arc welding amp drop question

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Is it normal that the DC ampere reading on an arc welder positive lead would be different than what is shown on the rheostat dial while welding?I have a gas powered portable arc welder that is rated at 180 amps, and I did a test with an inductive DC amp meter and an assistant.  At each setting of 100, 120, 140, 160 and 180 amps, the readings were between 20 and 28 amps below the settings on the panel.  For instance, at 180 amps on the rheostat dial, I got 156 amps on the meter.  Is that typical?  What is the reason/science for this? Is the rheostat dial "optimistic" in this situation?Thanks for your expertise!Jordan
Reply:Quite common.  Some are optimistic, some read low.  Machine calibration, electrical loss through your leads, etc can all affect actual output.
Reply:Doe any part of your cables get hot?  If they do, this could be where the missing amps are.Good connection at the ground and stinger?David Real world weldin.  When I grow up I want to be a tig weldor.
Reply:Would the length of his leads effect that?Yup
Reply:Originally Posted by BurnitWould the length of his leads effect that?
Reply:As I understand it, added resistance in the weld circuit, from a poor connection or excessive lead length, will not affect current from a constant current power supply.  Added resistance will cause the power supply voltage to increase in order to push the same current through a increased resistance.That said, I don't know what would cause your engine driven power supply current to be low, maybe lower than normal rpm?
Reply:You are also not recording the actual amperage in the plasma colum itself either. --Gol'
Reply:The welders's scale probably indicates maximum potential.Clamp a large carbon rod between the leads, or short a large electrode directly to the work; that will probably raise amp readings to where you like ‘em.Good Luck
Reply:Originally Posted by Go1lumYou are also not recording the actual amperage in the plasma colum itself either.
Reply:In a 'plain' series resistance circuit, the amperage is the same throughout the circuit and each resistive 'element' has a voltage drop across itself.  If you add up all the voltage drops throughout the circuit, they will total to the 'supply' voltage.In a 'plain' parallel resistance circuit, the voltage is the same throughout the circuit and each resistive 'element' flows current through itself based on that voltage and the resistance of the element.  If you add up all the currents through each element, they will total to the current coming from the supply.pulser,Small detail.  You could have a measurement of the voltage across the arc, but the current measurement would be through the arc.  Voltage is 'across' and current is 'through'.  That's why with a typical multimeter you put the probes 'across' something to measure the voltage, but you have to put the meter in the circuit and measure the current flowing 'through' the meter/circuit.  Clamp meters are different in that you don't have to 'break' the circuit to get a current measurement, but you are still measuring the current flowing 'through' the wire when you clamp around that wire.Back to the original question.  The machine can only put out so many amps at so many volts.  If the cables/connections have 'losses' (resistance due to corrosion on a contact, undersized wire for the desired current flow, etc), then the resistance in the circuit (machine+connections+leads+electrode+arc and then back to the machine via workclamp+leads+connections+machine) would be higher.  The higher resistance in the overall circuit means that the current in the circuit (anywhere in the circuit here, because it is pretty much a series resistive circuit at that part of things and as such the current ANYWHERE in that circuit will be the same.  So you could put a big clamp meter around a welding lead and measure the current in the circuit.) would be lower for any given voltage from the supply.  That's Ohm's Law E=IR.Your readings are probably a mix of optimistic dial, error in the meter, machine variations, and losses in the connections/leads/etc.And 24 amps out of 180 is pretty close to a 10% error (13%  in this case), which is a general tolerance value for things by me.  If your meter is off 5% and the machine is off 5%, you have your 10% error right there.  The best laid schemes ... Gang oft agley ...