AC - DC Rectifiers

AC - DC Rectifiers

AC-DC Rectifiers - An AC-DC rectifier can supply any type of current needed, although some transformer-rectifiers are designed to supply only DC. In former machines, by turning a switch, the output terminals can be changed to the transformer or to the rectifier and give either AC or DC straight or reverse polarity current. Such machines are essentially transformers containing an electrical device known as rectifier which changes AC to DC. The rectifier may consist of metal plates coated with a selenium compound or silicon diodes, each unit having the special property of allowing the current to flow in one direction only.
Rectifier units are designed to provide a choice of low voltage for MIG and submerged welding or a high open circuit with drooping voltage characteristics for TIG and flux shielded metal arc welding. A rectifier unit is installed with a fan to cool the transformer. A rectifier unit may be designed for a single phase or three phase power supply. Rectifier units may be of (A) Constant current characteristics. (B) Constant voltage characteristics.
	(A) Constant Current Machines The static volt-ampere characteristics of constant current rectifier machine: (a) Open circuit voltage usually is in the range of 70 to 80 volts. (b) System to adjust welding current is usually in the AC section of the machine before the rectifiers. The control of the current uses the principle of variable inductance or impedance. The methods of varying the impedance for current control are: (i)Moving shunt (ii) Moving coil (iii) Saturable reactor (iv)Tapped reactor (v) Moving reactor core.
In DC rectifier type welders if arc current is controlled by resistance instead of by the traditional inductive reactance system, (i) There is no time delay in current response after a change in arc voltage. (ii) Absence of this time delay and comparatively higher maximum current value achieved result in more pinch effect. (iii) It tends to prevent cold shorting (between the electrode and the workpiece) and lost arc phenomenon (arc disappears because of cold shorting). (c) A stabiliser is used in the DC circuit to minimize excessive current surges in load current. (d) Constant current type rectifiers are employed for
	(i) Shielded metal arc welding (ii) TIG welding (iii) Carbon arc gouging (iv) Submerged arc welding (v) Stud welding (vi) Plasma arc welding. (B) Constant Voltage Machine (a) The first machines in this classification had a relatively flat V-I characteristics of about 1 volt drop per hundred amperes. Some machines have a slightly rising V-I curve* designed to match the arc length - arc voltage characteristics of the arc and is adjustable from 0 to 6 volts rise per hundred amperes. In some welding processes, easier arc starting and better arc stability could be obtained with the rising-voltage type machines. At present a variable inductance. (or stabilizer) is included in the machines to improve welding with low voltage arcs.

(b) The voltage ratings range from 25 to 50 volts. Open circuit voltages go as high as 75 volts. Current ratings are from 200 to 1000 amperes.
(c) The duty cycle rating is usually 100%.

(d) Constant voltage type rectifiers are employed with semi-automatic and automatic\ welding processes using the gas shielded are, flux cored electrode, small wire submerged arc and electroslag processes.

Advantages of AC-DC Rectifiers
The advantages of these rectifier sets are:
(i) They can provide both straight and reverse polarity.
(ii) They possess all the advantages of an AC transformer.
(iii) They have good performance even at low welding currents.
(iv) Their current can be adjusted by using a remote control kept near the operator.

Disadvantages of Rectifier sets
1. The life of the selenium plates is limited to about seven years under normal usage.

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