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Letters and emails should contain complete name, address and daytime phone number. Letters to the Editor are submitted on the condition that Silicon Chip Publications Pty Ltd may edit and has the right to reproduce in electronic form and communicate these letters. This also applies to submissions to "Ask SILICON CHIP" and "Circuit Notebook".

Mains AC waveform distortion was not an April Fool story

I am a member of the CMCA (Campervan and Motorhome Club of Australia) and regularly follow the CMCA members’ forum. Recently on the forum there have been discussions about problems with running some loads on portable generators of the “non inverter” variety. The loads that sometimes give trouble are appliances with electronic control circuits that have switchmode power supplies.

At one point, I mentioned that the problem might be caused by the fact that such power supplies might be introducing “distortion power factor” (DPF) into the AC power circuit due to the fact that they (along with gas-discharge lighting etc) present a non-linear load to the power source, thus distorting the AC waveform.

Now, I’m unsure how this might affect the operation of the power supply (maybe it is a victim of its own DPF) or perhaps the introduced distortion is affecting the AVR in some generators. On a small “closed” system, like a washing machine as the only load on a generator, the effect could be quite severe. In most cases, the addition of a purely resistive load, like a small heater, will at least allow the washing to be done but unnecessarily makes a lot of heat in the process.

I’m not even sure of the mechanism which allows this “fix” to work. I would love to have the facilities at home to do some experiments in order to investigate further (I am now retired). Linear power factor mismatch, such as capacitive or inductive loads, does not distort the power waveform but does, of course, adversely affect the efficiency of the generation and distribution system, as you suggest.

I am well aware of the problem with cheap generators and their poor regulation and generally “filthy” output. I own one myself to keep the fridge going during our all too frequent power failures!

However, I think there is more going on here. Some of the generators involved are quite expensive well-known brands such as a 5kVA Onan (American) running a microwave oven and the most recent incident involved a $3000 or so 4.5kVA Honda-powered unit (I forget the brand name) which quoted output harmonic distortion at less than 5%. That Honda would not start a little Dometic (designed for RV use) washing machine (800W) without a fan heater running at the same time. It can be quite a frustrating problem as sometimes you never know what appliance will run with which generator without a trial run.

However, I read your piece entitled “Why is the AC Mains Waveform Distorted?” in the April 2012 issue of SILICON CHIP and I made reference to your article on the forum and mentioned a few of the main points in it. One member replied to the effect that he would tread warily as he knows the Publisher of SILICON CHIP has a sense of humour and that the month of publication of the article may be significant.

I would really like to know if the article was indeed serious journalism or not. It certainly sounded serious but then I guess that is the mark of a good “April Fool” gag.

Rod Goodwin,

Tinana, Qld.

Comment: no, it wasn’t an April Fool’s Day gag.

Clock radios gain time with grid-feed inverter

As an aside to Leo Simpson’s article on “Why is the 50Hz AC mains waveform distorted” (April 2012) I recently had a solar system installed that has produced an interesting side effect on the mains supply that remains unresolved. In April we had a 1.56kW solar panel system with a 4.2kW Growatt inverter (MTL) installed at my home. A couple of days after the system was installed I noticed that the two clock radios in our house had each gained about 12 minutes. We have had the radios for a number of years and they have never gained before.

I reset the clocks at approximately 1800 hours (6pm). At 0700 hours the clocks were correct but at 1800 hours they were both approximately six minutes fast again.

Given that the clock radios use the 50Hz mains supply as a reference for the clock function, it seems logical that the inverter is causing the problem since it is the only new item connected to the system. The frequency display on the inverter hunts between 49.9Hz and 50.0Hz and this is as I would expect.

It seems to me that the inverter has a fault (possibly a latent defect in the design) that is imposing some form of harmonic distortion that is superimposed onto the sinewave that it is designed to produce (if indeed it is a pure sinewave inverter). Since the system is under warranty it is beyond my scope to investigate inside the “box” but a review of the mains frequency shape using my 20MHz oscilloscope shows some distortion and clipping to the peaks of the sinewave but that distortion (similar to that shown in Leo Simpson’s article) is there even when the inverter has switched off at the end of daylight.

I think that more sophisticated test equipment than I have access to is required to get to the bottom of this mystery.

The matter was referred to the company that installed the system but their remedy was to offer me $100.00 to buy two new clock radios and would do nothing to verify the cleanliness of the inverter output. My next action was to contact the supply authority and their initial response was that the inverter was not their responsibility and I would have to go to the company that installed the system to have it fixed.

I then explained that if enough of these inverters were connected to the main supply system, the resulting noise on the 50Hz waveform could interfere with their control signals and surely they, as the supply authority, were concerned that any equipment connected to the mains only produced a “clean” 50Hz. At this stage the matter became too technical for the phone operator and she promised that the matter would be raised with the supervisor on the next working day and they would get back to me.

I guess the bottom line is I am getting a reduction in my electricity bill for connecting a solar system to the network and nobody is really interested if there are all sorts of harmonics on the waveform as long as it is 50Hz. I am waiting with some interest to see what happens next.

Gordon Dennis,

Mill Park, Vic.

Comment: the way in which grid-feed inverters generate a sinewave is similar to that employed by the Induction Motor Speed Controller (SILICON CHIP, April & May 2012) and that means that there will be switching artefacts in the waveform, possibly at frequencies above 20kHz.

It is possible that these switching artefacts from the inverter are interfering with the clocks’ function and it may well be that if you fed them via a mains filter then all would be well.

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