Mercury-arc valve
A mercury vapor rectifier is a today obsolete power rectifier electric current that converts three-phase alternating current or direct current. Mercury arc rectifiers are available in both uncontrolled execution of the power -operated with direct current paths as trams, as well as in controlled version ( Ignitron, thyratron ) for phase control (for example, in locomotives for single-phase, 50 Hertz) or converter.
Historical
The American Peter Cooper Hewitt patented in 1902 a mercury vapor rectifier for three-phase. The gas-discharge tubes were developed for thyratron later and dominated until the early 1970s, the power electronics.
Mercury arc rectifiers were used until 1960 for the production of higher DC voltage, eg power converters for power supply of trams and suburban trains, for the power supply of tube amplifiers of major broadcasters.
For controllable drives of machine phase angle controls were used with controlled mercury vapor rectifiers. Since 1960 mercury arc rectifiers and mercury thyratrons of silicon rectifiers, thyristors and insulated gate bipolar transistor ( IGBT) has been replaced because they are advantageous in every respect: lower forward voltage, smaller volume, not fragile, non-toxic. The last domain of the mercury vapor rectifier was the high-voltage direct -current transmission until 1975.
Construction
A mercury -vapor rectifier consists of a glass envelope, in which a sea of mercury is at the bottom as a cathode. In addition to warp a glass dome, condensed on the inner wall of the evaporating mercury in operation again and flows down. Just over the sea of mercury heating electrodes are attached to evaporate some mercury and to make the rectifier ready to start. Laterally, glass extensions melted, the poor and the same on its end graphite electrodes as anodes. Their number depends on the purpose of use: are generated nominal direct current from alternating current, so one or two "arms" are used, depending on whether one or both half cycles of the alternating current to be used. With induction, there are three or six "arms". Six " arms " = anodes are used to reduce the ripple of the resulting voltage ( six-pulse circuit ). When six-phase arrangement, the three secondary windings of a three-phase transformer are decomposed by halving in two parts, the center terminals of the secondary windings are connected to form the negative DC voltage connection, while the cathode of the rectifier represents the positive terminal. For rectifier with particularly high performance a metal housing is used with bushings made of glass instead of the glass bulb.
Operation
Principle
Mercury arc rectifiers are among the subgroup of electron tubes with mercury cathode and as such are a special form of cold cathode tubes.
Electron source (also called plasma cathode) in this tube type is the foot of the standing operating mercury vapor arc, the so-called cathode spot. The inflowing to the cathode with high kinetic energy positive mercury ions form a concentrated space charge and a strong electric field, which can emit in conjunction with the kinetic shock of the ions increases electron from the liquid mercury (cathode ) in the region in front of the cathode spot. By special design measures such as angled arms anode has to be ensured that no mercury droplets can directly reach the anode circuits. If this were the case, it would by the mercury ions in front of the anode to the so-called re-ignition between anode and cathode or between adjacent anodes.
For the construction of the ion current (arc ) an ignition is necessary. Einanodengleichrichter must be re- ignited in each period, in multi- anode rectifiers, the arc jumps from one anode to the next.
Excess mercury vapor condenses in operation Piston dome, so that adjusting a stable equilibrium between liquid mercury and the mercury vapor in the flask.
Startup
A mercury vapor rectifier must be similarly launched a fluorescent lamp. This is done by starting electrode which is mounted just above the sea of mercury. For starting, the electrode is energized, and immersed in the mercury (for example, by an electromagnet or by tilting of the glass bulb ). As soon as it loses contact with the mercury, a spark, causing the mercury vapor is ionized and the main current can flow to the anodes is formed. Once the rectifier operates, sufficient ions can be constantly replenished to maintain the space charge in the area of the cathode spot of the mercury cathode. The starting electrode is then no longer needed for multi- anode rectifiers.
For mercury vapor rectifiers that work with small load currents, there is a danger that the gas discharge and thus the cathode spot extinguishes. To prevent this, an auxiliary discharge, and thus the cathode spot is maintained by an auxiliary device. It consists of a short-circuited circuit M2 (rectifier) , which alternating current is inductively limited. This auxiliary device is necessary even if Einanodengefäßen.
Operation
Due to the continuous discharge inside the glass bulb produces heat, which ensures that the mercury evaporates continuously. Vaporized mercury is reflected in the glass dome down, condenses and flows back into the lake. So there is an easily ionizable mercury atmosphere. Through a connected DC load is generated at the cathode ( sea of mercury ) an electron pressure / electron excess. The electrons ionize the mercury atmosphere and migrate to the anode, which is currently loaded positively. The mercury atoms emit a bluish light. In the opposite direction and between the anode current hardly flows, as they are inclined by their passivation little donate electrons, whereas they emerge easily from the sea of mercury.
In mercury vapor controllable rectifiers in order to achieve a discharge, a positive voltage on the control electrode ( called a grid ) are required. After ignition, the current flow through the mercury vapor rectifier can not be controlled with the help of the grid - it flows to the zero crossing of the alternating current and is re- ignited shortly after the beginning of the next half-cycle. In this way the current may be only one, but not eliminated (as with a thyristor).
Mercury arc rectifying the Nelson River HVDC ( reverse voltage: 150 kV, maximum current: 1800 A )
Mercury vapor rectifier Siemens, for 560 V, 1330 A
Efficiency
Almost regardless of the anode current, the voltage drop between the anode and cathode, the so-called burning voltage, about 12 V. This means that with a current of 500 A, a loss of power of 6000 W, which must be removed by water cooling. At lower flow air cooling may be sufficient.
These comparatively very high leakage is low efficiency and could not be tolerated in high-voltage equipment ( about 100 V ), but not at low voltages, such as in battery chargers. The invention of the semiconductor rectifier with significantly lower voltage drops ( about 1 V ) and thus significantly better efficiencies replaced the mercury vapor rectifier in no time.
Particularity
The electrical excitation of the mercury and its ability to emit visible light, and flashes an ongoing mercury vapor rectifier in light blue light (see fluorescent lamp). Here you can watch the cathode spot as migratory, green glowing spot on the surface of the mercury lake. At this point emerges from the electron current.
Others
The largest mercury arc rectifiers ever used were used in the Nelson River Bipole.
Controllable types of mercury arc rectifiers are the thyratron, the Ignitron and Excitron.
Was used for charging other types of batteries rectifiers, including the so-called " Tungar tubes ", which were capable of operating at the relatively low charging voltage (eg, 12 volts) to supply the required current.