Author Topic: Total Harmonic Distortion and generator heads  (Read 12650 times)

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rearden

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Total Harmonic Distortion and generator heads
« on: October 16, 2012, 06:43:21 AM »
I have a question about Total Harmonic Distortion and its relationship to generators.
I have learned that THD can be dynamically increased by the type of (non linear) load on a system, but what I am interested in is the THD which is caused by the alternator itself.
From what I have read most modern devices with electronics, which would be nearly everything in your home especially large appliances, require a power source with THD <5%.  Better sine wave inverters and home standby generators advertise that their output power falls into this category.  Generators you buy at home improvement stores generally do not have THD numbers (if they even post them) of <5%.  My questions are:

1.   What characteristics of an alternator cause it to produce power which is not 100% X hz sine wave and to produce significant harmonics?
2.   What is different (if anything) in the alternator heads of generators with high and low THD?  An example would be Generac XP (and home standby) and GP(or XG) series.
3.   Is the increase in THD simply due to less heavy fly wheel or more poor governor and RPM regulation?
4.   Given an alternator with x% THD, what practical methods can be used to lower the THD?

I have an off grid solar set up with a generator to provide cloudy day and extra power.  This portable generator would also provide backup power for an on grid house and random portable power needs.  I donít want to fry my wifeís fancy refrigerator, furnace controls or LCD monitor, but I am trying to understand what engineering (and marketing guano) is behind the 3-4x price increase (besides better motor), what can be accomplished with DIY to get lower THD and what to look for when obtaining a generator head.

rearden

bob g

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Re: Total Harmonic Distortion and generator heads
« Reply #1 on: October 16, 2012, 09:34:17 AM »
generally you will get what you pay for, afterwhich there is little you can do about it.

the prevalent factor in thd is within the design of the alternator itself and less to do with the governor.

things like type of excitation have a large effect on thd, as does slot type, pole faces, stator skew,  and a few other details

the devil is always in the details, and attention to those details cost money, money which most folks are unwilling to pay for in my opinion.

so you go to a big box store, buy a generator and then live with the power it makes, or you go and so some homework, talk to the manufacture about your concern and the amount of thd you will accept and they can then recommend a product that will meet that requirement.

expect to pay maybe 20% or more for a low thd generator, with very tight frequency and voltage control.

the other option is to go with one of the new inverter generators, these products generally have low thd, accurate and stable hz and voltage, but they cost more, substantially more.

if you need a portable to go shoot some screws or cut some boards, then it is unlikely you need or will want an expensive generator, on the other hand if you have delicate electronics you want to run, you might want a much better unit,

in my opinion, there is probably a case to be made for having one of each.

bob g
research and development of a S195 changfa based trigenerator, modified
large frame automotive alternators for high output/high efficiency project X alternator for 24, 48 and higher voltages, and related cogen components.
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ChrisOlson

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Re: Total Harmonic Distortion and generator heads
« Reply #2 on: October 16, 2012, 09:55:30 AM »
We bought a Honda portable for a standby gen for our off-grid system.  It is not an inverter unit but its power output is so close to perfect that it can easily match an inverter unit for clean power.  It uses Honda's "iAVR" system with an electronic governor.  It runs even our HDTV without a single glitch and will maintain perfect voltage and frequency even at 25% overload for up to 10 seconds.
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Flux

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Re: Total Harmonic Distortion and generator heads
« Reply #3 on: October 16, 2012, 11:03:55 AM »
The waveform of a single coil of an iron cored alternator is nearer a square wave than a sine wave, various tricks are used in the design to make the waveform acceptable. With machines with large numbers of slots the waveform can be made very good indeed, but with small alternators and a limited number of slots there are less options but if it is really necessary then even then the distortion can be kept very low.

What is really required dictates how much money it is worth spending on this aspect of design. THe thing that affects most loads is a virtually square, steep rising waveform. Square wave inverters with exceedingly fast rising edges mess up some critical electronic loads that use a form of power factor correction such as clever battery chargers and some fluorescent lighting HF units.

As long as the fundamental has no rapidly changing edges most loads really aren't that fussy, but the common so called modified sine inverter is not a lot better than a square wave on in this respect, it just has a more defined rms and mean waveform  to suit peak rectified loads and rms depending things such as lamps and heaters.

Even a cheap poor quality alternator will not have this rapid rising edge and even a dreadful looking waveform is unlikely to exceed 10%THD. I really doubt that there are many loads that will not tolerate this/

I think that more important than waveform is voltage stability ( long term and transient). If there are spikes in the waveform during step load changes then this can be damaging to some critical loads. Load application causes a voltage dip and on small alternators this can be very large. It still happens with machines with an AVR but the recovery is very much facter. This dip can be a problem starting motors.

Much worse occurs during a load shed, the transient voltage spike before recovery can result in an up to 50% overshoot spike.

You can still have these problems on an alternator with near perfect waveform so except for a few strange applications waveform is no real issue. The more you spend on a machine the better all these factors tend to become but the issues are really not the same as comparing a sine inverter with a modified sine one.

Although the long term voltage stability of an AVR alternator and an inverter are virtually identical, the invertter will have a much lower voltage fluctuation during transients as long as they are short ( fraction of a second). What happens long term depends on the inverter design and again it is very cost related.

I suspect nearly all damage from alternators comes from the capacitor excited types when subjected to large transient loads, especially highly inductive loads such as welders and moror starting. If you avoid large step load changes these problems don't happen.

There may be a few odd loads that lock on to the higher harmonics of a bad waveform so something like an electronic clock relying on line frequency may do odd things. Things like thyristor speed controls may also get confused if they reference the phase angle from the crossing point of the waveform, but these things have other problems that make it near impossible for them to work properly on small alternators so waveform may not be the main factor here.

Surprisingly cheap AVRs often bite a big lump out of the waveform if they use a half wave control and although this waveform looks awful there seems to be little evidence that it affects most loads and this crude type of AVR controlled alternator usually performs much better than the capacitor excited type.

Flux

Flux

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Re: Total Harmonic Distortion and generator heads
« Reply #4 on: October 16, 2012, 11:18:14 AM »
Perhaps I ought to reply to the last part of your question regarding engine speed regulation as it does have some effect on the alternator.

No generator is going to be completely satisfactory with a rampant engine governor, so avoid some of those small cheap 2 stroke powered things where the off load speed is all over the place.

With a reasonable engine that governs fairly well, an AVR alternator will maintain constant volts and the small changes in frequency don't affect most loads ( clock based things excepted).

Self excited capacitor alterntotrs are rather more affected by engine speed and even if you can hold the engine speed exact the voltage regulation is not too good. With a change in engine speed the voltage change is very significant and a poor engine governor with this type of alternator may let the voltage variation become very excessive. Usually you will have trouble keeping the voltage regulation within 10% from no load to full load if the engine speeds moves more than about 2%.

These cheaper machines are mainly aimed at site work where saws, drills grinders etc are not very fussy, again kettles, heaters etc are far from fussy.

Flux

ChrisOlson

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Re: Total Harmonic Distortion and generator heads
« Reply #5 on: October 16, 2012, 11:39:00 AM »
With a reasonable engine that governs fairly well, an AVR alternator will maintain constant volts and the small changes in frequency don't affect most loads ( clock based things excepted).

I haven't had good luck with straight AVR controlled generators.  Our old Generac was a straight AVR unit and its power output caused problems with the electronics in my wife's induction range.  We tried everything under warranty and finally Generac just gave on it and said there's something wrong with the range - the generator is fine.  But the range would work perfectly on just inverter power - it only had a problem on the generator.

Our new generator we got has a microprocessor in it that's tied in with the engine throttle and ignition timing, plus what they call a "CT" (Current Transformer) sensor.  This CT sensor senses load changes on the output of the alternator and even with spike transients maintains voltage and frequency +/- 1%.  There's not even any exposed throttle linkage on it - the engine's ECU is integral with the carburator so it can instantly change throttle valve opening and ignition timing to react to load changes.

It was not a cheap generator and is probably a showcase of Honda's engineering prowess.  But it puts out more stable power than you can buy from the utility in most cases and is an example of what they can do with age-old system of driving an alternator with an internal combustion engine these days.
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Flux

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Re: Total Harmonic Distortion and generator heads
« Reply #6 on: October 16, 2012, 01:17:48 PM »
Sadly there are exceptions to every rule and sometimes a certain load presents problems. Sometimes it is worth investigating the problem, other times you have to opt out for changing the load or trying another generator. In my early days when TV came to our part of the world there was a big problem with the fact that our TV system used mains frequency lock for the frame timebase. When running on individual generators the slight difference in frequency caused many problems. Certain TVs were hardly affected by this but othere suffered badly. It was a bit ironic that one of the cheapest TV sets worked much better than many more expensive ones under this condition. I saved a lot of problems for generator installers by suggesting that they took one of these cheap tellys round and showed how the problem was easiest cured.

One remote farm had an old dc plant with batteries and a vibrator pack to run the telly. When they changed to an alternator and tried the tv direct it caused all manner of troubles and this was an expensive and highly prized manufacturer of TVs. The farmer accepted that the cheap tv we brought cured the problem but his wife wouldn't accept the solution, we ended up repairing the ancient vibrator unit and fitting a battery charger to keep the old batteries charged just for one stupid badly designed tv.

Flux

rearden

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Re: Total Harmonic Distortion and generator heads
« Reply #7 on: October 16, 2012, 03:38:10 PM »
What I hear so far is: there are a lot of things which can effect THD in a generator,
Automatic Voltage regulation is important,  once you have it Ė you canít really upgrade it, there are some really cool engineering tricks you can do to maintain smooth power, especially from Honda.

I would be fine with paying 20% more for a low THD, AVR generator.  But what I see is more like 250%+.  A Generac GP750 ~$1k <=> XP8000 $2.5k,  A Generac GP6500 ~$900 <=> XP6500E $2k+  Hondas can be significantly more than that.

This price difference raised my intellectual curiosity and hence my questions about what engineering differences exist between a well rated generator and a well rated generator with lower THD.

How much can you deal with the transients and THD with filter caps and MOVs?

I have another generator, a Coleman Powermate Vantage 3500 (240/120vac w/12dc).  It is about 15yrs old, but supposedly a premium portable generator at the time.   I have noticed that with a low load the voltage is 130v+ but as you start adding loads the voltage keeps dropping and even long before the engine will bog the voltage will go below 110vac.  My UPS refuses to run on the power from this generator.  Can anything be done?  Filter caps, MOVs, add AVR, adjustments, etc.?

rearden

ChrisOlson

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Re: Total Harmonic Distortion and generator heads
« Reply #8 on: October 16, 2012, 03:48:21 PM »
Sadly there are exceptions to every rule and sometimes a certain load presents problems. Sometimes it is worth investigating the problem, other times you have to opt out for changing the load or trying another generator.

Yes, for us changing out the generator with the new Honda "fixed" the problem.  The electronics in my wife's induction range have programmable timers and outfits that control the temperature of the cooking vessel by varying the power to the litz coils.  I think it is more of a sophisticated computer in that range than anything else.  The control panel is all digital and touch screen.  And it requires stable power.

Speaking of TV's, we also used to get horizontal lines and "fuzz" on our HDTV with the Generac.  That is totally gone with the Honda generator.  So there is a difference between generators, and both units were in the same price range (~$4,000).

I am convinced that a "bad" generator can cause damage to certain sensitive electronics.  Most of these small generators are made for home standby power in the event of a grid power outage.  They are built cheap so they put out "good enough" power, and are true standby units designed to run at 50% rated load.  They are designed to only run once in a great while for a few hours.

Off-grid generators have to be rated for prime power applications at continuous full rated load, and that takes a different design generator head, and usually an engine that is de-rated from what the same engine would be rated at for standby power, so it can deliver over rated power for full surge load and still maintain proper voltage and frequency.

After having bad luck with our first generator I did a lot of research, and tried one, before we bought the Honda.  The Honda is a true prime power unit, designed to run continuously at its full rated load.

There are some companies out there - Champion Power Equipment is one - that builds a small generator and gives it a 3,000 "running watt" rating and 3,500 "maximum watts".  I have one of these too.  However, it's maximum continuous power output is only 1,800 watts if you need nominal voltage and freq.  It WILL put out 3,000 watts but the voltage is 10% under nominal and the freq is down to 54 Hz.  It WILL deliver 3,500 watts surge, but it just about kills the engine to do it.  That generator might work for portable power tools and so on, but it will not work for an off-grid system because the inverter will "spit it off" due to being out of spec from a simple thing like the 'fridge kicking in.

So for off-grid standby, with the different requirements from grid standby power,  it is wise to try before buying to make sure you know what you're getting, otherwise you could end up being disappointed.
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bob g

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Re: Total Harmonic Distortion and generator heads
« Reply #9 on: October 16, 2012, 04:20:25 PM »
when i stated "20% or more" i should have qualified that i suppose

the increase in cost is at the manufacturing level, and once it goes through a half dozen hands to the end user, what started out as a moderate increase turns out to be significantly higher.

also generally speaking, the manufacture is not going to go to the trouble of engineering, designing and manufacturing a low distortion genhead and then couple it to an air vane governor briggs and stratton.

the higher quality head will typically be coupled to a much higher quality prime mover, then mounted in an enclosure to reduce sound, add a bunch of electronic controls, safeties etc.  basically a high quality low distortion head is going to be coupled with the best other parts they can assemble into a value added package.

that is the difference between a prime power unit and a contractor grade unit or worse a home owner grade unit.

most of the home owner grade stuff is built to a price point, the genhead will be built as inexpensively as possible, using the least amount of iron, least amount of copper, harmonic/capacitor excitation, no skew/few slots so there will be no distributed windings or other things to help shape the sine wave... all in an effort to ease winding the stator core and keep costs down... the result at times can be adequate for most needs as Flux alludes to, all the way to significantly worse but acceptable for infrequent use such as powering a drill, saw, or some lights.

usually 5% is good enough for thd for most stuff, excepting of course the computer controlled high techie stuff that might have trouble with it. lights, and most motors deal with this level of distortion adequately well for backup use.

there are some import units that are a bit better and some significantly worse, i have seen over 10%thd and it would cause some heating of motors run on it, but even still if it were intermittent short term operation like running a skill saw to cut 2x4's while framing a house?  probably good enough.

Flux made a very good point to as it relates to the compromises made not only for cost but because of physical size of the stators, small stators don't allow for many slots, many slots are needed to distribute the windings to aid in shaping the wave form.

so one could imagine why high quality low distortion heads cost more money, they are more costly to manufacture.

then you have to consider the market and the competition
very difficult to build a low distortion unit and compete with a china built machine of higher distortion, when the buying public has no clue what thd is to  start with.

of course then you have the whole "sears" rated power thing to contend with too.

so i would expect a manufacture wouldn't even mess with the home owner market when it comes to low distortion heads, why build a cadillac when the public only wants to pay for a yugo?

bob g
research and development of a S195 changfa based trigenerator, modified
large frame automotive alternators for high output/high efficiency project X alternator for 24, 48 and higher voltages, and related cogen components.
www.microcogen.info and a SOMRAD member

Frank S

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Re: Total Harmonic Distortion and generator heads
« Reply #10 on: October 16, 2012, 04:25:45 PM »
One thing I have noticed is that goods made specifically for the US market is less tolerant than things sold on the world market. When we moved over here the computer monitors the television and the wife's stereo all acted quirky the stereo DVD player part was worthless even though on the label is clearly stated 120 60 hz/ 240 50 hz
 The TV died good riddance as it was a Magnavox CRT and weighed a ton Monitors were CRT as well so no loss the stereo system I could care less since I do not listen to music gave me good excuse not to have to listen to it.
 Almost all new HD TVs that I've seen are now controlled by an internal power supply that will work on anything from 100-250 v 50-60 hz
 I have a dozen 32" HDTVs used as monitors in the company and the wife carried 2 back to Indiana with her last year one wound up in her sister's motor home and another wound up in the house they love them But maybe since the motor home has an Onan which is owned by Cummins it may be higher quality than a generac I don't know
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ChrisOlson

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Re: Total Harmonic Distortion and generator heads
« Reply #11 on: October 16, 2012, 07:09:29 PM »
I would be fine with paying 20% more for a low THD, AVR generator.  But what I see is more like 250%+.  A Generac GP750 ~$1k <=> XP8000 $2.5k,  A Generac GP6500 ~$900 <=> XP6500E $2k+  Hondas can be significantly more than that.

Generac's GP generators are junk.  I had one about two years ago and it was terrible.  The engine on the GP's are imported OHV, made by Shanghou (sp?) Machinery in China (in fact the whole generator is a Chinese unit).  They will run as high as 25% THD with non-linear loads.

When you size any generator, it should be sized so that no more than 1/3 of it's rated output capacity is used by non-linear loads.

I have no direct experience with the XP Generacs, nor do I know of anybody who has one.  We had a Generac EcoGen and it was really no better (worse, in fact) than the GP, and all the electronics in it were Chinese.  We replaced just about every electronic part in that EcoGen in the 18 months that we ran it, and it was on its third AVR when we finally pulled it out.

IMHO, Generac's Customer Service is as horrible as the GP-series generators.  If you have a problem with a Generac you'll get better response if you go out to the woods and talk to a tree than you will calling Generac Customer Support.  That's just based on my experience with them.

On the problem with your Coleman Powermate, the AVR is not increasing the field current like it should, or is defective.  The first thing to check is the brushes and slip rings to make sure they're clean.

You are not going to get a generator that puts out high quality power at the price point that most of the cheap portables like the Generac GP compete at.  There's a phrase used to describe this phenomenon - "you get what you pay for".

Edit:
Hondas can be significantly more than that.

When you buy a Honda you can tack on an extra Grand for the name and the reputation.
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« Last Edit: October 16, 2012, 07:29:40 PM by ChrisOlson »

XeonPony

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Re: Total Harmonic Distortion and generator heads
« Reply #12 on: October 17, 2012, 12:23:53 PM »
or do what I do, go to the junk yard buy a scrap Honda, pay 600 bucks for an engine rebuild and have a high quality bullet proof generator cheaply!, next is I am going to over hual the distrobox! Hell I didn't even have to get the engine rebuilt for 3 years! and all it took to get it running in the first place? A twist tie up the orifice tube in the bowel assembly lol!
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ChrisOlson

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Re: Total Harmonic Distortion and generator heads
« Reply #13 on: October 17, 2012, 12:36:10 PM »
XeonPony makes a good point - I don't recall ever hearing about Honda building a bad generator.  I've seen them on Craigslist for sale and it says "needs work" or "don't run".  What happens is that people buy them, run them once or twice, then it gets parked in a corner with a bunch of junk piled on top of it.  The gas goes bad in it and then one day they discover it there and find out it don't run anymore because it's been neglected.

I saw a liquid cooled (gasoline) Honda ES6500 generator laying on its side in a pile of junk at the local salvage yard about two weeks ago.  I went and looked at it and I could not see a single thing wrong with it and the hour meter said it only had 61 hours on it.  That generator has a twin cylinder overhead cam (chain drive like a motorcycle engine) liquid cooled engine and I've seen many of those back in the day run 10,000+ hours without even shimming the valves.  They were very big, bulky and heavy because of the radiator and whatnot - but they were one of the most bullet proof generators ever built.

Somebody threw it out in the junk just because it was 20 years old and probably wouldn't start after sitting for who knows how long.
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ghurd

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Re: Total Harmonic Distortion and generator heads
« Reply #14 on: October 18, 2012, 07:54:14 PM »
Somebody threw it out in the junk just because it was 20 years old and probably wouldn't start after sitting for who knows how long....

.... with 20 year old gas in it!
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