MERCURY ARC RECTIFIER

The mercury-arc rectifier comprises an evacuated glass vessel having a pool of mercury at the bottom (cathode) and graphite block (the anode) at the top. The device is connected to circuit through wires sealed into the glass which serves as terminal. When the conduction starts, ionization produces the characteristic blue luminosity. The cathode spot emits electrons and mercury vapor, the anode attracts the electrons; the electrons in their flight shock-ionise the molecules of mercury vapor ; the enriched electron stream proceeds to the anode ; the ionized mercury vapor molecules are driven downwards to the cathode spot. The upward electron stream and downwards iou stream constitute an electric current passing from anode to cathode internally. The action of the the rectifier requires the cathode spot to be produced first by some auxiliary device before the arc can be established. This is usually done by introducing arc between cathode and an auxiliary anode - the process is termed as ignition. Further the arc will be maintained only when the current is above certain level.

Total internal losses in rectifier are given by

Losses = Voltage drop at anode + Voltage drop at cathode + drop in the arc
= 9V + 5 V + (0.1 L) V (approx.)

where
L is the length of arc in cm.

Single-phase full wave mercury arc rectifier:

The two anodes a and b are connected to two terminals T₁ and T₂ of a center-tapped secondary of a transformer. The DC output is taken from cathode and center tap of secondary of transformer.

Three phase mercury arc rectifier:

Here there are 3 anodes placed at equidistance in a circle and at equal distances from the cathode.

THREE ANODE RECTIFIER:

The arrangement of three anode rectifier is shown in the figure given below. The three secondary windings are star connected and the star point forms the negative output terminal. The voltages of the three secondary phase are equal, but displaced in lime by one third of a period. As the conduction will take place between the cathode and that anode with the maximum positive voltage, consequently the arc will change from anode to anode, thus remaining with each anode for one third of the cycle period in the middle of its positive half cycle.



As shown in Figure below, the output waveform comprises the successive tops of the positive waves. The output waveform is still smooth when size anodes are used.

Advantages of mercury arc rectifier:

1. Less chances of faults as there is no moving part in it
2. Maintenance cost very low.
3. Simple in operation as compared to other rectifiers.
4. Vaporized mercury not wasted.
5. Less costly as compared to rotary converter.
6. Efficiency high (95-96%).

Disadvantages of mercury arc rectifier:

1. It cannot be used as invertor.

2. Always power loss in the excitation anode circuit when working without load.