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Generators & Electric Motors General Discussion Antique Generators and Old Electric Motors: Questions and answers about restoring and showing old power generation systems.

Generators & Electric Motors General Discussion

Total Harmonic Distortion of Portable Generators

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Old 02-19-2018, 09:54:52 PM
Vanman Vanman is offline
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Default Re: Total Harmonic Distortion of Portable Generators

That would be so much better running 1800 rpm (with a four pole generator), making ~6 kw, using ~1/2 as much fuel, lasting orders of magnitude longer, and still providing plenty of power for a house. Oh, and being pleasant to listen to, too.
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Old 02-19-2018, 10:37:35 PM
GeneratorNewb GeneratorNewb is offline
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Default Re: Total Harmonic Distortion of Portable Generators

That's the one but with a 16 circuit outdoor transfer switch instead.

]Well either way the standby will operate at 60hz with sub 5% thd.

I know that's ideal but I have no desire to stock diesel.
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Old 02-24-2018, 01:38:32 PM
dkamp dkamp is offline
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Default Re: Total Harmonic Distortion of Portable Generators

Originally Posted by GeneratorNewb View Post
They said that the generator is electronically governed so it will maintain a steady 60hz. Unlike a portable generator that is mechanically governed and goes to 62hz no load and 58-60hz with a load.
Okay, so I ignored the THD, power output, wave considerations and furnace control board issues, but this argument is absolute crap.

First of all, an electronic governor and a mechanical governor have the SAME JOB... they observe output, and alter input, to maintain a steady state.

yeah, an electronic has the ability to maintain tighter under some circumstances, but I absolutely positively assure you that it will NOT make a hoot-owl's difference in the end, and when things go wrong, it will NOT result in a functioning generation system... rather, it will go to zero, or WFO.

What you're experiencing amidst this whole thread concern, is anxiety and misunderstanding of the realities of generating power. IF you are dedicated to the concept of havnig your own ability to reliably maintain electric power under loss of utility, then the ultimate answer is not in what you invest in dollars and machinery, but rather, what you invest between your ears.

That being said, I'm not making this point to chastize you, or anyone else, for considering all the above concept, as ANY forum's ultimate purpose is to facilitate the sharing of knowledge and experience, and that's exactly what you seek. Henceforth, I'll shorten up the tail here with some basic truth:

First... advertising THD figures, with no other circumstances identified, is absolute garbage and circumstancial marketing tactics. We went through this with hi-fi systems of the 60's-80's, and as time went by, everybody just talked about THD, slew rate, separation, and wow-and-flutter without having the slightest clue what it all meant.

Total Harmonic Distorion is irrelevant without identifying the LOAD attached... because the LOAD is a reactive component to the circuit. Your utility company has it's own guidelines to which it attempts to maintain power quality, and THD is one of the measurements they'll use to determine wether the power is 'clean'. In order to understand harmonic distortion, you have to understand AC power.

A rotating generator is naturally going to create a sine wave. From there, the noise comes from OTHER FACTORS. Let's say you've got a circuit breaker that makes marginal contact below 5 volts or so. No matter WHAT the system voltage is, every time that waveform crosses zero, it'll get below five, and then above five, and each time, it will put a little 'fizz' on the line. Now, that 'fizz' will go to the far end of a wire span, where another transformer steps it up or down, or through breakers which will couple it to a bus going to other segments of a power system, and along that way, SOME of the 'fizz' will radiate out as radio frequency interference. Drive under power lines with your AM radio on, and you'll HEAR it. As you drive along the line, you'll hear that the FIZZ gets louder and softer... it's because a harmonic component of the noise is occurring in a wavelength that alters in current vs. voltage at regular intervals respective to the transmission impedence and applied frequency. You're hearing it highest at one point, but not the other.

By the time that FIZZ gets to the next transformer, it will have lost a substantial amount of it's energy. once it crosses that transformer, however, it's totally different... because the transformer is an substantially inductive device, optimized for the AC line frequency (USA typically 60hz). Any activity at substantially higher or lower frequencies basically don't pass... therefore, any NOISE that was present at the incoming end, is heavily (very heavily) supressed.

Now, the process starts over again, until it gets to that transformer right outside your house.

If I were to bring MY scope to YOUR utility panel, and look for THD, I'd guarantee you that all but 0.01% of the noise we see, is appearing ON YOUR PREMISES. MOST will be from devices in YOUR house, the rest will be from marginal connections at the utility panel, inside the meter box, in the connections of the overhead or underground feeder, or leakage across the insulators at your pole transformer.

This leads us to the simple design of simple generators and simple transmission systems. The basic AC generator, is nothing more than a coil of wires being acted on by a magnetic field... one is spinning, the other stationary. Generators either use a stationary field, and spinning AC windings, or a spinning field, and stationary AC windings. Stationary wiring connections are easy- connect wires... but In order for power to be coupled from whatever's spinning, to the rest of the pseudo-stationary world (btw, the earth is actually flat)... requires either BRUSHES, or SLIP RINGS, which means electricty is being transmitted through contacts which are actually MOVING. This creates NOISE... which requires SUPPRESSION to prevent the HIGHLY technical term I identified above as "FIZZ". Lousy brushes and slip rings generate FIZZ. Even GOOD ones generate a little FIZZ. If you're wondering why there's a capacitor connected across the leads of a generator's componentry somewhere, it's to suppress "FIZZ". You can use my very expensive digital recording oscilloscope and some of it's handy measuring features and a computer spreadsheet, along with calculus (integrals and derivatives are handy for this) to calculate Total Harmonic Distortion, or you can cut to the chase, grab your Radio Shack Flavo-Radio and an earphone, tune it to some blank spot in the dial, and call it a Purple Fizz-O-Meter, and experience the audible indication without lifting a finger.

This is all totally irrelevant to Governance and Frequency. Regardless of HOW it's done, the spinning frequency has to be maintained to some level of precision, because the FREQUENCY is determined by the rotating speed. the frequency stability is determined by how closely the GOVERNOR is able to DETECT change of speed, and how WELL the engine is able to react to the governor's input. If the engine is weak, it doesn't matter WHAT kind of governor you have... and if your load is very high, the result is same. If you have an engine that generates 732 ft-lbs of torque at 1200rpm driving a six-pole (1200rpm) generator head that can put out 3400w, and you load it to 3400w, there is NOTHING on earth that will cause your frequency to change. Why? Because you've got so much shaft torque available, that the generator head will melt down from electrical overload before coming up with enough load to affect the prime mover's speed. The engineering description, is that you've an overabundance of power input for the power output. My dad says... it's a fart in a windstorm. Of course, you can accomplish the same feat, by having a 55kw generating plant driving a 3.8kw load... it won't even know you're there.
Now, when it comes to INVERTER generators, the reality is that there IS NO SINE WAVE. An inverter generator generates DC, and a solid-state inverter circuit converts it to AC by nothing more than 'switching' it alternately forward and back. The advantage here, is that the AC frequency is set by an oscillator, and once running, that oscillator ain't gonna drift much, because it's load independant.

On the other end, is an engine driving a DC generator, and yeah, it has an efficiency feature... you can vary the engine's SPEED, and modulate the DC generator's FIELD, to produce the same DC output (for the inverter) but by doing so, use a little less fuel... and when demand comes up, throttle the engine to a higher speed, and reduce field excitation, to get the output capacity back up where it belongs...
..which in my opinion, is clever for a tiny portable, but totally pointless for someone who has a house on standby power with constant loads. Why? Because you're right back to GOVERNANCE... but this time, it's CURRENT that you're trying to measure and respond...

...and you've replaced a simple mechanical gadget and hard-iron engineering with a laptop computer's worth of integrated circuitry, a bunch of diodes, capacitors, and other stuff that...
..really hates THD.

Did I mention that inverters really aren't 'sine waves'? They're switchers. At the start, they're 100% HARMONIC DISTORTION. The ONLY way an inverter can generate LOW distortion, is by FILTERING OUT ALL THE CRAP. oh, and the crap is FREQUENCY and LOAD sensitive. So in order for the filtering to actually 'work', it has to be able to be modulated based on LOAD... and therefore, any THD figure that you see coming out of an INVERTER, is total hogwash UNLESS the load constraints under which it's tested, are also indicated... and of course, any deviation from those constraints, results in something entirely different.

I'll tip my hat to Peter Sellers:
Peter: "Messieur- I sought you say your doog he does not bite..."
messieur: "Yes... but zees is nota my door."
ow, let's go back to the COST of things:

You're looking at bids for machines in the what... $5-7K range... and you're being told that the furnace control board is a grand. Yeah. Now go look up the ICM series of aftermarket furnace control boards. They're about two-fifty or so... and IMO, these guys do an incredible job of building a control board. Frankly, I'm surprised that the company-or-two that makes the fourteen major brands of tin furnaces hasn't just given up making their own boards and bought them from ICM instead, because the ICM's componentry is a whole lot tougher than the OEM stuff. If you're worried about smoking the furnace, keep one of these on hand for the sunday-night-chrismastime failure. Get a spare ignitor, flame sensor, and blower motor capacitor, put 'em in your utility closet, and hope you never need them, and when you do, you'll be glad you did.

Switching transients will kill your furnace controls long before THD ever will.

Switching power supplies from your laptop computer, TV, and every other piece of household consumer junk, will cause more THD than your generator ever will. Problem is, that if your generator uses fancy electronic govornance and regulation, that the THD that all those OTHER things put on the line, will confuse the generator's governor and regulator, and the end result will be poor performance of EVERYTHING.

So after you look up prices on furnace controls, go look up the price for a 50's era liquid-cooled ex-commercial/industrial/government stationary generator on the used classifieds. Find one for $500, bring it home, exercise the net to get docs, exercise your hands and mind cleaning it up, and wire it to some sensible method to safely connect it to the loads that are MOST CRITICAL to you when there's an outage (well, sump pumps, furnace, refrig/freezer, some lights, etc)...

and realize that it's what's between your ears that makes the most difference.
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Old 02-25-2018, 09:39:26 PM
Larry Rusch Larry Rusch is offline
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Default Re: Total Harmonic Distortion of Portable Generators

Well said Mr. dkamp, well said. (clap, clap, clap)
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Old 01-14-2019, 10:35:01 AM
HouseBuilder328 HouseBuilder328 is offline
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Default Re: Total Harmonic Distortion of Portable Generators

Originally Posted by GeneratorNewb View Post
I considered the eu6500 but they still command roughly $3k and aren't fuel injected like the eu7000is. I guess I'm having a hard time spending $4k on something that's rarely going to be used (I bought the wh7500e and haven't used it once in 3 years). Maybe I'll just wait until they come down in price.
Had to post in an old thread, but because I used to think like GeneratorNewb and now instead of the price going down or even steady, the price for the Honda eu7000is has gone UP! Check out this great article.

Is it safe to assume that Honda probably won't come out with another update to the eu7000is for several years to come?
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