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Stupid things we (ok I ) do

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oztules:
Stemming from an answer to Wolv regarding driving resistive loads:

Here is one experience I had with it. ;D

========================================================= from fieldlines some years back

Well I had sorted through the battery lot that I was given and found that I had 13 good ones. It was one of those blustery mornings and so I had the bright idea of "lets see what she'll do mister" type of thing going on in my head.


I had also pondered what to do with the excess power that the chainsaw blade mill seemed to be producing. I had read some of Daveb's posts on water heating... so I got the bright idea that I would use it as a dump load, and/or use the mill to heat up hot water.


The obvious conclusion was that in order to have reliable water heating, I would use the batteries to heat the water while the mill was charging. This would allow me to use say 5kwh to heat up some water, and if the mill was in reasonable wind, it would carry the load some of the time, and top up the batteries after the heating was done.... thats the plan.


So I dutifully fired up the mill, and proceeded to charge the batteries a 72v configuration from the 48v windings of the mill... It seemed to be putting in a fairly steady 10 or so amps, and so while it was doing that I hatched a plan to test the water heating idea.... funny how we think sometimes.


Armed with a plastic 1 gallon (4.5 litres) icecream bucket and a 4 ohm load, I wandered back to the battery bank and proceeded to fill the icecream bucket with cold water, and with the aid of two microwave transformer primaries, freshly stolen from two unsuspecting transformers, I put them in series, and threw them into the bucket of water. (The transformer primaries come out complete from the transformers with 6mm spade connectors, and were about 2 ohms each) I spaced them a few inches apart, and hooked them up to the batteries in series with an amp meter.


The 72v bank was now at about 80v, I hooked up the resistor (coils) to see what would happen.


I had forgotten that they were actually wire coils and was thinking only in terms of resistance...


You'd be surprised how fast they move together when you dump 80v@20A into two coils in a bucket....they were now tightly  stuck to one another in the center of the bucket... I wiped the water from my surprised face ... they moved through the water very fast and smashed into each other erupting the water everywhere.... and I  mused...oh well, no harm done... just frightened the hell out of me.


We were now putting about 1600 watts into the water. I shifted the mill input to the coil side of the meter. That way I could see how much the mill was putting in versus the batteries. ie no mill power meter means  20A into the coils from the batteries, as the mill gets some wind the amps contributed by the battery drop, and the meter moves towards zero.


A gust of wind came along and the meter dropped to zero, which meant the mill was carrying the full load, it then pushed the meter well below zero, with the meter hard on the stops, this means running the heater and charging the batteries.


It was at about this time I had the bright idea of dropping the batteries from the circuit, and see if the mill could do a reasonable job of driving the 4 ohm load by itself.


I repositioned the meter wiring to reflect this change, and the mill was putting out a nice steady 15amps or so.


By this time, the water was starting to get quite hot, and the wind a little stronger and it moved between 15 and 20 amps... all was going swimmingly.


Suddenly a larger gust came along, and the amps went up to about 25, and I got a little concerned, but it subsided as soon as it got up.... then it happened, a bigger stronger more lasting gust came along that just seemed to increase and keep increasing, the amp meter went hard over the 30A mark, until it was rammed up against the end of the meter, A quick look at the volts said 145,-150 the amps were well over 35A, and things were getting a bit interesting.


This lasted for an eternity (probably a minute or two really)... I dashed around the corner to see how the mill was coping... it was running perfectly, no vibration at all, no sign of trouble.... except the blades were just a blur.


I raced back to the batteries, to find steam pouring out of the icecream bucket, and hot water bubbling over the side.


At last the wind eased up, and I quickly re-attatched the batteries, which calmed the mill further, then shorted the 3 ph and the mill stopped nearly instantly.... it was over.


Luckily Zubbly's blade balancing method worked,... but the yaw didn't. Daveb was right about the furling part into resistors. It didn't look like furling.


So it was just (well)  over 5kw into a resistive load in I would say a 30 odd mile per hour wind on a 3 meter pole. Now it is over it seems good really, but I didn't care for it at the time. The stator was absorbing around 500-600 watts (.39ohms in star @35A) the load was copping about 5kw (4 ohms) so the stator could have done this all day. The blades worked well, and I walked away with the mill all in one piece.


And that's how silly I can be....


............oztules

philb:
If I don't make dumb, 'I should have known better' mistakes, I don't learn. :)

birdhouse:
geez, you could have at least used a five gallon bucket...   ;D

great story!

niall:
running away is my preferred option ...

and then maybe throw a few stones at it a bit later from a safe distance to shut it down ..... :P

great write up there indeed ... :)

Wolvenar:
I would still stick by my first assessment that the resistance was changing as the elements were not staying cooled enough.
Also it seems very likely there was not nearly enough load to handle the mill you were driving with.
It may be possible to handle this by adding additional load as voltage rises with a controller monitoring such things.
Then you can keep it out of stall until needed?

Unless I am missing something completely, as far as I understand batteries they just naturally increase resistance as input voltage rises ( but decreases as battery voltage does ( to a point).
Where as resistive loads like nichrome acts in reverse as heat builds

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