Electrical Fundamentals in Arc Welding
Electrical Fundamentals in Arc Welding
Electrical Fundamentals in Arc Welding
INTRODUCTION
Electrical energy has many advantages over other forms of energy. Probably the greatest of these is the ease with which it may be transformed into heat energy. An electric arc from a welding power source may be considered a transformer of electrical energy to heat energy because the electrical energy is converted into heat during arc welding. Arc welding involves low-voltage, high current arcs between the electrode and the work piece. An electric arc is a very bright, luminous glow that assumes a nearly conical shape when it is not constricted or compressed. This arc is the result of the successful effort of an electric current to jump across or bridge an air or other gas (argon in GTAW etc.) gap that is introduced in its circuit. This electric arc may be formed when two conductors of an electrical circuit are brought together forming an electrical contact or a short circuit. When they are separated, and if sufficient voltage is available to maintain the current flow, the arc is established and maintained. There is considerable heat created by the resistance introduced into the flow of current by this air/gas gap (which is a poor conductor of electricity). These heated and ionized gases (gas atoms in the presence of an electrical current), are called arc plasma or sometimes called the arc flame. Heat distribution in an arc depends upon the density of current (the amperes per square-inch of the cross-sectional area of an electrode) in the electrode. Seventy percent of the total amount of electrical power converted into heat is concentrated at the positive electrode at the densities used in welding. This is at the point where the electric current passes from the solid medium of the electrode to the gaseous medium of the arc plasma (arc flame). The balance of the heat is produced in the arc plasma (arc flame) and on the negative work connection. It is this power massed or concentrated at the point of the arc that brings the arc to such a high temperature that makes it useful for welding. This dynamic arc phenomenon may vary widely as the voltage between the electrode and work piece changes as the arc length is increased or decreased.
SMAW Welding Arc In electric arc welding the arc is struck or established between a carbon or a metal electrode connected to one of the two secondary output terminals of an electric arc welding power source. The other connection is the metal to be welded, which is connected to the other terminal. When there is a continuous direct current arc the conductor, from which the current flows, is called the negative electrode or cathode. The conductor, to which Page 1
the current flows, is called the positive electrode or anode. The size of the arc (the height and width) can be varied by increasing or decreasing the arc length (distance across the gap), by increasing or decreasing the amount of current (amperage) or by placing a varying resistance in the circuit. In arc welding, the amount of heat produced in the welding arc will depend upon the current (amperage) flowing in the welding circuit. If more heat is needed in the weld (increased penetration), the amount of current must be increased. If the weld is too hot (too much penetration), the current must be decreased. The amount of electrical energy present in the arc, is calculated by multiplying the voltage (across the arc) times the current (in amperes) flowing in the circuit (VOLTS x AMPERES = WATTS). This product is measured in watts. The total amount of heat in a weld is also dependent upon the travel speed of the arc.
Electrical Fundamentals
VOLTAGE or VOLTS is the electrical pressure that causes the current to flow in an electrical conductor. The electromotive force (EMF) or voltage coming from a welding power source or primary utility power company sets up or creates a pressure that causes electrons to flow through a conductor. Voltage does not flow. Voltage is also called the potential difference. This is the difference in potential energy between the two ends of an electrical circuit (terminals). It is this difference of potential between the terminals of a welding power source or a dry cell battery that causes electrons to flow in a completed circuit. AMPERAGE is another name for electrical current flow. Amperage and current are synonymous and means, electricity in motion, or the flow of electrons through a conductor. Voltage has the most effect on the height and width of the weld. The strength of the current is known as its amperage and is measured by the electrical unit called the ampere. It is measured by an ammeter on the machine or a tong meter that is placed around a current carrying conductor. Polarity has very little effect on the amperage chosen for a welding procedure. Amperage has the most effect on the depth of penetration into the base metal. RESISTANCE is the opposition to electrical current flow. It is measured in OHMS with an ohm meter or volt-ohm meter (VOM). ALWAYS turn the electrical power off before checking continuity or the resistance of fuses, cables, rheostats or switches. Substances or materials vary in their ability to conduct electricity. Those that allow the flow of electrons are called conductors. Those that resist the flow of electrons are called insulators. The amount of resistance in a conductor depends upon (1) length of the conductor (the longer the conductor, the greater is the resistance), (2) cross-sectional area (the greater the diameter or size of the conductor, the resistance is less), (3) conductor material (certain metals have greater resistance than others, for example; carbon steel and stainless steel have greater resistance than aluminum which has 65% of the electrical conductivity of copper), (4) temperature (heat increases the resistance of metals). If resistance losses are excessive in a welding circuit, the results can be weld defects such as; lack of penetration, lack of fusion and cold lapping.
Summary
The illustrations shown on the following pages are intended to demonstrate the functions of voltage, amperage, and resistance in an electrical circuit. They use the example of water flowing through a pipe as a method of understanding the electrical terms covered here.
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The illustration shown below is intended to be a pipe filled with water. It could also be thought of as a piece of copper wire with the electrons being shown.
Water/Electrons The force needed to add another drop of water to the already full pipe is similar to the function of voltage which is force that causes electrons to move in a copper wire.
Pressure/Voltage
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The pressure created by adding one more drop of water to the pipe causes a drop of water to leave the pipe. In a similar manner the pressure caused by voltage in an electrical circuit causes current to flow in a copper wire.
Water Flow/Amperage If a narrow spot were added to the pipe resistance to water flow would be created. In an electrical circuit, a damaged wire or loose connection creates resistance which causes heat to be generated at the point of resistance.
Resistance/Heat Page 4
Measuring Single Phase Primary Power To measure three phase voltage, three different voltage readings will need to be taken. As illustrated below the meter leads will be placed at the bottom of the fuses for each of the readings. The voltage reading in each case will be the actual voltage for each phase of the system.
If the primary voltage fluctuates higher or lower the open circuit voltage (OCV) may be affected accordingly. That is, Input affects output! if the welding machine does not have a solid state, electronically controlled welding output (primary or utility line voltage compensation).
LL
LL
LL
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200 VOL TS
230 VOL TS
460 VOL TS
110
90
45
35
150
125
70
50
175
90
70
6 129 (39) 6
8 114 (35) 8
10 308 (94) 10
12 296 (90) 12
4 118 (36) 6
4 156 (47) 6
8 276 (84) 8
10 290 (88) 10
3 125 (38) 6
8 182 (55) 8
8 284 (86) 8
S-0092-J
Fuse/Breaker Chart
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Measuring Open Circuit Voltage LOAD VOLTAGE is measured with a voltmeter at the output terminals on the machine while welding or under a load. The load voltage, at a given load or welding current, is responsive to the rate at which a consumable electrode is fed into the arc. The arc length, the type and diameter of electrodes used, will determine the load voltage. It is the total voltage load, including arc voltage and the voltage drop through the welding cables and the material being welded, that the power source senses. When a nonconsumable electrode (tungsten in GTAW) is used, the load voltage is responsive to the electrode-to-work distance.
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Measuring Load Voltage ARC VOLTAGE is measured with a voltmeter across the arc between the electrode tip and the base metal surface while welding. Arc voltage has a direct relationship to the arc length. Arc Length is the distance through the center of the arc from the end of the electrode to the point where the arc contacts the surface of the work. If the welding circuit remains constant, the arc voltage increases as the arc is lengthened and decreases as the arc is shortened. This may vary due to many conditions which include, the temperature and gaseous content (ionization potential) of the arc. The arc voltage of a given arc length may also vary with current changes. Arc length for a flux-coated SMAW electrode is usually greater than what is apparent to the eye. This is because the end of the electrode core wire burns away more rapidly than the flux-coating. And this allows the flux-coating to come closer to the molten pool than the actual end of the core of the electrode. A short arc length may result in: (1) porosity in the weld, or (2) poor fusion. A long arc length presents the likelihood of: (1) poor penetration, (2) excessive exposure of the deposited metal to oxidation, (3) difficulties in concentration of welding heat, (4) wild and erratic arc action and (5) undercut. Voltage has the most effect on the height and width of the weld deposit.
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DC Weld Circuit THE POLARITY of the direct current welding arc or the direction of electrical current flow is very important. The Shielded Metal Arc Welding (SMAW) process was first used with bare or lightly flux-coated metallic electrodes with the electrode cable, and electrode holder, connected to the negative (-) terminal (pole). The work connection was then made to the positive (+) terminal (pole) of the welding power source. This is Electrode Negative and is called "Straight Polarity." When the electrode cable and electrode holder were connected to the positive (+) terminal (Electrode Positive) and the work cable connection was made at the negative (-) terminal. It was then said to be "Reversed Polarity", this is also known as "Reverse Polarity." It is important to make the connections so the current flow is in the right direction for the specific welding process and procedure being used.
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In a DC arc approximately 70% of the heat will be concentrated at the positive side of the arc. Electrode Positive produces deeper penetration into the base metal when welding with the consumable electrode processes. Electrode Negative produces a higher electrode melting rate (deposition rates) with the consumable electrode processes. Conversely, Electrode Negative produces deeper penetration into the base metal with a nonconsumable electrode such as the tungsten used in the GTAW process. Because the polarity of DC is not always changing like Alternating Current the arc is more stable with less fluctuation. When alternating current is used there is no difference in the heat developed or produced at either pole and polarity ceases to be important.
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Alternating Current
ALTERNATING CURRENT (AC) is an electrical current that has both a positive and a negative half-cycle value (polarities) alternately. Current flows in a specific direction for one half-cycle, stops at the "zero" line, then reverses direction of flow the next half-cycle at regular intervals. The AC sine wave represents the current flow as it builds in amount and time in the positive direction and then decreases in value and finally reaches zero. The current then reverses direction and polarity reaching a maximum negative value before rising to the zero value. This alternating repeats as long as the current is flowing. The number of cycles of alternating current that is completed in one second of time is called the frequency of the alternating current. The unit of frequency is the hertz (Hz). One hertz equals one cycle per second. The phrase "60 cycles per second or hertz" means that the particular alternating current completes 60 cycles in one second. One half of a cycle is called an alternation. There are two alternations in a cycle: one in a positive direction, the other in a negative direction. There are 120 alternations per second in a current of 60 cycles or hertz.
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AC Welding Circuit
AC Frequency
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10 Thru 100% Duty Cycle 3 1 1/0 2/0 3/0 4/0 4/0 2-2/0 2-3/0 2-4/0 2-4/0 1000 1000 2-750 2-750 2-1000 2-1000 2 1/0 2/0 3/0 4/0 2-2/0 2-2/0 2-3/0 2-4/0 1000 1000 1000 2-7500 2-750 2-1000 2-1000 1 2/0 3/0 4/0 2-2/0 2-3/0 2-3/0 2-4/0 1000 1000 2-750 2-750 2-750 2-1000 2-1000 1/0 3/0 4/0 2-2/0 2-3/0 2-3/0 2-4/0 1000 1000 2-750 2-750 2-1000 2-1000 2-1000 1/0 3/0 4/0 2-2/0 2-3/0 2-4/0 2-4/0 1000 2-750 2-750 2-1000 2-1000 2-1000
*Weld cable size (AWG and MCM) is based on either a 4 volt or less drop, or a current density of at least 300 circular mils per ampere
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Load In Watts 4
600 840 1200 1800 2400 3000 3600 4200 4800 5400 6000 400 (122) 300 (91) 225 (68) 175 (53) 150 (46) 125 (38) 112 (34) 100 (30) 87 (26)
Maximum Allowable Cord Length In Feet (Meters) For Conductor Size (AWG)* 6 8
350 (106) 400 (122) 275 (84) 175 (53) 137 (42) 112 (34) 87 (26) 75 (23) 62 (19) 62 (19) 50 (15) 250 (76) 175 (53) 112 (34) 87 (26) 62 (19) 50 (15) 50 (15) 37 (11)
10
225 (68) 150 (46) 112 (34) 75 (23) 50 (15) 37 (11) 37 (11)
12
137 (42) 100 (30) 62 (19) 37 (11) 30 (9)
14
100 (30) 62 (19) 50 (15) 30 (9)
Load In Watts 4
1200 1680 2400 3600 4800 6000 7000 8400 9600 10,800 12,000 800 (244) 600 (183) 450 (137) 350 (107) 300 (91) 250 (76) 225 (69) 200 (61) 175 (53)
Maximum Allowable Cord Length In Feet (Meters) For Conductor Size (AWG)* 6 8
700 (213) 800 (244) 550 (168) 350 (107) 275 (84) 225 (69) 175 (53) 150 (46) 125 (38) 125 (38) 100 (31) 500 (152) 350 (107) 225 (69) 175 (53) 125 (38) 100 (31) 100 (31) 75 (23)
10
450 (137) 300 (91) 225 (69) 150 (46) 100 (31) 75 (23) 75 (23)
12
225 (84) 200 (61) 125 (38) 75 (23) 60 (18)
14
200 (61) 125 (38) 100 (31) 60 (18)
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Duty Cycle
The DUTY CYCLE of a welding power source expresses, as a percentage, the actual operation time that it may be used at its rated load without exceeding the temperature limits of the insulation of the component parts. The rated load is the rated amperage at the rated load voltage. This is calculated by multiplying the rated amperage times the rated load voltage, the product is measured in watts. In the United States, duty cycles are based on a ten minute period of time. In some other areas, notably Europe, the duty cycles are based on a five minute period of time. This may be shown as a 100% duty cycle at a reduced rated load. Factors which contribute to lower performance include high ambient temperatures, insufficient cooling, air quantity, and low line voltage.
Chart Definition 0 10 Duty Cycle is percentage of 10 minutes that unit can weld at rated load without overheating.
Minutes
2 Minutes Welding
8 Minutes Resting
sb1.2 8/93 ST-086 727-A
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60
VOLTS
40
20
100
200
300 AMPS
400
500
Volt/Amp Curve Welding Amperage or Load Current is adjusted or set on Constant Current (CC) type welding machines. They are also referred to as variable voltage (V.V.) or Drooper type welding machines. They are called droopers because of the significant downward slope of the V/A curve. These machines are normally used for manual Shielded Metal Arc Welding (SMAW or Stick), Gas Tungsten Arc Welding (GTAW, TIG or Heliarc), or Air Carbon Arc Cutting & Gouging. They may also be used for Submerged Arc Welding (SAW), and Flux Cored Arc Welding (FCAW) with gas shielded or self shielded wires. Also Gas Metal Arc Welding (GMAW) Spray Transfer Mode may be possible if welding with Eighty (80) Per Cent or greater Argon in the shielding gas mix. These wire feed processes require an arc voltage driven (arc voltage sensing) type of wire feeder to maintain a constant stable arc length when welding with a constant current machine.
Constant Current (CC) type welding power sources set amperage. They are also referred to as variable voltage (VV) or drooper type power sources.
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Constant Voltage (CV) type power sources set voltage. They are also referred to as constant potential (CP). They provide a relatively flat volt-amp output characteristic
Constant Current Volt/Amp Curve Load Voltage is set on Constant Voltage (CV) welding machines. They are also referred to as Constant Potential (CP) or Variable Amperage. They provide a relatively flat volt-ampere weld output characteristic. The load current at a set load voltage is responsive to the rate at which the consumable electrode wire is fed into the arc. The current at the arc will be approximately proportional to the wire feed speed for all wire diameters. With a constant speed wire feeder, a continuous fed consumable electrode wire, and a constant voltage power source this is essentially a self-regulating system. The constant voltage welding power sources are used for the continuous electrode wire processes such as Gas Metal Arc Welding with any of the metal transfer modes, Flux Cored Arc Welding, Submerged Arc Welding and Air Carbon Arc Cutting and Gouging. It is also used as the power source for a multiple operator grid system for Shielded Metal Arc Welding, Air Carbon Arc Cutting and Gouging, and FCAW or GMAW Spray Transfer with arc voltage sensing wire feeders.
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T1 Pri. Sec. E
(VOLTS)
(AMPS)
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T1 Pri. Sec. E
(VOLTS)
(AMPS)
T1 Pri. Sec. Z E
(VOLTS)
(AMPS)
T1 Pri. Sec. E
(VOLTS)
(AMPS)
Tapped Reactor (Stabilizer) Current Control The voltage that the secondary side of a transformer will deliver depends upon the number of turns of wire in the primary and secondary coils and the amount of voltage received by the primary. The voltage that will be delivered by the secondary side of the transformer has a direct relationship to the electric current in the secondary side. If the voltage has been stepped down (decreased), the current is stepped up (increased); if the voltage is stepped up (increased) the current is stepped down (decreased). In the primary side of a step-down transformer that is used for welding both the voltage and the number of turns are high but the current is low and the wire size is small. The primary coil is wound with many turns of small diameter wire. In the secondary side, both the voltage and the number of turns are low but the current is high and the wire size is large. The secondary is wound with fewer turns of larger diameter wire. Therefore, large secondary welding currents can be obtained from relatively low primary line currents. In a step-up transformer, such as is used in the high frequency arc starter circuits these conditions are reversed. The primary coil is wound with a few turns of large diameter wire and the secondary with many turns of smaller wire. The secondary voltage is very high and the current is very low.
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Rectifiers
Rectifiers are devices that change alternating current into direct current. They permit or allow the passage of current in one direction only through the circuit. Rectifiers may be selenium, silicon diode, silicon controlled (SCR) or a bridge rectifier. Most rectifiers are made of silicon because of economy, current-carrying capacity, reliability, and efficiency. A single rectifying element is called a diode, which is a one-way electrical valve. The SCR is a diode variation with a trigger called a gate. SCRs can be used to directly control welding power by altering the welding current or voltage waveform. Because the output characteristics are controlled electronically, automatic line voltage compensation is easily accomplished. This allows welding power to be precisely set and held at that value even if the input line voltage varies. The SCR can also serve as a secondary contactor allowing the welding current to flow only when commanded. The AC/DC and DC static welding power sources usually incorporate both a transformer and a rectifier. The transformer - rectifier type arc welding power source has a "stabilizer" or "inductor" added to the DC portion of the power source circuitry. (The term "stabilizer" and "inductor" are synonymous in electrical terms). A stabilizer is an iron core with a current carrying coil wrapped around it. It is placed in the DC portion of the power source output. The function of the stabilizer is to provide inductance or choke in the dc welding circuit to improve arc stability. This slows down the rate of response of the power source to changing arc conditions.
()
AC
(+)
LOAD
DC OUTPUT
AC INPUT
AC ()
AC INPUT
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AC INPUT
DC OUTPUT
LOAD
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Electrical Control
Electrically controlled machines like the constant current DIALARC 250 AC/DC and the Gold Star SRH series use a low voltage and low amperage dc circuit to change the effective magnetic characteristics of reactor cores. They are referred to as a magnetic amplifier because a relatively small control power change will produce a sizeable output power change. They use a rheostat to control the welding amperage output. They permit remote welding output control and normally have fewer moving parts than the mechanically controlled machines. Like the mechanically controlled machines the electrical controls are subject to atmospheric corrosion and there is no primary line voltage compensation.
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*While idling.
Sample Power Source Specification Chart POWER FACTOR CORRECTION applies to AC single-phase primary power. Power factor is the amount of electrical power used with reference to the total amount of power supplied. It is the ratio of the actual or usable power in watts to the volt-amperes provided in an ac circuit. Power Factor = Watts Volts x Amps
It is the measure of time phase difference between the voltage and amperage in an alternating current circuit. When the amperage and voltage are in phase, the power factor is unity.
Good vs Poor Unity Power factor takes place on the primary side. It has no affect on output. It is not related to rise or drop in the primary line voltage. Page 31
Customer's reason for buying power factor corrected power source: 1. Reduced primary amperage draw, this means: a. Smaller primary wire size required* b. Smaller primary fuses * To verify, check Owner's Manual or input conductor fuse size card for specific model designation comparison. 2. Less kilovolt amps (kVA) required per power source, this means: a. Can place more power factor corrected power sources on existing plant wire size** b. Can place more power factor corrected power sources on existing plant transformers**
** To verify, check catalog specifications or Owner's Manual on specific model desiring comparison.
3. Can cost less for primary power because: a. If customer has good power factor rating, he can get a better rate per kWH from power company than if the power factor were poor.*** b. Power company may charge penalty fees added to normal electrical bill for poor power factor.***
***The local power company the customer is using must be checked to see how they handle their poor power factor billings. Some power companies may not even allow a poor power factor device to be connected to their systems.
The following data is based on a Syncrowave 250 AC/DC with 200 amp 28 volt rated output secondary load:
Power Factor Correction Without With Without With 60 Hz Amps Input at AC Balance, Rated Load, 1-phase 200V 230V 460V 575V KVA KW 88 3.3* 60 55.3* 110 3.3* 82 55.3* 77 2.8* 52 49.5* 96 2.8* 71 49.5* 38 1.5* 26 24.5* 48 1.5* 35 24.5* 31 1.1* 21 19.6* 38 1.1* 28 19.6* 17.6 .59* 12.06 11.2* 21.98 .59* 16.32 11.2* 8.6 .29* 8.11 .39* 11.76 .29* 11.81 1.93* Welding Amp Range 5 310 A 5 310 A 5 310 A 5 310 A
Rated Output 200 A, 28 VAC, 60% Duty Cycle, NEMA Class I (40) 250 A, 30 VAC, 40% Duty Cycle, NEMA Class II (40)
Max OCV 80 V 80 V 80 V 80 V
*While idling.
Proof of 1. Standard power source 74 amps at 230 volt primary Power Factor Corrected power source 48 amps at 230 volt primary Proof of 2. Standard power source 17.0 kVA Power Factor Corrected power source 11.0 kVA This means if the customer had a 150 kVA transformer supplying power to weld stations, only 8 standard power sources could be run, whereas 13 power factor corrected units could be used at rated output. 8.82 17 150 13.63 11 150
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Proof of 3. Most power companies base their charges on kWH and PF. The power factor is expressed in hundredths, such as .51 or in percent such as 51%. The power company would then like to have power factors of 1.00 or 100%. Since this is virtually impossible, they normally settle on a livable amount, let's say approximately .70 or 70%. To calculate power factor, use the following formula: % Power Factor (PF) = Example: Standard Power Source % PF = PF Corrected Power Source % PF = 8.3 kW x 100 = 48% PF 17 8.3 kW x 100 = 75.4% PF 11 Pri kW x 100 kVA
Again, the local power company must be checked on what they consider poor power factor and what they do about it. Customer's reason for not buying power factor corrected power source: 1. Costs more money. (The points above must be pointed out.) 2. The power factor corrected power source uses the same amount of wattage so is not more electrically efficient. 3. It is not primary line voltage compensating. The power source will have this as standard if it is a solid state controlled power source 4. Has virtually no effect on arc characteristics. Proof of 1. Power factor correction can add an additional 2-20% to the cost of a power source. The specific power sources being considered should be compared and the above information used to encourage or discourage it based on the facts. Proof of 2. Consult the Owner's Manual or catalog specification of kW. The kW shown will be primary kW at the rated secondary load shown. Do not multiply primary amperage draw times the primary voltage. This will yield volt amps not watts. You must use the wattage specified or measured wattage with a watt meter. To calculate electrical efficiency, use the following formula: % Efficiency = Sec. kW Pri. kW x 100
Since the secondary wattage is not given in the Owner's Manual or catalog specifications, it must be calculated by using the following formula: Sec. kW = Amps x Volts 1000
This example has been based on a 200 amp arc at 28 volts = 5.6 kW for both the standard power source as well as the power factor corrected model: Sec. kw = 200 amps x 28 volts = 5.6 kW for both 1000
Volts X amps in this case equals watts because a welding arc is a resistive load.
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The primary kW draw is also the same for both power sources: 8.2 kW. % Efficiency = 5.6 kW 8.3 kW x 100 = 67% efficiency
Thus proving the power factor corrected power source uses the same amount of wattage on the primary as does the standard power source to produce the same wattage in the arc. Both power sources have the same electrical efficiency. Proof of 3. Since a capacitor is a storage device and cannot create energy, it will not compensate for line voltage fluctuations. Example: If the power source is designed to operate on 230 volt primary and this voltage drops below this value, the output power to the arc will also drop. If the primary voltage goes above 230 volts, the output power to the arc will also go up. Since the capacitor is being charged from the primary voltage it will follow the primary voltage very rapidly and cannot possibly correct any long term primary voltage fluctuations. If you can read the primary voltage fluctuations on a standard volt meter, the capacitor will do nothing to correct for this and the arc is going to fluctuate. Proof of 4. In the matter of arc characteristics there are no meters or devices that can accurately measure what is felt in the mind of the welder watching an arc. However, it has been shown to be virtually impossible for a welder to determine any difference in arc characteristics between a standard power source and one that is power factor corrected. This can be easily set up by using both types and not allowing the welder to know which type of power source is being used. Then run a series of trials to see if the welder can determine which is the power factor corrected model.
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