Small generator update & more ques.

[Eric R Snow]

Greetings All,
Thanks to who responded with answers to my last generator questions. I
finally just now got the time to check things. I also have a new
generator that I 'scoped this morning. The small generator now puts
out 60 cycles after adjusting the governor. Both generators have a
waveform that looks jagged. I think these must be harmonics after
reading about them last night. Both generators are built like a car
alternator with brushes powering the rotating field coils. I thought
that this type of ac generation would yeild a pure sine wave. Boy was
I wrong. The harmonics get smaller on the 'scope screen when the
generators are powering an induction motor. Is there a fairly easy was
to filter these harmonics? Also, since computers and modern TVs all
seem to use switching power supplies why would they be sensitive to
harmonics? Stuff I have read about generators warn about sensitive
equipment like computers. My very limited understanding of computer
power switching supplies is that they convert the ac input to dc and
then chop up the dc into high frequency ac to do all the voltage
changes using smaller inductors. How wrong am I? Finally, I am using a
step down transformer to keep the voltage lower when measuring the
generator waveforms. As a test I first looked at the ac coming out of
the wall. I expected to see a pure sine wave there but instead the
crests of the wavform are bent over a little. Is this the transformer
doing this?
Thanks,
Eric

 

[James Beck]

Greetings All,
Thanks to who responded with answers to my last generator questions. I
finally just now got the time to check things. I also have a new
generator that I 'scoped this morning. The small generator now puts
out 60 cycles after adjusting the governor. Both generators have a
waveform that looks jagged. I think these must be harmonics after
reading about them last night. Both generators are built like a car
alternator with brushes powering the rotating field coils. I thought
that this type of ac generation would yeild a pure sine wave. Boy was
I wrong. The harmonics get smaller on the 'scope screen when the
generators are powering an induction motor. Is there a fairly easy was
to filter these harmonics? Also, since computers and modern TVs all
seem to use switching power supplies why would they be sensitive to
harmonics? Stuff I have read about generators warn about sensitive
equipment like computers. My very limited understanding of computer
power switching supplies is that they convert the ac input to dc and
then chop up the dc into high frequency ac to do all the voltage
changes using smaller inductors. How wrong am I? Finally, I am using a
step down transformer to keep the voltage lower when measuring the
generator waveforms. As a test I first looked at the ac coming out of
the wall. I expected to see a pure sine wave there but instead the
crests of the wavform are bent over a little. Is this the transformer
doing this?
Thanks,
Eric

I wouldn't worry about the waveform unless you just need to waste some
time. I can't think of anything in the home that would even notice.
Things like modern computers use switching power supplies (as you said)
that could care less about that distortion.

Jim

 

[Eric R Snow]

I wouldn't worry about the waveform unless you just need to waste some
time. I can't think of anything in the home that would even notice.
Things like modern computers use switching power supplies (as you said)
that could care less about that distortion.

Jim

Thanks for the reply Jim,
It seems to me that maybe there's some hype out there (no surprise).
But it makes good sense to me to ask about things I don't know. As far
as the waveform shown on the oscilloscope goes, I just want to know
why. Monkey curiosity. Besides, the more I understand about the way
the world works the happier I am.
Eric

 

[Aly]

Greetings All,

Thanks,
Eric

Hi Eric,

From past experience I've found that a small UPS tends to tidy things up.
The sort of UPS you can buy from a computer store or find in skips.

A number of years ago I worked on a market stall during the summer holidays
that printed T-Shirts at outdoor trade shows. Before I joined they were
powering the computer kit straight off of a gennie with the
computer/equipment crashing all of the time. A plotter was also had it's
PSU burnt out.

The UPS solved all of those problems. Also good for when the load increased
significantly or when the fuel started to get low.

Hope that gives you some ideas Some UPS's are better than others so try
a couple under load if you can. It was the second brand that we tried that
worked. The first one would continually trip.

 

[default]

Greetings All,
Thanks to who responded with answers to my last generator questions. I
finally just now got the time to check things. I also have a new
generator that I 'scoped this morning. The small generator now puts
out 60 cycles after adjusting the governor. Both generators have a
waveform that looks jagged. I think these must be harmonics after
reading about them last night. Both generators are built like a car
alternator with brushes powering the rotating field coils. I thought
that this type of ac generation would yeild a pure sine wave. Boy was
I wrong. The harmonics get smaller on the 'scope screen when the
generators are powering an induction motor. Is there a fairly easy was
to filter these harmonics? Also, since computers and modern TVs all
seem to use switching power supplies why would they be sensitive to
harmonics? Stuff I have read about generators warn about sensitive
equipment like computers. My very limited understanding of computer
power switching supplies is that they convert the ac input to dc and
then chop up the dc into high frequency ac to do all the voltage
changes using smaller inductors. How wrong am I? Finally, I am using a
step down transformer to keep the voltage lower when measuring the
generator waveforms. As a test I first looked at the ac coming out of
the wall. I expected to see a pure sine wave there but instead the
crests of the wavform are bent over a little. Is this the transformer
doing this?
Thanks,
Eric

A few harmonics is nothing to worry about. Severe harmonics like a
square wave, and more heat will be generated at the point of use like
a motor or transformer; some diodes in power supplies don't respond
well to square waves.

With an AC generator if the field isn't uniform - the output won't be
sinusoidal. If your regulator is changing the field that will
contribute to the lack of a pure sine wave.

The pole pieces of the rotor are probably designed to be easily
assembled and not to produce the optimal sine wave.

The output could be filtered to make a sine wave. It isn't often done
unless you really need a sine wave - for instance to form a Lissajous
pattern on a scope, or to poll a sine cosine position indicator.

For power uses a ferroresonant voltage regulator on the output may
make it a sine wave (and waste some power in the process).

 

[Eric R Snow]

Hi Eric,

From past experience I've found that a small UPS tends to tidy things up.
The sort of UPS you can buy from a computer store or find in skips.

A number of years ago I worked on a market stall during the summer holidays
that printed T-Shirts at outdoor trade shows. Before I joined they were
powering the computer kit straight off of a gennie with the
computer/equipment crashing all of the time. A plotter was also had it's
PSU burnt out.

The UPS solved all of those problems. Also good for when the load increased
significantly or when the fuel started to get low.

Hope that gives you some ideas Some UPS's are better than others so try
a couple under load if you can. It was the second brand that we tried that
worked. The first one would continually trip.

Greetings Aly,
I have been thinking about a UPS. Your post confirms this. Thanks.
Eric

 

[Alison]

Greetings Aly,
I have been thinking about a UPS. Your post confirms this. Thanks.
Eric

Hey again

They *should* work. As said, it may take a couple of go's as the first one
we had hated it. We ended up with a little 400VA one from what I remember,
that had two golf caddy type batteries in it. It may be worth approaching
your big bad rips everyone off PC World or alike, and saying you've had
trouble with UPSs in the past and electricity spikes, could you return for
another as you know some are better than others.

An old one would be fine as long as the batteries keep the kit running for a
few minutes. The aim for us was to use the gennie to charge the batteries,
and for the UPS to generate the sine wave. Any spikes from the gennie were
absorbed by the batteries. Some UPSs use slightly different methods and are
wired up differently.

Aly

 

[Eric R Snow]

Hey again

They *should* work. As said, it may take a couple of go's as the first one
we had hated it. We ended up with a little 400VA one from what I remember,
that had two golf caddy type batteries in it. It may be worth approaching
your big bad rips everyone off PC World or alike, and saying you've had
trouble with UPSs in the past and electricity spikes, could you return for
another as you know some are better than others.

An old one would be fine as long as the batteries keep the kit running for a
few minutes. The aim for us was to use the gennie to charge the batteries,
and for the UPS to generate the sine wave. Any spikes from the gennie were
absorbed by the batteries. Some UPSs use slightly different methods and are
wired up differently.

Aly

Greetings Aly,
I spoke to my wife about this type of setup after reading your
previous post. How it can be advantageous in some situations. I have
been considering using lithium batteries instead of lead acid. I know
lithium batteries are more expensive and need to be charged with care.
But there are several types available from surplus outlets. A web
search turned up many inexpensive lithium battery charging ICs and
discharge controllers. So even if the batteries don't have built in
overcharge and over discharge protection it's something simple enough
for me to build. And lithium batteries don't need to be float charged
the way lead acid batteries do. AC is way different than DC and I
don't understand harmonics and many of the other ways the desired pure
sine wave can be distorted. The UPS idea would work well for light
loads, say 600 watts total. The distorted AC from the generator gets
better when running induction motors, less so with a resistave load.
I ran several motors in my shop with the generator and they don't get
significantly hotter than when running off the mains. So the
generator could be used to run the fridge, freezer, and well pump and
the UPS can run the computer and similar types of equipment.
Thanks for your replies.
Cheers,
Eric