ch-08 welding electricity _1

7/31/2019 Ch-08 Welding Electricity _1

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8. WELDING ELECTRICITY

8.1 WELDING ELECTRICITY

The electrical arc welding circuit is the same as any electrical circuit. In the simplest electrical

circuits, there are three factors:

Current: flow of electricity

Pressure: force required to cause the current

Resistance: force used to regulate the flow of current

Current is a rate of flow. Current is measured by the amount of electricity that flows through a

wire in one second. One ampere (A) is the amount of current per second that flows in a circuit.

The letter I is used to designate current in amperes.

Pressure is the force that causes a current to flow. The measure of electrical pressure is the volt.

The voltage between two points in an electrical circuit is called the difference in potential. Thisforce or potential is called electromotive force (EMF). The difference of potential or voltage

causes current to flow in an electrical circuit. The letter E is used to designate voltage or EMF.

Resistance is the restriction to current flow in an electrical circuit. Every component in the circuit,

including the conductor, has some resistance to current flow. Current flows easier through some

conductors is less than others. Resistance depends on the material, the cross-sectional area, and

the temperature of the conductor. It is designated by the letter R. The unit of electrical resistance

is the ohm. Copper is widely used for conductors since it has the lowest electrical resistivity of

common metals. Insulators have a very high resistance and will not conduct current.

The simple electrical circuit shown in Fig 8.2 includes two meters for electrical measurement, a

voltmeter, and an ammeter. It also shows a symbol for a battery. The longer line of the symbolrepresents the positive terminal. Outside of a device that sets up the EMF, such as a generator or a

battery, the electron current flows from the negative (-) to the positive (+). The arrow shows the

direction of current flow.

The ammeter is a low-resistance meter, shown by the round circle and arrow adjacent to the

letter l. The pressure or voltage across the battery can be measured by a voltmeter. The voltmeter

is a high-resistance meter, shown by the round circle and arrow adjacent to the letter E.

The resistance in the circuit is shown by a zigzag symbol. The resistance of a resistor can be

measured by an ohmmeter. An ohmmeter is never used to measure resistance in a circuit when

current is flowing.

The relationship of these three factors is expressed by Ohms law as follows:

R

EIor

ohms

voltsamperes

or

resistance

pressurecurrent


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Where I = current in amperes (flow)

E = pressure in volts (EmF)

R = resistance in ohms

Ohms law can also be expressed as

By simple arithmetic, if two values are known or measured, the third value can be determined.

Fig. 8.1: Simple electrical circuit.

A few changes to the circuit can be made to represent an arc welding circuit. Replace the battery

with a welding generator since they are both a source of EMF (or voltage) and replace the resistor

with a welding arc, which is also a resistance to current flow. The electron current will flow from

the negative terminal through the resistance of the arc to the positive terminal.

In the early days of arc welding, using bare metal electrodes, it was normal to connect the

negative side of the generator to the electrode and the positive side to the workpiece. This wasknown as straight polarity. When deeper penetration was required on the base metal, the polarity

would be reversed. This connected the electrode to the positive pole of the generator and the

workpiece to the negative pole.

In those days, to change the polarity it was necessary to remove the cables from the machine

terminals and replace them in the reverse position. The early coated electrodes for welding steel

gave best results with the electrode positive or reverse polarity; however, bare electrodes were

still used. It was necessary to change polarity frequently when using both bare and covered

electrodes.

To meet this condition, welding machines were equipped with switches that changed the polarity

of the terminals and with dual reading metes. Thus the welder could quickly change the polarityof the welding current. In marking welding machines and polarity switches these old terms were

used and indicated the polarity as straight when the electrode is negative, and reverse when the

electrode is positive. In these lectures, to avoid confusion, whenever polarity is discussed the term

electrode negative (DCEN) is used instead of straight polarity (DCSP) AND ELECTRODE

POSITIVE (DCEP) is used instead of reverse polarity (DCRP).

The ammeter used in a welding circuit is a millivoltmeter calibrated in amperes connected across

a high current shunt in the welding circuit. The shunt is a calibrated, very low resistance

conductor. The voltmeter shown in the figure will measure the welding machine output and the

I

ERorIRE


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voltage across the arc which are essentially the same. Before the arc is struck or if the arc is

broken, the voltmeter will read the voltage across the machine with no current flowing in the

circuit. This is known as the open circuit voltage and is higher than the arc voltage or voltage or

voltage across the machine when current is flowing.

Fig. 8.2: Welding electrical circuit (Straight Polarity).

Another unit in an electrical circuit, and important to welding, is the unit of power. The rate ofproducing, or of using, energy is called power and is measure in watts. Power in a circuit is the

product of the current in amperes times the pressure in volts, or

power = current x pressure

or watts = amperes x volts

or

P = I x E

where P = power in watts

I = current in amperes

E = pressure in volts

When welding using a 3.2 mm electrode at 100 amperes and an arc voltage of 25, the power

would be 2500 watts (W), 2500 W can be expressed as 2.5 kilowatts (kW). Power is measured by

a wattmeter, which is a combination of an ammeter and a voltmeter.

In addition to power, it is necessary to know the amount of work involved. Electrical work of

energy is the product of power times time and is expressed as watt-seconds or joules or kilo-watt-

hours.

Work = power x time or W = Pt

Where W = work in watt-seconds or joules or kilowatt-hours

P = power in watts or kilowatts

T = time in seconds or hours

Cost-of-welding calculations involve these work units since the watt-hour or kilowatt-hour are

commercial units of work and are the basis of charges by the electric utility companies.


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So far, we have dealt exclusively with direct current electricity, electricity that flows continually

through the circuit in the same direction. Alternating current electricity is also important since it is

the power furnished by utility companies.

Alternating current is an electrical current which flows back and forth at regular intervals in a

circuit. When the current rises from zero to a maximum, returns to zero and increases to a

maximum in the opposite direction, and finally returns to zero again, it is said to have completedone cycle. For convenience, a cycle is divided into 360 degrees. Fig 8.5 is a graphical

representation of a cycle and is called a sine wave. It is generated by one revolution of a single

loop coil armature in a two-pole alternating-current generator. The maximum value in one

direction is reached at the 90 point and in the other direction at the 270 point. The number of

times this cycle is repeated in one second is called the frequency and is measured in hertz. When a

current rises to a maximum in each direction 60 times a second it completes 60 cycles per second

or has a frequency of 60 hertz (Hz).

The principle of electrical generation states that when a conductor moves in a magnetic field so

as to cut lines of force an electromotive force is generated. The lines of force run between the

north and south magnetic poles of the generator. The single turn coil rotates within these lines of

force or magnetic filed and as the conductor cuts the lines of force at right angles the maximumvoltage is generated (i.e., at 90 and at 270). When no lines of force are being cut as at position

0, 180, and 360, there is no EMF generated. The EMF generated in the one loop coil is taken

from the rotating armature by means of slip rings. In welding generators there are usually more

than two poles and many hundred loops of wire in the coil.

Fig. 8.3: Sign wave generation.

Alternating current (ac) for arc welding normally has the same frequency as the line current. The

voltage and current in the (ac) welding arc follow the sine wave and go through zero twice each

cycle. The frequency is so fast that the arc appears continuous and steady to the naked eye. The

sine wave is the simplest form of alternating current. It is always assumed that alternating current

has a sine wave shape unless otherwise stated.

Alternating current and voltage are measured with ac meters. An ac voltmeter measures the value

of both the positive and negative parts of the sine wave. It reads the effective voltage, called the

root-mean-square (rms) voltage. The effective direct-current value of an alternating current or

voltage is 0.707 times the maximum value.

An alternating current has no unit of its own, but is measured in terms of direct current, the

ampere. The ampere is defined as a steady rate of flow, but an alternating current is not a steady


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current. An alternating current is said to be the equivalent to a direct current when it produces the

same average heating effect under exactly similar conditions. This is used since the heating effect

of a negative current is the same as that of a positive current. Therefore, an ac ammeter will

measure a value called the effective value of an alternating current which is shown in amperes. Al

ac meters, unless otherwise marked, read effective values of current and voltage.

Ohms law also applies to alternating-current circuits. This is because Ohms law deals only withvoltage, current, and resistance. In alternating current welding circuits there are other factors, and

one of the most important is inductance. To understand inductance we must refer to magnetism.

Fig. 8.4: Transformer Principle.

A magnet has a north pole and a south pole, which have identical strength. Between these poles

there are lines of force. This effect can be shown by sprinkling iron filings on a sheet of paper and

placing it over a magnet. The distinct pattern shows these lines of force running from one pole to

the other. Similar lines of force exist around electric conductors that carry direct current. This can

be proven by placing a small compass near a current-carrying wire. The needle will deflect when

the current is turned off and on. Magnetic lines of force create physical forces between magnets or

magnetic fields around current-carrying wires. This is the principle of operation of an electric

motor. The magnetic properties of a ferromagnetic material such as iron when wrapped with a coil

of wire are such that the combination will produce a much stronger magnetic field that the

magnetic field produced by the coil alone. The coil of wire around an iron core is a magnetic

circuit. Magnetic circuits will have a specific inductance. Inductance expresses the results of a

certain arrangement of conductors, iron, and magnetic fields. Inductance involves change since it

functions only when magnetic lines of force are cutting across electrical conductors. Inductance is

important only in alternating current circuits or in direct-current circuits when they are connected

or disconnected. When the current is turned off, the magnetic field collapses and the lines of force

cut across the wires and induce current in the wires in the same direction as it had been flowing. If

the coil is connected to alternating current the lines of force build up the maximum and then

collapse and then build up in the opposite direction to a maximum and collapse each cycle. Ifanother coil is placed on the same iron core and close to the first coil the magnetic lines of force

will cut across the second coil and induce the EMF in it. The closer the coils, or the stronger the

magnetic lines of force, the greater will be the induced EMF. This is the principle of the

transformer and is shown in Fig.8.6. By changing the magnetic coupling of the two coils we can

control the output of the second coil (the secondary) and thus the output of the welding

transformer. This coupling can be changed by moving the coils closer together or by increasing

the strength of the magnetic field between them. The strength of the magnetic field can be

changed by putting more or less iron in the area between the coils or by adjusting the availability

of the magnetic field in other ways.


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The output of a transformer-welding machine is alternating current of the same frequency as the

input power. A rectifier is a device that conducts current easier in one direction than the other. It

has a high resistance to current flowing in one direction and a low resistance to current flowing in

the opposite direction. A diode vacuum tube is an efficient rectifier but will not carry sufficient

current for welding. Another type, the dry disk rectifier, employs layers of semiconductors such as

selenium between plates. The newest and most popular rectifier is the silicon diode. These are

made of thin wafers of silicon that have had small amounts of impurities added to make themsemiconductors. The wafers are specially treated and then assembled in holders for mounting in

welding machines. The diodes are connected to the output of a welding transformer to produce a

rectifier-welding machine with dc output.

This brief discussion of electricity is presented to help explain the electricity of welding.

ch-08 welding electricity _1

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