[larry lee]

Multiplaz-3500 Evaluation, Part 08: Electrical Power Issues

DISCLAIMER!
Let me emphasize that I will not be able to tell you whether the Multiplaz-3500, or any other piece of equipment, will be a good investment for you. Only you can decide that. My intent is to provide as much factual information as I can about the Multiplaz-3500 so that others in our company can make an informed decision about that. The company has no objection to my sharing the information with you as long as I leave their name out of it and make it clear that I am not endorsing any particular product.
DISCLAIMER!

This evaluation started because of our interest in self-contained (i.e., only electricity needs to be provided) plasma cutters and welders. As part of that more general evaluation, I now have access to a Miller Spectrum 125C plasma cutter for evaluation and I will soon have access to a Hobart AirForce 250ci plasma cutter as well. Whether or not I will get access to a Lincoln Tomahawk 375 Air plasma cutter remains to be seen. I will report on those evaluations later in separate posts. To the extent that some of those results are relevant to “Comparing the Multiplaz Cutting Torch”, I will also include them in later posts of the Multiplaz Evaluation. In the meantime, while the Multiplaz welding torch is being tested, I would like to talk about AC power quality, as it is a factor in the evaluation of all of these machines.

Ideally, the AC power that is provided to us by the power company is a pure sine wave in terms of voltage with no distortion. The frequency of the sine wave and the standard voltage depend upon the geographical location. As electronic devices proliferate and the world as a whole becomes more and more dependent on commercial AC power, there is more and more concern about “electrical pollution” and its effects on the AC electrical system itself and on devices connected to the system. At present, the largest contributor to this “electrical pollution” is the “spikey” current switching in the AC-to-DC converter in most power supplies, especially switching-mode power supplies (SMPS). SMPSs are incorporated in almost all modern electronics, including welding power supplies. (The “inverter welding power supply” is a SMPS.)

The “spikey” current switching introduces lots of harmonics into the power system. These harmonics are undesirable because:
1. The harmonics (multiples of the fundamental AC frequency) cause more heating in conductors because of higher eddy current losses.
2. The AC power system has a finite impedance, so the harmonics in the current waveform get incorporated into the voltage waveform and it becomes distorted. Significant overvoltages and undervoltages can result.
3. In 3 phase, 4 wire systems (common in most commercial buildings), the harmonic currents can add (instead of cancel) in the neutral wire to almost twice the current in each of the other three lines. All four wires are normally installed with identical sizes, so the higher current in the neutral wire can cause it to melt, with catastrophic results.
4. Voltage waveforms with harmonics (spikes and glitches) can cause malfunction in many types of electronic equipment, with loss of time, money, data, and sometimes life.
An excellent reference document, called “Understanding Power & Power Quality Measurements” is available from AEMC Instruments at
http://www.aemc.com/techinfo/techworkbo ... _power.pdf

AC power quality is expressed in terms of several parameters;

Power Factor (PF) = (Actual Power in watts) / (Apparent Power in volt-amperes).

For a pure resistive load, the power factor is 1.00.
For loads in which the current waveform is not identical (in time and shape) to the voltage waveform, the power factor is between 0 and 1. A low power factor means the AC current ratings for the generator, conductors, transformers, circuit breakers, etc. must be larger than actually necessary by a factor of 1/PF. The AC power companies usually charge commercial and industrial customers more for their power if the power factor is less than 0.9. The charge increases with lower power factor. At present, there is no additional charge to residential customers for low power factor, as the design of the traditional revolving-disk kilowatt meter is such that it only measures actual power and not apparent power. I do not know whether the new SmartMeters are capable of measuring both actual power and apparent power.

Total Harmonic Distortion (THD)

is a measure of the distortion due to all of the harmonics in the (current or voltage) waveform. It is calculated as the quadratic sum of (Magnitude of Each Harmonic / Magnitude of Fundamental). The result is usually expressed as a percentage. A perfect sine wave has a THD of 0 percent. A very distorted waveform may have a THD of more than 100% (i.e., there may be more harmonics than fundamental frequency).

Switching-mode power supplies do not necessarily have to have “spikey” current switching. That is just the cheapest and simplest way to do it. And since there was no law against it, that is what most manufacturers did. The effects on the AC power system were not considered, at least not for most items. Because of the details of the AC power distribution systems in other parts of the world, the average amount of “electrical pollution” is often much greater than it is in the USA. Hence, other countries have taken the lead in limiting the amount of “electrical pollution” that a device may put into the AC power system. The European Directive IEC 61000-3-2, and later IEC 61000-3-4, specify limits on the harmonics that may be at the “point of common connection” to the AC power system (usually the AC input cord). The strictest limits are on consumer electronics and lighting (because of the huge number of such devices), but with time the limits are being extended to a wider range of devices. Devices covered by the directives are not permitted to be sold in the European Union unless they have been tested and shown to have harmonics within the specification. Many other countries have adopted (or are adopting) the same or stricter requirements.

You may have noticed that some welder manufacturers (for example) have two versions of the same machine: one labeled as “CSA” for sale in the Americas and one labeled as “CE” for sale in Europe and Asia. The “CE” machine usually weighs a little more and it costs more. The difference is not as a result of the different AC voltage and frequency input, but rather that the “CE” machine incorporates additional circuitry to reduce harmonics and (usually) increase the power factor.

PF and THD are not the same, but they are related. And there is a difference in perspective as to the best way to reduce the “electrical pollution”. In the USA, the EPA Energy Smart requirements are usually stated in terms of minimum power factor, whereas the IEC Directives are stated in terms of maximum harmonics. A properly designed power supply will have both low harmonics and high power factor. Initially it may cost a little more than a “spikey” one, but it need not. Once power-factor correction (PFC) is the norm, improved designs and economies of scale are likely to make the cost essentially the same.

A useful approximate formula that relates power factor (PF) and total harmonic distortion (THD) for most SMPS designs is

(1 + THD*THD) (PF*PF) = ~1

So, what has all of this got to do with the evaluation of the Multiplaz-3500? The Multiplaz-3500 power supply is a SMPS and it is widely sold in Europe and Asia. It is an outstanding example of a well designed power supply with low “electrical pollution”. It operates with any AC voltage input from 100 V (Japan) to 253 V (other countries), either 50 Hz or 60 Hz. The maximum actual input power is 2.0 kilowatts at 110 volts and 3.5 kilowatts at 220 volts. The duty cycle is 100%. The power factor with any AC input at any rated load (3 A to 9.5 A DC) is 0.99. This was verified by our measurements. The actual AC power to DC power conversion efficiency is ~0.84. We also measured the THD for both current and voltage.
The THD of the voltage at the outlet with the Multiplaz off was 0.6% at 121 V. The THD of the input current to the Multiplaz when operating is 6.8% at 12.5 A. The impedance of our AC power system at this outlet is such that the AC voltage drops about 1 V for this load current. Therefore, one expects the contribution to the voltage THD to be ~(1/121)(6.8%) = 0.06%. Combining 0.6% and 0.06% quadratically gives 0.6%, no significant increase. The actual measurement of the voltage THD was 0.6%.

Just for comparison, look at some of the preliminary values for the Miller Spectrum 125C plasma cutter. This is a good plasma cutter, but the SMPS was not designed to conform to the CE requirements. As a result, the machine is designated as “CSA only”. We measured the power factor as ~0.70 at rated load (12 A DC). The actual AC power to DC power conversion efficiency is ~0.68. The THD of the voltage at the outlet with the Miller off was 0.6% at 123 V. The THD of the input current to the Miller when operating is ~100% at 26 A. The impedance of our AC power system at this outlet is such that the AC voltage drops about 2 V for this load current. Therefore, one expects the contribution to the voltage THD to be ~(2/123)(100%) = 1.6%. Combining 0.6% and 1.6% quadratically gives 1.7%, a significant increase. The actual measurement of the voltage THD was 1.8% (i.e., 3 times the original THD).

By contrast, more modern designs from Miller, Hypertherm, and others include power factor correction (PFC) and have power factors from 0.97 to above 0.99. However, they all still have much lower duty cycles than the Multiplaz, even though the actual AC power input and DC power output to the plasma are almost exactly the same. I don’t know why the difference in the duty cycle.

Another example is the energy-efficient refrigerator that I purchased a few years ago. It has a SMPS that converts 60 Hz to higher frequency AC so that the compressor motor can be smaller and more power efficient. The PF is 0.56 and the THD of the current is 140%. It uses less actual AC power and less apparent power than the old refrigerator, but the THD contribution to the AC voltage waveform is greater.

I urge you to consider PF and THD when you purchase any electrical device. It is especially important for devices that are to run off of an AC generator. The generator must be able to provide the apparent power (not just the actual power) and must also be able to withstand the additional harmonic currents. For this reason, most specifications for non-PFC electrical devices (including welders and plasma cutters) recommend that the generator have a capacity in watts at least twice the actual power drawn by the device.

to be continued

larry lee

[Billbong]

SHREEEEEK.....at about this point my brain shut down and I wandered off in a daze, wondering if I had made a bad decision in getting the 'Plaz, but as soon as I plugged it in and started the welding cycle I just forgot what THD & PF meant, so now I'm happy just using it straight out of the box.

Anyone reading the above "essay" might get a warm glow from the technical side of the bed, but when it comes to using the 'Plaz, it's hands on and fiddle around until you get it right.....worked for me right from the word go.

It does help a bit if you've already got some welding knowledge with other gear, otherwise it takes a bit longer.

I get depressed now 'cos all the other threads on stick, Mig, Tig and Oxy/Acc have no interest for me, but living with a specialist piece of equipment is much better than continually battling against factors I cannot control.

Factors like different welding rods, gas mixes for different materials and whether DC is better than AC for some job or other....too much to continually worry about.

Now, if it'll melt with heat I can weld it....end of message.
Ian.