Author Topic: mechanical burst generator  (Read 29668 times)

jzeeff

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mechanical burst generator
« on: August 12, 2009, 10:00:21 PM »
Inverters that will produce lots of current, say enough to start a 1.5HP well pump are expensive (or at least the brand names ones are).   But this current is only needed for 1/2 second or so and then it drops to reasonable levels.

So is there a cost effective way to store 1/2 second of AC current?  Flywheels suffer from the fact that as they output energy, they slow down and lose sync with the inverter (unless the inverter could track the flywheel/generator frequency like a grid-tie inverter does). 

Maybe a very small compressor, a storage tank, a feedback controlled pneumatic valve, an air motor and an AC generator? 

I suppose an alternative is get a 3 phase well pump and then use a variable frequency drive (VFD, 240VAC single phase in) to slow start it 


Summary answer:  mechanical isn't very practical, inverters are coming down in price, use a resistor to limit motor starting current.









« Last Edit: September 12, 2009, 07:27:00 PM by jzeeff »

compig

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Re: mechanical burst generator
« Reply #1 on: August 12, 2009, 10:55:12 PM »
Wouldn't it be easier to connect an air motor supplied by an air receiver to the well pump then arrange a valve system to pulse the air motor at start up ?
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jzeeff

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Re: mechanical burst generator
« Reply #2 on: August 13, 2009, 12:14:22 AM »
Sounds hard to retrofit to a submersible well pump - but would work.

.2 sec is only 6 revolutions of a 1800 rpm generator.  Maybe a large rubber band that is twisted up by a small motor and then released and regulated by a brake and electronic switch?

« Last Edit: August 13, 2009, 12:24:51 AM by jzeeff »

apogee_man

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Re: mechanical burst generator
« Reply #3 on: August 13, 2009, 12:18:50 AM »
Ah.....

One of the biggests hassles of all...

Many years ago I met a guy who had a bed and breakfast that was fully off grid.  No big deal until you consider that it was about 4000 square feet in size.  Then, on top of that, he was a french chef and had a full commercial kitchen in the place...

So we were talking about his setup and one of the biggest issues that he had was exactly what you describe.

He solved it by building a large capacitor bank.

Completely solved the motor starting issue and allowed him to use commercial grade appliances in his kitchen with no modifications.

He also ran a hot tub off of the system.

Heat was from used motor oil donated from diesel trucks.

It was a very, very cool setup!

Inverter was a large surplus unit from a telephone company.  There was also a diesel genset for backup if the solar couldn't keep up.

He was the person that made me realize that off-grid could be done here in the northwest and it would really work.  This bed and breakfast had five rooms that they rented, plus the owner and his wife's living quarters.  If he and I hadn't talked about it, I would have never known we were off-grid!

The place is located down near Mt. St. Helens and is called The Farm.  I was planning to go back several years ago and found it was for sale then.  I'm not sure what has happened to it since.  When I last spoke to him, he mentioned he was getting too old to stay on top of the place and that was why he'd decided to sell it.

Tom

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Re: mechanical burst generator
« Reply #4 on: August 13, 2009, 12:39:19 AM »
I think that the easiest way to start the pump is to get one that is easy to start to begin with. We use a Grundfos SQFlex pump that is soft start and runs efficently on a wide range of AC and DC voltages.
Tom
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jzeeff

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Re: mechanical burst generator
« Reply #5 on: August 13, 2009, 01:00:02 AM »
How do capacitors storing DC help with starting an AC motor (other than the PF correction)? 
« Last Edit: August 13, 2009, 05:16:09 AM by jzeeff »

panaceabeachbum

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Re: mechanical burst generator
« Reply #6 on: August 13, 2009, 03:07:34 AM »
I would think it might be cheaper to buy a smaller , lower volume , pump and a larger storage tank. This way you can reduce the starting load with a smaller pump . It will take longer to fill your tank and as a result will have to run longer but the larger tank will reduce the number of start cycles per day and allow for long showers etc. Looking at the cost of an inverter that will start a 1.5hp motor I would just buy the 1/2hp dc  submersible pump and attach it directly to my Batt.

If your water level is close to the well head , less than 10feet or so you can buy a much cheaper pump if you dont need the submersible.

We have a 1/2 hp pump and a 120 gal tank and have no loss of pressure or flow at any point with a 4 person household .

Another thought might be a large non pressurized tank , a small low powered lift pump to fill the tank , float valve to control the fill pump and the a pressure pump to provide the water pressure you need on demand when a spigot etc is opened
« Last Edit: August 13, 2009, 03:15:14 AM by panaceabeachbum »

Irish Artist

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Re: mechanical burst generator
« Reply #7 on: August 13, 2009, 03:54:31 AM »
Panacea, I think you're on the right track.

In my situation, I'm pulling water from 100 feet down with a 1 hp motor, might have to go deeper in the future as more folks tap into the same source around me.

My plan is to have a 300+ gallon holding tank at ground level that I'll fill from the well when I have my gen set running. Then, when I'm running on battery bank, I'll use a low volume pump to keep the pressure tank up to snuff.

Just have to plan your loads and schedule it accordingly. Keep it simple and easy to maintain.

I have to say though, I'm curious to hear more about that large capacitor bank system.

Murph'

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compig

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Re: mechanical burst generator
« Reply #8 on: August 13, 2009, 10:11:30 AM »
How do capacitors storing DC help with starting an AC motor (other than the PF correction)? 

Yes , the same occurred to me !!
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apogee_man

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Re: mechanical burst generator
« Reply #9 on: August 13, 2009, 06:08:25 PM »
Keep in mind this was a long time ago so my memory is fuzzy...

I believe that the cap bank was across the AC side.  Besides correcting PF, it also added additional surge capacity.

Shortly after I saw his setup, I purchased a surplus industrial inverter setup that had been scrapped.  To this day, I'm still storing it my garage...  Anyway, one of the main reasons that I bought it was that it had several capacitor banks that closely resembled what I'd seen at his place.

I do clearly remember him saying that the caps made all the difference and allowed him to be able to start motors that he otherwise wasn't able to.

I have to dig one of mine out, but I seem to recall the caps in my banks being rated for AC also...  Please don't hold me to that though, it's been a long time since I've looked at them.

Food for thought...

Steve

compig

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Re: mechanical burst generator
« Reply #10 on: August 13, 2009, 06:18:18 PM »
Must have something to do with phase angles or surge limiting because capacitors can only hold a DC charge , they pass AC with varying resistance dependant on cap size and frequency.
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apogee_man

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Re: mechanical burst generator
« Reply #11 on: August 13, 2009, 06:51:39 PM »
Like I said, my memory is very fuzzy as it was about 15 years ago...

Perhaps they were on the DC side but not sure about that.  I seem to remember asking him about it and him saying they were on the AC side.  However, I also could be wrong about that...

Point well taken regarding them only storing charge using DC.  Maybe he didn't have enough DC capacity to properly handle the surges and the caps were enough to push it over the edge.

All I know is it worked and worked well.

The kicker was his kitchen.  It was identical to what you'd see in a big restaurant and I distinctly remember him saying that he was able to run everything unmodified.

That is what started me down the off-grid path.  Up to that point, I figured the solar/off-grid stuff was only a hippie thing that was a pipe-dream.  Once I saw what he was doing, I realized it was for real and it could work, even here in the northwest with all of our cloudy weather...




jzeeff

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Re: mechanical burst generator
« Reply #12 on: August 14, 2009, 02:02:03 AM »
I have read that motors can have a terrible power factor while they are starting, so capacitors could help quite a bit in this way.  Wouldn't surprise me if his inverter didn't like non-linear loads (dealing with back EMF is tricky) and so by correcting PF, things worked.

I suppose it is a good reminder - fix PF first, then look for ways to  increase surge capability.  But I'm not sure how to measure PF for a .2 second period - storage scope and voltage and current probes I guess.









panaceabeachbum

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Re: mechanical burst generator
« Reply #13 on: August 14, 2009, 04:08:19 PM »
I know a little less than zero about electrical things but I did add a third 30hp motor to the load on my electrical service and anytime all three big machines were starting and stopping quite frequently it was causing a couple of my neighbors all sorts of fits , causing flourescent lights , tvs stereos etc to shut off . 

The power company came out to change the large can out on the service pole that I have always called a transformer and the engineer explained to me that it was not a transformer  but instead a large capacitor to handle surges etc of various devices starting . 

Again I have no idea how the electron Geenie does her magic , but I would like to know more about starting big ac motors

compig

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Re: mechanical burst generator
« Reply #14 on: August 14, 2009, 05:31:02 PM »
Very basically , a purely resistive load supplied by an AC voltage consumes power (volts x amps) without changing the relative phase of the voltage and current sine wave forms thus it represents a unity power factor , or power factor of 1. Inductive loads , like motors , lighting ballasts (they have a choke which is an inductor) etc change the relative phase between voltage and current , so for a given load power they consume more current to achieve the rated power of the motor. Practically , that means the actual power consumed is more than the power the meter will read. This is a power factor of less than 1. Utility companies provide supply on an assumption of a power factor of no less than 0.95 , but a consumer with lots of electric motors will have a factor of around 0.7 so they will a/ be charged on a higher tariff as they actually consume more power than a standard meter reads and b/ will need a larger capacity service connection to cope with the current.
So , on start up the current surge of a large induction motor is massive because the inductance value of that motor before it starts to turn is very large , this means the resistance the supply see's is almost a short circuit. To handle that start surge without power factor correction would need a massive supply rating. To reduce this surge a capacitor is placed across the supply which cancels out some that surge by bringing the phase of the voltage and current sine waves closer together. The value of that capacitor has to be calculated using all relative factors.
This is why you will see motors rated in KVA.
Difficult to simplify a very complicated subject !
 
« Last Edit: August 14, 2009, 05:36:12 PM by compig »
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