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The inverter as a welding power source.

The inverter as a welding power source

The inverter welding power source has the potential to revolutionize the welding industry. In fact, the inverter can appropriately be called the power source of the future. Although the inverter is becoming a common piece of welding equipment there is still much mystery surrounding this radically different type of power source.

What is an inverter?

It is a specific type of power converter. Power converters change electrical power from one form to another. There are four basic types of converters:

* DC to DC: choppers

* DC to AC: inverters

* AC to AC: converters

* AC to DC: rectifiers

An inverter then is a power converter that changes DC power into AC power. In the broadest sense of the definition, any welding power source that operates from 50 to 60 Hz AC input power is either an AC converter (AC to AC for AC weld output power) or rectifier (AC to DC for DC weld output power). These power sources take the high-voltage, low-current input (primary) power and transform it into low-voltage high-current isolated secondary power. This secondary power is then converted and conditioned into usable welding power through the use of thyristors and appropriate control circuits (Figure 1).

What is commonly called an inverter welding power source is really three converters in one. The actual inverter portion merely converts DC to AC, but before that happens the primary AC must first be converted to the necessary DC, and after the inverter operation the AC must be converter back to DC welding output power (Figure 2).

Why use inverter technology?

If a conventional machine can convert AC to DC welding power why use one that first converts AC to DC, DC to AC, and then AC back to DC? The answer lies in the following basic relationship that applies to all transformers: V = NAfK

The magnitude of voltage to be transformed (V) is proportional to the number of turns of wire on the transformer coil (N), the cross-section area of the transformer core (A), the frequency of the AC voltage to be transformed (f), and various design constants (K).

So what does this all mean? Well, for a transformer designed to transform a given voltage, if the number of turns on the primary coil were doubled, the cross sectional area of the core could fbe cut in half and the overall operation would be the same. In like manner, if the core area were doubled, the turns could be cut in half. Also, if the frequency of operation were doubled, either the turns or core area could be halved. What, then, if the frequency were increased 10 times, 100 times, or even 400 or 500 times? Imagine how the number of turns and/or the core area could be reduced!

Inverters do just that. They operate at frequencies from a few kilohertz to 100 kHz. Inverters create their own operating frequency to take advantage of drastically reduced transformer size possible when operating so much above 60 Hz. While some inverters do operate in the 2 to 10 kHz region, there are definite advantages to operating above 20 kHz. One advantage is elimination of audible noise. Another is faster response time. Hence, better performance is possible at higher operating frequencies.

Types of inverters

There are numerous ways to construct an inverter, and there are various types of power semiconductors that may be used as solid-state switches necessary for inverter implementation. Topology refers to the type of inverter design and how various components are interconnected. Industry recognized topologies include the flyback, forward, full- and half-bridge, push-pull, and series reasonant inverter.

Inverter topologies best suited to the high power requirements of welding power sources are the forward, full- or half-bridge, and the series resonant designs. Typically, forward inverters use transistors as power-switch components, and the series resonant inverter uses thyristors. The transistors could be either bipolar or power MOSFET, and the thyristors could be fast-switching SCRs, ASCRs (asynchronous SCRs that block voltage of one polarity), or GTOs (gate turn-off SCRs).

How do inverters work?

Inverters change a DC voltage into an AC voltage. They do so by the on/off action of high-power solid state switches. This on/off action alternately connects and disconnects the transformer primary fromf an energy source and has the same basic effect on the transformer as applying a regular sinusoidal waveform. The methods of producing this AC and the means of controlling the output voltage of the power source are significantly different in the various topologies. It is not the intent of this article to discuss fthe intricate details of each topology but rather to explain general inverter operation. Therefore, I will limit discussion to the Miller Arc Pak 350 as an example of a high-performance inverter type of welding power source.

The transistorized inverter

The power module. Referring to Figure 3, the basic schematic for the Miller ARC Pak 350 transistorized inverter, you can see that, when the power switches Q1 and Q2 are on, the transformer primary A-B is connected to the DC buss voltage. "A" is connected to the positive side of the buss, and "B" to the negative side. This action causes a proportional voltage to be generated in the secondary winding. Whatever load is present at the output causes current Is in the secondary and a proportional current Ip in the primary.

When the transistor switches turn off, A is disconnected from the (+) buss and B is simultaneously disconnected from the (-) buss. As the switches turn off, circuit inductance maintains current in the transformer primary; diodes D7 and D8 begin to conduct; and point A is now connected to the (-) buss and B to the (+) buss.

The voltage across the primary is of opposite polarity as to when the switches were on, and Ip decreases toward zero. When the transistors again switch on at the beginning of the next cycle, the process repeats. Note that the two switches turn on and off together. Although current to the load is delivered by the inverter power stage in pulses, the load current IL is continuous. During the time the power switches are off, IL continues through the free-wheeling diode D10 as a result of the inductance of L1 and that of the output cables.

Control circuits

Referring to Figure 2, we can see that it is the control circuitry which turns the power switches on and off in response to the output demand of the load as sensed by the feedback circuits. The basic operating frequency, 20 to 50 kHz for state-of-the-art high-performance designs, is set by components in the control circuit. The amount of power the unit can supply to the arc is dependent upon the DC bus voltage, transformer primary current, and the ratio of ON time to OFF time of the power switches. For a given bus voltage, a high level of output power requires a much shorter ON time (Figure 4). Thus, output power is varied by changing or modulating the ON time of the transistors and consequently the width of the voltage pulse applied to the transformer primary. This technique of power control is appropriately called pulse-width modulation (PWM).

The transformer

The Transformer used in an inverter, aside from being incredibly small in size and weight, also has other interesting characteristics. Core material usually is not laminated steel as found in traditional 60-Hz machines, but rather a material called ferrite. Ferrite is ceramic-like material molded into various forms from a liquid slurry. Ferrite is used in inverter machines because the high operating frequencies would cause laminated-steel cores to overheat as a result of eddy-current and hysteresis losses.

Conductors used in the transformer coil quite often are not made of regular magnet wire, but rather extremely thin copper strip or a specially woven wire called litz wire. These conductors also are used because of high operating frequency.

Benefits of using inverters

Precise, high-speed response. Referring to Figure 1, we see that conditioning and controlling output power of a conventional machine is accomplished by phase control of the thyristors in the output rectifier, Figure 5. The earlier in the AC cycle the thyristors are turned on, the greater the output. The longer turn-on is delayed in the cycle, the lower the output. Once turned on, however, the device remains on until the end of the cycle.

In a 60-Hz machine, the power available in any one cycle is relatively large. Also, the unfiltered output of the rectifier bridge has considerable ripple, particularly at low outputs. For suitable welding, this output must be filtered (smoothed) by a large choke, and perhaps even one or more capacitors.

The inductance of a choke determines its smoothing capability; the larger the inductance, the greater the smoothing effect. Because of the filtering effect of the choke and capacitors, the rate at which the control circuits can force the output to respond is relatively slow.

In addition to smoothing, the output choke also limits the rate of current rise when the power-source output is shorted. In short-circuit-transfer GMAW (Gas Metal Arc Welding), the electrode actually contacts the workpiece about 100 times/sec. Each time this happens the electrode places an effective short across the machine output.

The electrode remains in contact with the work until it is sufficiently heated by the current passing through it to melt free. It is the output choke in conjunction with the control circuit that determines the behavior of the arc when the electrode burns free of the work. Because each size and type of electrode (mild steel, stainless, aluminum, etc) presents a different type of short to the power source and reacts differently when the short is cleared, it is very difficult to obtain optimum welding characteristics for applications from a 60-Hz power source with a fixed value of filter inductance.

In an inverter machine, most of the control and conditioning of the output power takes place in the inverter section, on the primary side of the transformer. The amount of power available in a single cycle of an inverter machine is drastically less than in a single cycle of a thyristor phase-controlled unit. This is because of the must higher operating frequency and the correspondingly shorter time duration of each cycle. Consequently, the output power can be controlled much more precisely and also much quicker. because there is such a short time interval between each cycle, the stabilizer (choke) requirements are minimal and the stabilizer can be very small, both in inductance value and also physical size.

Electronically variable inductance. In a thyristor machine, the purpose of the stabilizer is to maintain load current between power cycles and to limit the rate of current rise when the machine output is shorter by the electrode. In an inverter machine, operating in the 20 to 50 kHz range, the operating cycles are much shorter, the control-circuit response time is much faster, and the degree of output control is much more precise.

Because it is primarily the control circuit that dictates the behavior of the output current, the manner in which the output current responds to a short can be controlled accurately--and even predetermined. Thus, electronically variable inductance is easily implemented. The current can be made to increase faster or slower by a simple setting of a control potentiometer, thereby simulating a lower or higher value of inductance. Thus, optimum weld characteristics can be obtained for various wire sizes and types.

Smooth, clean arc starting

The ultra-fast-circuit response and variable-inductance feature also provide smooth, clean, blast-free arc initiation. Three factors that greatly affect the quality of arc initiation are the initial rate of rise of current in the electrode, the amount of inductance in the stabilizer, and the response time of the power source. Starting is enhanced by rapid but controlled current rise, a minimum of inductance, and circuits that responds quickly to the rapidly changing conditions at the output of the machine during starting. An inverter machine possesses all three characteristics, hence it has the potential to provide clean, stumblefree starts.

Pulse welding capabilities

Pulse welding, both GTAW (Gas Tungsten Arc Welding) and GMAW, is continuing to gain popularity and is an application for which the inverter machine is ideally suited. The fast response and fine control of an inverter permit all four pulse parameters (peak, background, pulse time, and pulse frequency) to be continuously varied and precisely controlled.

A variety of pulse shapes can also be produced, from almost perfectly rectangular to trapezoidal or even triangular. Pulse current can be adjusted continuously from machine minimum to maximum. Background current can be set to any value, independent of peak settings. Pulse duration can be adjusted from fractions of a millisecond to hundreds of milliseconds. Pulse frequency can be adjusted continuously from a minimum of one pulse every several seconds to several hundred pulses per second.

Virtually any inverter power source can be used for pulse welding, but maximum pulse frequency and pulse quality is limited by the topology used. Generally, higher pulse frequencies and better pulse waveforms are possible with topologies that allow inverter operation at higher frequencies, say 20 to 50 kHz.

Other benefits

Size and weight. Inverter machines are much smaller and lighter than conventional machines of comparable output rating, allowing easy movement and minimizing floorspace requirements.

Primary power requirements. An inverter machine can readily be used on either three-phase or single-phase input power and either 50 or 60 Hz operation. Because the input is immediately rectified and converted from AC to DC, the type and quality of input power is less critical than with conventional machines.

Multiprocess capability. The fast response time of inverters operating in the 25 kHz range and the ease with which such units can be made to respond can make them ideal for multiprocess operation. Unlike conventional multiprocess machines, which may have to compromise performance in one or more areas, a high-performance inverter machine can excel in all areas of operation.

Cooling means. Because of an inverter's small size and efficient design, the units are easy to cool with relatively small fans. It is possible, though prudent design, to construct a machine that isolates cooling air and associated harmful contaminants from electronic components.

Conclusion

The inverter welding power source, although appearing to be very complicated, is really fairly easy to understand. It is basically a piece of electronic equipment that converts "raw" primary power into manageable power specifically suited to the requirements of the welding arc. Inverters take advantage of the latest innovations in power and control electronics fand electromagnetic materials to offer a welding power source of benefit to the industry.
COPYRIGHT 1989 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Author:Schiedemayer, Marv
Publication:Tooling & Production
Date:Mar 1, 1989
Words:2430
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