What is a welding transformer?

The transformer, housed in a welding machine, is used to convert the high-voltage input or, primary power, from the wall plug and this is usually from 208 to 600 volts, with a low alternating current (AC) from 15 to 55 amps. This is transformed in the secondary power side to a lower voltage of up to 80 volts and a range of welding currents up to 1000 amps AC or more, depending on the process and equipment.

Figure 1 shows the typical shielded metal arc welding (SMAW) hook up of a welding machine illustrating the main power supply at the primary side, and the output to the electrode holder from the secondary side of the transformer.

Figure 1, Hook up diagram for a typical shielded metal arc welding process

The transformer performs the task described by “stepping down’ “from the high voltage/low current primary side, where we use a large number of turns of smaller wire (N1 in the schematic) and a lesser number of large wires turns (N2 in the schematic) on the secondary side. This outputs low voltage/higher amperage based on the turn’s ratio, or the number turns of wire on the secondary side as illustrated in Figure 2.

Figure 2, Schematic of the step-down transformer

The wires are wrapped around an iron core which creates a magnetic flux from the movement of electrical energy through the transformer. The amount of amperage output determines the size of the transformer. The higher the amperage output the larger the transformer, and the heavier and larger the machine becomes. Figure 3 shows a typical transformer, the AC high voltage/low current enters at the input conductor and AC low/voltage/high amperage exits at the output conductor.

Figure 3, Actual step-down transformer

The first welding machines were alternating current (AC) only and alternated between electrode positive and electrode negative up to 60 times per second as per Figure 4.

Figure 4, Image of a balanced AC wave

The advancement of the processes required transforming the AC current to direct current (DC) to provide for a more stable welding arc and a variation in weld bead penetration with electrode polarity selection. To achieve DC output a rectifier diode was used as per Figure 5.

Figure 5, Typical diode

The diode works by allowing the AC current to flow through the diode but not allowing the AC current to flow back, thus creating direct current (DC) which is used on most welding machines today. These transformer rectifiers would use a series of diodes in a bridge to generate DC output, as shown in Figure 6. The AC inline power would go through the welding transformer and out through a series of rectifying diodes in the bridge and transform to a smooth DC current output.

Figure 6, Transformer-rectifier technology

The welding transformer for the typical AC/DC welding processes was very large and heavy and many advancements were made to reduce the transformer size. In the late 1970’s the initial welding inverters began to appear. This inverter technology was being introduced with several advantages. One of these was a way to transform the high voltage/low current input to low voltage/high current output that would reduce the size and weight of the welding transformer. Figure 7 shows how the inverter technology works inside a power source.

Figure 7, Schematic of Inverter technology

Inversion technology is the opposite of rectifying, the inversion process converting direct current into alternating current at a high frequency using a switching type of regulation made up mostly of transistor devices.

The switching of currents is performed on the high voltage primary input side of the transformer rather than the more conventional secondary output side as described previously. The schematic in Figure 7 shows the high AC voltage entering and being inverted to DC, switched to a high frequency pulsating AC square wave, and then “transformed” to low voltage, high current rectified DC at the output. This is the way many welding machines are today using this inverter technology which reduces the need for very large and heavy welding transformers and thus significantly reducing the size and weight of equipment.

The technology also reduces the amount of energy (electrical power) the inverter technology uses compared to the older transformer rectifier machines.

Bill Eccles, VP PPC and Associates

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