[ARC5] Question...

[Scott Johnson]

The biggest problem with toy generators is they have poor speed regulation,
and typically poor voltage regulation and droop compensation.  Inverters
cure some of these ills, but I doubt the SMPS in the computers cares all
that much about harmonic content, rather is is probably more sensitive to
voltage droop and variation.  
Some very small gensets use a Lundall or "claw pole" generator, much like an
automotive alternator.  The machine intentionally produce a squared off
waveform, which , in the case of the alternator, is advantageous since it is
more efficient to rectify.
Most small gensets are two pole machines running at 3600 RPM (60 Hz), with
very little rotating mass to help short term speed variations, So as Ben
said they spend more time below 60 Hz than above.  The inverter addresses
most all of the problems, IF it is sophisticated enough to have a proper
high rate PWM output (many are not).  I recently had experience with a
Capstone 1 MW inverter derived turbine generation system, and the power it
produces has lower harmonic content that the grid to which it was connected.
I can see that very soon, most likely all small gensets will be inverter
sets, they are rapidly improving.  I look forward to replacing my 10 kVA MEP
generator when a decent one is available.


Scott V. Johnson W7SVJ
5111 E. Sharon Dr.
Scottsdale, AZ 85254-3636
H (602) 953-5779
C (480) 550-2358
scottjohnson1 at cox.net
scott.johnson at ieee.org

-----Original Message-----
From: arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> On Behalf
Of Ben Hall
Sent: Sunday, November 1, 2020 8:57 AM
To: arc5 at mailman.qth.net
Subject: Re: [ARC5] Question...

Hi Ken,

First things first - put the genset on the scope and see if you've got a
problem.  There are a lot of people out there who like to scream "all
generators are dirty, they are going to screw up your computers" who have no
idea what they are talking about.  (and generally are in the business of
selling filters and whatnot)

I've been inside a lot of gensets, fixed more than I can count, and have
learned a lot.

It's likely that your genset, being 7500 watts, is NOT an inverter genset
known for being extremely dirty.  Most generators of that size have a
standard generator head where 3,600 RPM of mechanical rotation is converted
to 120 VAC 60 Hz AC without any trick circuitry.

(when you run it, does it run at a stead speed regardless of the load? 
Or does the speed change as you plug things into it?  The former is not
going to be an inverter unless it has some sort of "smart idle" 
circuitry, and the later is definitely an inverter type)

Theoretically, these non-inverter generators should generate a sine wave due
to the mechanical motion being converted directly into AC without any
inverter trickery.

However, in practice, theory doesn't match reality.  Your generator is
likely single cylinder...and a four-stroke single cylinder at that...so for
three our of every four strokes, the rotational speed is slowing down...and
during the power stroke, it's speeding back up.

So what you get, is not a pure sine wave...but it's not going to be horribly
dirty, either.  Those military gensets are likely multi-cylinder, powered by
a mil-spec engine like a 2A016, 4A032, or whatever multi-cylinder diesels
they are using these days...making the RPM much more steady...plus they are
going to have good output filters on them.

I'd put the scope on it (carefully, of course) and see what you've got both
at no-load and whatever load you can put on it before doing anything...

Now to the "screw up your computer" thing...modern computer power supplies
are multi-voltage, 100 (Japan) to 240 VAC (Europe) input, 50 to
60 Hz input, and the first thing they do with the incoming AC is rectify it
to DC and filter it...  Very hard to kill these.

Thanks much and 73,
ben

> I have a "small emergency generator", 7500 watts, brand new. > >  From my
reading, I find that the output waveform is very "dirty" 
as it > is some sort of "stepped" square-wave. I have not yet looked at the
> wave-form with an oscilloscope, but I am not expecting it to be very
pretty.

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