silicon, Si, 14 Chemical series metalloids Group, Period, Block 14, 3, p Appearance dark gray, bluish tinge Atomic mass 28. ...
Circuit symbol for a thyristor The thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. ...
HVDC or high-voltage, direct current electric power transmission systems contrast with the more common alternating-current systems as a means for the bulk transmission of electrical power. ...
1975 (MCMLXXV) was a common year starting on Wednesday (the link is to a full 1975 calendar). ...
They are still used in some South African mines (2006)
 Mercury arc valve operating in 2006 Image File history File linksMetadata MercuryArc-Rectifier. ...
Image File history File linksMetadata MercuryArc-Rectifier. ...
Design
 A glass envelope mercury arc rectifier valve One type of mercury vapour electric rectifier consists of an evacuated glass bulb, with a pool of liquid mercury sitting in the bottom as the cathode. Over it curves a glass bulb, which condenses mercury evaporated in the course of operation of the device. The glass envelope has one or more arms with graphite electrodes as anodes. Their number depends on the application. If direct current is to be produced from single-phase alternating current, then two anodes are used, each connected to the outer ends of a centre-tapped transformer secondary winding. With three-phase alternating current three or six anodes are used, to provide a smoother direct current. Six-phase operation can improve the efficiency of the transformer as well as providing smoother DC, by enabling two anodes to conduct simultaneoulsy. During operation, the arc transfers to the anodes at the highest positive potential (with respect to the cathode). Design of the arms and envelope is intended to prevent an arc from forming between the anodes; such a condition is called "backfire" and is a critical factor in the design of mercury arc rectifiers. Image File history File links A schematic of a glass envelope mercury arc rectifier. ...
Image File history File links A schematic of a glass envelope mercury arc rectifier. ...
General Name, Symbol, Number mercury, Hg, 80 Chemical series transition metals Group, Period, Block 12, 6, d Appearance silvery white Atomic mass 200. ...
Diagram of a copper cathode in a Daniells cell. ...
Diagram of a zinc anode in a Daniells cell. ...
Glass envelope rectifiers can produce up to hundreds of kilowatts of direct-current power in a single unit. A 6-phase rectifier rated 150 amperes has a glass envelope approximately 600 mm high by 300 mm outside diameter. These rectifiers will contain several pounds of liquid mercury. The large size of the envelope is required due to the low thermal conductivity of glass. Mercury vapor in the upper part of the envelope must give up heat through the glass envelope to condense and return to the cathode pool.
The current carrying capacity of a rectifier is limited in part by the size of the wires fused into the glass envelope for connection of the anodes and cathode. Development of high-current rectifiers required leadwire materials and glass with very similar coefficients of thermal expansion, to prevent leakage of air into the envelope.
For larger valves, a metal tank with ceramic insulators for the electrodes is used, sometimes with a vacuum pump system to counteract slight leakage of air into the tank around imperfect seals. The design patented by Uno Lamm of ASEA is one example of this type which includes grading electrodes between the anode and cathode to prevent backfire. Metal-tank rectifiers were built with ratings of 2000 A and 125 kV per unit. August Uno Lamm (May 22, 1904 – June 1, 1989) was a Swedish electrical engineer and inventor, sometimes called The Father of High Voltage Direct Current power transmission. ...
ASEA (Allmänna Svenska Elektriska Aktiebolaget) was a Swedish industry company. ...
Both glass and metal envelope rectifiers may have control grids inserted between the anode and cathode. This allows the conduction of the rectifier to be controlled, for example to delay the instant at which the arc transfers to the anode on the alternating current waveform, thereby giving control of the mean output voltage produced by the rectifier. Such grid-controlled valves are an essential part of a static inverter.
The temperature of the envelope must be carefully controlled, since the working pressure within the envelope is set by the coolest spot on the enclosure wall. A typical design maintains temperature at 40 degrees Celsius and a mercury vapor pressure of 7 millipascals. The pascal (symbol: Pa) is the SI unit of pressure. ...
Function
Principle Operation of the rectifier relies on an electrical arc discharge between electrodes in a sealed envelope containing mercury vapour. A pool of liquid mercury acts as a self-renewing cathode that does not deteroriate with time. The mercury emits electrons freely, whereas the carbon anodes emit very few electrons even when heated, thus rectifying action occurs. An electric arc can melt calcium oxide. ...
Once an arc is formed, electrons are emitted from the surface of the pool, causing shock-ionisation of mercury vapour along the path towards the anodes. The mercury ions are attracted towards the cathode, and the resulting ionic bombardment of the pool maintains the temperature of the 'emission spot', so long as a current of a few amperes continues to flow.
The mercury ions emit light at characteristic wavelengths, the relative intensities of which are determined by the pressure of the vapour. At the low pressure within a rectifier, the light appears pale blue / violet and contains much ultraviolet.
Starting A conventional mercury arc rectifier is started by a brief high-voltage arc within the rectifier, between the cathode pool and a starting electrode. By one of a number of means, the starting electrode is brought into contact with the pool and allowed to pass current through an inductive circuit. The contact with the pool is then broken, resulting in a high emf.
The momentary contact between the starting electrode and the pool may be achieved by allowing an external electromagnet to pull the electrode into contact with the pool; the electromagnet can also serve as the starting inductance. Alternatively, the electromagnet may be arranged to tip the bulb of a small rectifier, just enough to allow mercury from the pool to reach the starting electrode. An alternative system provides a narrow neck of mercury between two pools, and by passing a very high current at negligible voltage through the neck, displaces it by magnetostriction, thus opening the circuit. An electromagnet is a type of magnet in which the magnetic field is produced by a flow of electric current. ...
Excitation Since momentary interruptions or reductions of output current may cause the cathode spot to extinguish, many rectifiers incorporate a small local circuit to maintain an arc whenever the plant is in use. Typically, a two or three phase supply of a few amperes passes through small 'excitation' anodes. A magnetically-shunted transformer of a few hundred VA rating is commonly used to provide this supply.
Others The largest ever mercury arc rectifiers were used with the Nelson River Bipole high-voltage DC power transmission project. A 2000A 250kV thyristor valve at Manitoba Hydros Henday converter station, April 2001 The Nelson River Bipole is a system of two HVDC lines in Manitoba, operated by Manitoba Hydro. ...
Special types of mercury arc rectifiers are the thyratron, the Ignitron and the Excitron. For charging of secondary batteries, "Tungar" tubes were used, which were able to supply the necessary current with the relatively small charging voltages (e.g. 12 V). A thyratron is a type of gas filled tube used as a high energy electrical switch. ...
An ignitron is a type of controlled rectifier dating from the 1930s. ...
In 1919 the book "Cyclopedia of Telephony & Telegraphy Vol. 1" by Kempster Miller and others described an amplifier for telephone signals that used a magnetic field to modulate an arc in a mercury rectifier tube. This pre-dated the application of the vacuum tube to amplification of audio signals but was never commercially important. 1919 (MCMXIX) was a common year starting on Wednesday (see link for calendar). ...
For the British rock band of the same name, see Amplifier (band) An amplifier can be considered to be any device that uses a small amount of energy to control a source of a larger amount of energy, although the term today usually refers to an electronic amplifier. ...
The telephone or phone (Greek: tele = far away and phone = voice) is a telecommunications device which is used to transmit and receive sound (most commonly voice and speech) across distance. ...
Modulation is the process of varying a carrier signal, typically a sinusoidal signal, in order to use that signal to convey information. ...
In electronics, a vacuum tube (U.S. and Canadian English) or (thermionic) valve (outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. ...
Environmental hazard The use of large quantities of mercury in fragile glass envelopes presents a hazard of potential release of mercury to the environment should the glass bulb be broken. Some HVDC static inverter stations have required expensive clean-up to eliminate traces of mercury emitted from the station over its service life. Steel tank rectifiers frequently required vacuum pumps which continually emitted small amounts of mercury vapor. Mercury compounds are toxic, highly persistent in the environment, and present a danger to humans and the environment.
External links
References - A. H. Howatson, An Introduction to Gas Discharges, Pergamon Press, Oxford, 1965 - especially Chapter 8.
- The Project Gutenberg EBook of Cyclopedia of Telephony & Telegraphy Vol. 1 by Kempster Miller, George Patterson, Charles Thom, Robert Millikan, Samuel McMeen
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