MANUFACTURING PROCESS - ARC WELDING POWER SOURCES

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ARC WELDING POWER SOURCES

THE MAIN REQUIREMENT OF A POWER SOURCE IS TO DELIVER CONTROLLABLE CURRENT AT A VOLTAGE ACCORDING TO THE DEMANDS OF THE WELDING PROCESS BEING USED. EACH WELDING PROCESS HAS DISTINCT DIFFERENCES FROM ONE ANOTHER, BOTH IN THE FORM OF PROCESS CONTROLS REQUIRED TO ACCOMPLISH A GIVEN OPERATING CONDITION AND THE CONSEQUENT DEMANDS ON THE POWER SOURCE. THEREFORE, ARC WELDING POWER SOURCES ARE PLAYING VERY IMPORTANT ROLE IN WELDING. THE CONVENTIONAL WELDING POWER SOURCES ARE: 

POWER SOURCE SUPPLY
POWER SOURCE
SUPPLY
(I) WELDING TRANSFORMER
AC
(II) WELDING RECTIFIER
DC
(III) WELDING GENERATORS
AC OR DC (DEPENDING ON GENERATOR)
WELDING TRANSFORMERS, RECTIFIERS AND DC GENERATORS ARE BEING USED IN SHOP WHILE ENGINE COUPLED AC GENERATORS AS WELL AS SOMETIMES DC GENERATORS ARE USED AT SITE WHERE LINE SUPPLY IS NOT AVAILABLE. NORMALLY RECTIFIERS AND TRANSFORMERS ARE PREFERRED BECAUSE OF LOW NOISE, HIGHER EFFICIENCY AND LOWER MAINTENANCE AS COMPARED TO GENERATORS. SELECTION OF POWER SOURCE IS MAINLY DEPENDENT ON WELDING PROCESS AND CONSUMABLE. THE OPEN CIRCUIT VOLTAGE NORMALLY RANGES BETWEEN 70-90 V IN CASE OF WELDING TRANSFORMERS WHILE IN CASE OF RECTIFIERS IT IS 50-80 V. HOWEVER, WELDING VOLTAGES ARE LOWER AS COMPARED TO OPEN CIRCUIT VOLTAGE OF THE POWER SOURCE.
BASED ON THE STATIC CHARACTERISTICS POWER SOURCES CAN BE CLASSIFIED IN TWO CATEGORIES
•  CONSTANT CURRENT OR DROOPING OR FALLING CHARACTERISTIC POWER SOURCE.
• CONSTANT POTENTIAL OR CONSTANT VOLTAGE OR FLAT CHARACTERISTIC POWER SOURCE. 

CONSTANT VOLTAGE POWER SOURCE DOES NOT HAVE TRUE CONSTANT VOLTAGE OUTPUT. IT HAS A SLIGHTLY DOWNWARD OR NEGATIVE SLOPE BECAUSE OF SUFFICIENT INTERNAL ELECTRICAL RESISTANCE AND INDUCTANCE IN THE WELDING CIRCUIT TO CAUSE A MINOR DROOP IN THE OUTPUT VOLT AMPERE CHARACTERISTICS.
WITH CONSTANT VOLTAGE POWER SUPPLY THE ARC VOLTAGE IS ESTABLISHED BY SETTING THE OUTPUT VOLTAGE ON THE SOURCE. THE POWER SOURCE SHALL SUPPLY NECESSARY CURRENT TO MELT THE ELECTRODE AT THE RATE REQUIRED TO MAINTAIN THE PRESET VOLTAGE OR RELATIVE ARC LENGTH. THE SPEED OF ELECTRODE DRIVE IS USED TO CONTROL THE AVERAGE WELDING CURRENT. THE USE OF SUCH POWER SOURCE IN CONJUNCTION WITH A CONSTANT ELECTRODE WIRE FEED RESULTS IN A SELF REGULATING OR SELF ADJUSTING ARC LENGTH SYSTEM. DUE TO SOME INTERNAL OR EXTERNAL FLUCTUATION IF THE CHANGE IN WELDING CURRENT OCCURS, IT WILL AUTOMATICALLY INCREASE OR DECREASE THE ELECTRODE MELTING RATE TO REGAIN THE DESIRED ARC LENGTH. 

THE VOLT AMPERE OUTPUT CURVES FOR CONSTANT CURRENT POWER SOURCE ARE CALLED ‘DROOPER' BECAUSE OF SUBSTANTIAL DOWNWARD OR NEGATIVE SLOPE OF THE CURVES. THE POWER SOURCE MAY HAVE OPEN CIRCUIT VOLTAGE ADJUSTMENT IN ADDITION TO OUTPUT CURRENT CONTROL. A CHANGE IN EITHER CONTROL WILL CHANGE THE SLOPE OF THE VOLT AMPERE CURVE. WITH A CHANGE IN ARC VOLTAGE, THE CHANGE IN CURRENT IS SMALL AND, THEREFORE, WITH A CONSUMABLE ELECTRODE WELDING PROCESS, ELECTRODE MELTING RATE WOULD REMAIN FAIRLY CONSTANT WITH A CHANGE IN ARC LENGTH. THESE POWER SOURCES ARE REQUIRED FOR PROCESSES USING RELATIVELY THICKER CONSUMABLE ELECTRODES WHICH MAY SOMETIMES GET STUBBED TO WORKPIECE OR WITH NONCONSUMABLE TUNGSTEN ELECTRODE WHERE DURING TOUCHING OF ELECTRODE FOR STARTING OF ARC MAY LEAD TO DAMAGE OF ELECTRODE IF CURRENT IS UNLIMITED. UNDER THESE CONDITIONS THE SHORT CIRCUITING CURRENT SHALL BE LIMITED LEADING TO SAFETY OF POWER SOURCE AND THE ELECTRODE. 

SOME POWER SOURCES NEED HIGH FREQUENCY UNIT TO START THE ARC, WHICH MAY BE REQUIREMENT OF PROCESSES LIKE TIG AND PLASMA ARC. HIGH FREQUENCY UNIT IS INTRODUCED IN THE WELDING CIRCUIT BUT IN BETWEEN THE CONTROL CIRCUIT AND HF UNIT, FILTERS ARE REQUIRED SO THAT HIGH FREQUENCY MAY NOT FLOW THROUGH CONTROL CIRCUIT AND DAMAGE IT. HIGH FREQUENCY UNIT IS A DEVICE WHICH SUPPLIES HIGH VOLTAGE OF THE ORDER OF FEW KV ALONG WITH HIGH FREQUENCY OF FEW KHZ WITH LOW CURRENT. THIS HIGH VOLTAGE IONIZES THE MEDIUM BETWEEN ELECTRODE AND WORKPIECE/NOZZLE STARTING PILOT ARC WHICH ULTIMATELY LEADS TO THE START OF MAIN ARC. ALTHOUGH HIGH VOLTAGE MAY BE FATAL FOR THE OPERATOR BUT WHEN IT IS ASSOCIATED WITH HIGH FREQUENCIES THEN CURRENT DOES NOT ENTER BODY BUT IT CAUSES ONLY SKIN EFFECT I.E. CURRENT PASSES THROUGH THE SKIN OF OPERATOR CAUSING NO DAMAGE TO THE OPERATOR. 

DUTY CYCLE:
DUTY CYCLE IS THE RATIO OF ARCING TIME TO THE WELD CYCLE TIME MULTIPLIED BY 100. WELDING CYCLE TIME IS EITHER 5 MINUTES AS PER EUROPEAN STANDARDS OR 10 MINUTES AS PER AMERICAN STANDARD AND ACCORDINGLY POWER SOURCES ARE DESIGNED. IT ARCING TIME IS CONTINUOUSLY 5 MINUTES THEN AS PER EUROPEAN STANDARD IT IS 100% DUTY CYCLE AND 50% AS PER AMERICAN STANDARD. AT 100% DUTY CYCLE MINIMUM CURRENT IS TO BE DRAWN I.E. WITH THE REDUCTION OF DUTY CYCLE CURRENT DRAWN CAN BE OF HIGHER LEVEL. THE WELDING CURRENT WHICH CAN BE DRAWN AT A DUTY CYCLE CAN BE EVALUATED FROM THE FOLLOWING EQUATION; 

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