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Another cheap dump load
By frackers, Section Controls Posted on Mon Jun 30th, 2008 at 04:05:52 AM MST
but I need one crucial piece of data

I've been trying to characterise my mill but without a viable battery bank (the old truck batteries now have shattered plates!!) I need another form of load.
Having several 100's meters of high tensile fence wire I've started looking at making a dump load out of it since it is very cheap and has quite a lot more resistance than copper but without the expense of nickel-chrome.
The crucial bit of data I need is the resistivity of 2.5mm diameter HT wire. The nearest I have found is piano wire (which although HT is not coated with a zinc/aluminium galv protection) of 1.18 x 10-7 ohm-meters. Plugging that value into the calculator at this site gives me a length of about 40 meters per ohm. This value is about right for my testing of my 24 volt mill and if I put a couple of taps on it I should be able to fulling load it (say 40amps) at 28 volts.
This length of wire sounds like a lot until you imagine an open wooden frame 0.5m across (something like an elongated fencing contractors spider) would only require 20 turns - say about 0.5m long. Having lots of air round it I reckon that 1kw would disappear from this without scorching the wood.
So - does anyone have any numbers for the resistivity of high tensile wire?

Another cheap dump load | 14 comments (14 topical, 0 editorial)

Re: Another cheap dump load (3.00 / 0) (#1)
by Flux on Mon Jun 30th, 2008 at 12:09:15 AM MST
(User Info)

Loading into a resistor will not help you determine the characteristics of a battery charging windmill without a complicated constant voltage load regulator.

I doubt that your wire has any defined electrical characteristics and any figure you get for resistivity will be rough. If you have the wire why can't you find its resistance per foot or yard or something and work from the true value rather than an inaccurate calculated value.

Just pass a known current through a yard of it and measure the volt drop.

Flux

Re: Another cheap dump load (3.00 / 0) (#3)
by DamonHD (d@hd.org) on Mon Jun 30th, 2008 at 02:40:21 AM MST
(User Info) http://www.earth.org.uk/

Flux, the EU measurement police will be on you like a ton(ne) of bricks!

Obviously you meant to say 0.9144m. %-P

(My dad many years ago in a book of his wanted to refer to the standard 16oz can that you might find on a supermarket shelf, and he had to fight his editor not to replace it throughout with "454g can".)

Rgds

Damon

[ Parent ]

Re: Another cheap dump load (3.00 / 0) (#5)
by Flux on Mon Jun 30th, 2008 at 06:58:53 AM MST
(User Info)

It's the 25.4 mm worm that bothers me. Wouldn't want to find one in my marigolds.

Flux

[ Parent ]

Re: Another cheap dump load (3.00 / 0) (#6)
by DamonHD (d@hd.org) on Mon Jun 30th, 2008 at 07:12:11 AM MST
(User Info) http://www.earth.org.uk/

Eeek.

Next stop: millipedes and centipedes being locked up for false advertising. B^>

Rgds

Damon

[ Parent ]

Re: Another cheap dump load (3.00 / 0) (#9)
by frackers (g8ecj at *nospam* gilks dot org) on Mon Jun 30th, 2008 at 05:10:40 PM MST
(User Info) http://www.gilks.org

A guy I work with designs switch mode power supplies for the company. Anything from 1.8volts at 100mA to 28volts at 25amps from an input from 5volts to 240 volts :-) He has some prototype gear I'm going to use to make an MPPT.

Part of the characterisation I want to carry out is to see how the rpm changes as I allow the on load voltage to rise as I derive a maximum power curve for it (will need to make a coupling to my tractor PTO for this!). Got to make the genny into a Prony brake to see what is going in as well as measure what comes out...
Robin - Down Under (or are you Up Over)
[ Parent ]

Re: Another cheap dump load (3.00 / 0) (#2)
by antw on Mon Jun 30th, 2008 at 12:25:54 AM MST
(User Info)

Have you looked at Jaycar product WW4040 ? $NZ3.60 for four metres at 13.77 ohms per metre. A known resistance at a reasonable cost. Shop in Christchurch or online.

Antony
Off grid 21 months

Re: Another cheap dump load (3.00 / 0) (#4)
by imsmooth on Mon Jun 30th, 2008 at 06:51:14 AM MST
(User Info) http://www.mindchallenger.com

Why don't you buy some high wattage resistors on Ebay and make a dump-load from them? I got over 1600W of them for about $10 or $15 half a year ago. It is certainly easier than messing with hundreds of feet of wire, and probably cheaper.
Jonathan

Re: Another cheap dump load (3.00 / 0) (#7)
by RUFUS (RFRUFUS@AOL.COM) on Mon Jun 30th, 2008 at 08:52:36 AM MST
(User Info)

Hey F
Mr.Ghurd has 10 ohm 25 watt resistors
on his website 8 for 12 bucks
pretty nice, they make a neat setup
Rufus

Re: Another cheap dump load (3.00 / 0) (#8)
by jimjjnn (jimjjnn at yahoo dot com) on Mon Jun 30th, 2008 at 11:43:13 AM MST
(User Info)

Yep, Mr Ghurd is almost a Twelveionaire now. Chuckle
Jim Denver,CO
[ Parent ]

Re: Another cheap dump load (3.00 / 0) (#10)
by RUFUS (RFRUFUS@AOL.COM) on Tue Jul 1st, 2008 at 11:37:32 AM MST
(User Info)

Every little bit helps
Rufus

Re: Another cheap dump load (3.00 / 0) (#11)
by ainsea on Wed Jul 9th, 2008 at 03:52:16 PM MST
(User Info)

I'm wanting to dump 45amps at 24volts from the dump load terminals from my wind turbine controller. Between fencing wire, jaycar wire, headlights and high wattage resistors I'm ready to do something. Though dealing with ohms (I use it to see open circuit =1 or closed =0) and resistance is leaving me unsure of which way to head.
Does someone have the dumb ass guide to dumping?

Re: Another cheap dump load (3.00 / 0) (#12)
by ghurd on Wed Jul 9th, 2008 at 07:09:24 PM MST
(User Info)

Ohm's Law will give the ohms.
Watt's Law will tell the watts the resistor is taking.
They will have to be done over and over until they match what resistors you can find.

It is going to take more than 1 resistor. Probably at least 6.
G-

[ Parent ]

Re: Another cheap dump load (3.00 / 0) (#13)
by frackers (g8ecj at *nospam* gilks dot org) on Wed Jul 9th, 2008 at 07:56:46 PM MST
(User Info) http://www.gilks.org

It seems that every weekend has been either heavy rain or snow since I proposed this so I haven't yet had a chance to even measure the resistance of the fence wire.

Assuming I can knock up a rough wooden frame to separate the loops of wire I should be able to at least get as far as finding out how the wire behaves this coming weekend.

I've got about 1200m of soft iron "number eight" wire to wind up from a redundant fence in the next couple of weeks so I'll try measuring that as well (although I don't expect it to be consistent as its old and rusty in places where the galv has gone)
Robin - Down Under (or are you Up Over)
[ Parent ]

Re: Another cheap dump load (3.00 / 0) (#14)
by frackers (g8ecj at *nospam* gilks dot org) on Mon Jul 14th, 2008 at 05:14:09 AM MST
(User Info) http://www.gilks.org

Well, I've had a play with some 2.5mm high tensile fence wire recovered from a fence I dismantled this weekend with the following results.

Using a timber frame (a couple of cross pieces at each end with 4 lengths of 2 by 1 between) I've made an air spaced coil of 30 turns with a diameter of 0.5m. This is 1m high (so not too unwieldy but big enough to dissipate a fair amount of heat I reckon!). I didn't check the length of wire before I wound it on but it should be about 47m. If its still light when I get home tomorrow I'll try for a picture of it (not that its anything to look at!).

The measured resistance of the whole thing is 1.88 ohms so the fence wire comes out at approximately 25m per ohm.

When I drop the mill in a few weeks, to put it on the permanent mast, I'll run it up on the tractor PTO into this wire wound resistor to see what occurs :-)
Robin - Down Under (or are you Up Over)
[ Parent ]

Another cheap dump load | 14 comments (14 topical, 0 editorial)

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