Rectifier and Converter: Concepts ( sai saikumar jn )

Rectifier and Converter: Concepts

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.

ROTARY CONVERTERS:

The rotary converters, also known as synchronous converters are rotating machines for converting an a.c. supply to d.c. or vice versa. Such machines have constructional features similar to dc machines with stationary field magnet system and a rotating armature carrying a double lap or wave winding. The winding is connected to a commutator upon which the brushes are connected to the dc terminals. Also at the opposite end of the armature to the commutator are slip-rings connected to tapping in the armature winding and the brushes on these rings are connected to the ac terminals.

The supply on the ac side may be 1, 3, 6 or 12 phase. Accordingly the number of slip rings may be 2, 3,6 or 12. The no load ratio of the voltage E_a between successive slip rings (i.e. the armature phase voltage) to the commutator voltage E_d is

E_a / E_d = 1 / (√ (2 ) ) sin (π / N )

where

N is the number of slip rings.

N	Ratio Ea/ Ed
2	0.707
3	0.61
6	0.35
12	0.18

Rotary Converters:

The ac and dc armature powers will be equal when :

(i) losses are neglected.
(ii) power factor is unity.







In that case the ratio of slip ring (line Current) to the dc output current will be

I_L / I_D = (2 √ ( 2 ) ) / N

Where

N is the number of slip rings.

N	Ratio IL/ ID
2	1.41
3	0.94
6	0.47
12	0.24

For efficiency η and power factor cos φ, the ratio will be greater by a factor 1/ η cos φ.

SI. No.	Particular	Rotary Converter	Metal plate rectifier	Mercury arc rectifier
1.	Construction	Very difficult to construct	Most easy to construct	Easy to construct as compared with rotary converter
2.	Operation	Requires great care at the time of starting	Its starting operation is most easy	Operation easier than that of rotary converter.
3.	Sound and vibration	Produces sound and vibration	Soundless and produces no vibration	No vibration and soundless operation
4.	Fault	Chances of occurring faults are more due to rotating machine	Less chances of faults in it	Very less chances of faults
5.	Rating	Can be manufactured for high rating only	Manufactured in both small and high capacity	It cannot be made in low capacity
6.	Cost	Initial cost if very high	Least costly	Initial cost is less than rotary converter.
7.	Maintenance	Needs great care and maintenance as it is a rotating machine	Requires less maintenance	Needs less maintenance as compared to rotary converter.
8.	Efficiency	Fair	Good	Good
9.	Life	Long	Long	Very long
10.	Application	It can be used as a dc motor, ac motor, converter and inverter. It is specially used in sub-stations to convert ac to dc.	It is used for battery charging, electroplating, electrolysis, etc.	It is employed in substation to convert ac to dc and is superceding the rotary converters.

from ur's -- Bellapuri saikumar
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