Small generator over-voltage

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I have an old (mid 80's) Sears Craftsman generator - 4 cycle B&S engine  
& 120v, 1350W rated output.  (Jeff - model 580.328172)

I haven't used it in decades so I cleaned it up & checked it out with  
the intent to donate it to the Habitat ReStore.

Imagine my surprise when I measured the no-load output voltage at 134v  
(with Fluke 36 True RMS meter).  That dropped to 117v with a 10A load.

The no-load frequency is 62Hz, which is what the manual says it should  
be (3720 RPM).  That drops to 59Hz at 10A load.

The manual also claims a +-5% voltage regulation.

Seemingly unrelated, but ... the DC output is only 7v.  Also, there is  
1500Hz+-, 20v+- PP superimposed on the 120v 60hz output.

Note that I never before checked the voltage and I've never had a  
problem with it damaging loads (refrigerator, freezer, ??).  So either  
it's always been over-voltage & no-matter, or it's changed while sitting  
idle.

Any ideas about what's going on here?  And to diagnose & fix?

Thanks, Bob


The manual spec page:
https://imgur.com/phBAUpM
The circuit schematic:
https://imgur.com/vA6RIsa

Re: Small generator over-voltage
On 16/05/2020 11:07 pm, Bob Engelhardt wrote:
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Not familiar with USA products but a lot of the early ones were pretty  
basic and that sounds like normal to me :-)
So I don't think I'd worry about tinkering with the control circuitry,  
perhaps a quick check of any diodes or caps if any would be about it.

Re: Small generator over-voltage
On Sat, 16 May 2020 11:07:08 -0400, Bob Engelhardt

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From the schematic... it appears as if the battery would be an
integral part of the alternator regulation,  Do you have a good, fully
charged battery connected to the 12V terminals?  That, diodes and
brushes are pretty much all that can go wrong.

If it has been sitting awhile, you may want to check that the brushes
move freely in the holders and the slip rings are free of corrosion.
The rings should be shiny and may be dark colored, that's normal but
you don't want any encrustations, green stuff, white flaky stuff,
etc..  If you sand them use very fine sandpaper, avoid Silicon Carbide
abrasives, and blow out the dust it creates.

Re: Small generator over-voltage
On Monday, May 18, 2020 at 4:12:49 AM UTC-4, default wrote:
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Metal polish (EG: Brasso), or a large in eraser is better than sandpaper. It leaves a highly polished surface that only removes the oxides.

Small generators are intended for lighting, or power tools where you only have to be close. Also, without a load the output isn't a clean sine wave. Any inductive load helps to clean up their output.

Re: Small generator over-voltage
snipped-for-privacy@gmail.com says...
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I noticed on a 5 KW generator I have that the generator engine would  
tend to hunt around for speed unless I had a load of around 500 or more  
watts connected to it. Then it smoothed out some and got better with a 1  
kw load.  Using fresh gas without the ethanol in it seemed to help some  
too.  

This is a generator about 20 years old, but does not have very many  
hours on it.  I don't worry about the waveform. It does not have any  
voltage regulator circuit in it and just relies on the governor of the  
engine to hold it constnt.

 There used to be a big thing about the generators not suited for  
electronics in hte past.  I don't buy that for the general home  
electronics unless maybe some high end sterio equipment.  The computer  
supplies are designed for wide ranges of power and the digital TV sets  
do not depend on the frequency being 60 HZ.  Most electronics for the  
home now use switching power supplies and those things generate very  
dirty power theirsleves.


Re: Small generator over-voltage
On Mon, 18 May 2020 05:02:35 -0700 (PDT), Michael Terrell

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Those old-school ink erasers were the best IMO.  Don't see them
anymore...

Fine abrasives have the advantage of cleaning off the brush surfaces
and reseating brushes, so it may be more advantageous in a generator
that has been in storage for awhile. Otherwise I concur, a pencil
eraser is a good choice.

Re: Small generator over-voltage
On Monday, May 18, 2020 at 6:43:19 PM UTC-4, default wrote:
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. It leaves a highly polished surface that only removes the oxides.
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y have to be close. Also, without a load the output isn't a clean sine wave
. Any inductive load helps to clean up their output.
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   I used to rebuild vacuum clean motors as a sideline. The customer kept a
sking to see my armature lathe, since they commutators all looked brand new
. These were made by Lamb, and 120VAC. I would connect them to my 24V DC po
wer supply and polish the commutator with the motor running at a reduced sp
eed. He gave me all his bad motors. I sold them back to him at half the who
lesale price, with over a 50% recover rate. Many didn't run because the com
mutator was so nasty. I reground a blade for my Exacto knife to undercut th
e mica spacers. A lot of the time it only took five minutes to revive a mot
or. He never did figure it out, but he never brought back any of my repaire
d motors. Other had bad bearings or burnt windings so they were stripped an
d the good parts were used to make a good fan. I ended up with scrap alumin
um and copper from the scrap parts, as well. :)

   Ink erasers are still available, but you might have to go o an office su
pply store or buy them online.

Re: Small generator over-voltage
On Mon, 18 May 2020 23:36:52 -0700 (PDT), Michael Terrell

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Way back, I worked a part time job repairing consumer and commercial
electronics while going to school.  One of the sidelines of the
business was installing custom stereos in vehicles.  For that there
were high school kids with a mechanical bent that would do all the
work.  None of them could afford decent cars themselves so there were
a fleet of high-mileage junkers parked outside the building.

There was also this place "Ben's Armature," who'd built a reputation
for repairing car alternators.  Everyone in town swore by them being
the best, but they'd always diagnose a bad field winding (which they
called the armature) and the cure was always to "rewind the armature."
at a cost of $50-150.  This old Land Cruiser I used to haul my boat
around succumbed one day and I went there and listened to the
estimate.  

I decided to rewind it myself and save a buck.  How hard could it be?
I took it apart and there was nothing left of the brushes.  $4 for a
set of brushes (with springs and holder) and it could have been fixed
without even taking the alternator off the truck.

One by one the junkers the high schoolers used also went to Ben's.
(120-200K/Mi the brushes fail)  In no time the business had another
sideline.  We'd charge $20 labor and $4-15 for brushes (seldom had to
do more than remove a few screws and replace the brushes)

I did get to "rewind the armature" one day on an old 750 Honda
motorcycle.  The only hard part of doing it was in fabricating a
bobbin to hold the wire 'till the epoxy set - they used a "self
supporting coil" in the original alternator.

Re: Small generator over-voltage
On Tuesday, May 19, 2020 at 5:24:04 AM UTC-4, default wrote:
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per. It leaves a highly polished surface that only removes the oxides.
Quoted text here. Click to load it
only have to be close. Also, without a load the output isn't a clean sine w
ave. Any inductive load helps to clean up their output.
Quoted text here. Click to load it
t asking to see my armature lathe, since they commutators all looked brand  
new. These were made by Lamb, and 120VAC. I would connect them to my 24V DC
 power supply and polish the commutator with the motor running at a reduced
 speed. He gave me all his bad motors. I sold them back to him at half the  
wholesale price, with over a 50% recover rate. Many didn't run because the  
commutator was so nasty. I reground a blade for my Exacto knife to undercut
 the mica spacers. A lot of the time it only took five minutes to revive a  
motor. He never did figure it out, but he never brought back any of my repa
ired motors. Other had bad bearings or burnt windings so they were stripped
 and the good parts were used to make a good fan. I ended up with scrap alu
minum and copper from the scrap parts, as well. :)
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 supply store or buy them online.
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I've replaced a lot of brushes in Alternators and Starters. I had to junk o
ne Alternator because a weld was broken where a wire was attached to a slip
 ring. Solder wouldn't hold and I couldn't get it welded. I bought another  
alternator at a junkyard for $20. The owner read the GM markings and said i
t was a low current model. He as wrong.it was the highest current version.

I installed and repaired car radios right after I graduated. My first car w
as a '63 Pontiac Catalina convertable. It was seven yeas old, but some peop
le thought anything over two years old was junk.

Re: Small generator over-voltage
On Saturday, May 16, 2020 at 11:07:29 AM UTC-4, Bob Engelhardt wrote:
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Those numbers seem fine to me.  What's the voltage with a 100W  
incandescent as load?  (Or even a 10-20W resistor?)  

George H.  
who gave his old Sears gen. to his brother when he got a new one  
with a much bigger muffler!    
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Re: Small generator over-voltage
On 5/16/2020 11:07 AM, Bob Engelhardt wrote:
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My Fluke lies!!  I put a scope on the generator & got 210v p-p (or 150v  
p-p depending upon what the peak is with the superimposed 1500Hz),  
that's 71v (or 50v)RMS ... assuming sine wave, which it is, kind of.

https://imgur.com/AQlOpmA (2ms & 50v per)

I had it on the scope before, but I must have been in a fog to misread  
it so badly, or I didn't read the p-p at all, IDR.

So I was 180* out of phase with the problem: it's way, way under voltage  
instead of way over voltage.  Which is easier to understand possible  
causes - basically there's too little excitation current.  I couldn't  
understand how it could possibly be over voltage/too much excitation  
current.

The diodes check OK with a DMM, so I'll look at the slip rings.  Or for  
a poor connection.

Thanks for the replies.

Re: Small generator over-voltage
says...
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Is that a digital or analog scope ?

You need to just hang a good old filiment light bulb across the  
generator and see how it looks and measure the voltage with one of the  
old analog meters.  That digital crap often gets confused on oddball  
waveforms.

I don't recall exectally the way it works, but in the old days you put a  
piece of greased paper between 2 light bulbs.  One bulb on the generator  
and the other on a good sine wave from the power company and a meter and  
variack and adjusted the voltage so the bulbs were the same brightness  
and then you could read the voltage.


Re: Small generator over-voltage

[snip]

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It gives you a simple and crude way of comparing the
relative brightnesses of both sides of the paper..

Similar techniques have been used in photography:

The "Polaroid Swinger [tm]" camera, 1960's, used
a variant of this for its manually operated
light meter:

http://camera-wiki.org/wiki/Exposure_meter#Extinction_meters


--  
_____________________________________________________
Knowledge may be power, but communications is the key
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Re: Small generator over-voltage
On 2020-05-21 22:13, danny burstein wrote:
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That's called the "half shade" method, and is actually pretty good.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Small generator over-voltage
On 5/21/2020 5:41 PM, Bob Engelhardt wrote:


It doesn't lie - I'm an idiot.  The scope probe was bad ... a lot of my  
equipment is iffy, but the probe was junk.

So, I'm back where I started (and a little dizzy from the go-round).  
Going back to the fundamentals of the situation:
- the output really is 135v
- it has always been that way or it's changed & I can't see what could  
have changed
- there is no regulation.  The schematic had the stator & rotor  
reversed, here's the correct one:
https://imgur.com/REKKgHL
- the specs claim +-5% voltage - how can that be without regulation?  
They lie.  Maybe they mean +-5% at full load.

I found this at http://portable.generatorguide.net/avr.html
"Most cheap portable generators have fixed excitation. In such machines,  
when an alternator is loaded, its terminal voltage drops due to its  
internal impedance."

This is a low end generator from the mid 80's, so electronic regulation  
wouldn't have been as common as it is now.

Conclusion: I don't have a problem with my generator - I have a shitty  
generator

Re: Small generator over-voltage
says...
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With the 5% that is about 6 volts so the generator could put out from  
around 120 to 130 volts if set to 125 volts.  I have an old viberating  
reed frequency meter to use with my generator.  Hard to fool that thing  
frequency wise.  The best way for me with an old generator I have is  
just to monitor the frequency to see if the engine speed is correct.

The factory may set it high in voltage to allow for the wiring drop and  
internal impedance.


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