Author Topic: DIY Air resistive loads?  (Read 18261 times)

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Vortechs

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DIY Air resistive loads?
« on: December 08, 2013, 09:25:23 AM »
I've just upgraded my hydro pipe and so now need a better air resistive dump load that can handle upto 500W continuous (and is not a fire hazard). My system is 24v and at present I have a home made one constructed from 2 stainless steel welding rods wound into a coil. Is there anything that's easily available that would do this job? Something like an old fashioned cooker ring maybe, adapt an old electric fire? I have no idea how you work out what the correct resistance should be  ::)
Thanks

mab

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Re: DIY Air resistive loads?
« Reply #1 on: December 08, 2013, 12:33:16 PM »
I assume you are charging a 24v battery:

500W at 28V would be approximately 18A (I=P/V)
So at 28v you would want 1.55 Ohms maximum (R=V/I).

I think you would not find many grid voltage heating elements with such a low resistance.

More SS welding rods?

joestue

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Re: DIY Air resistive loads?
« Reply #2 on: December 08, 2013, 07:02:44 PM »
I see no reason to buy nichrome when it wouldn't be safe to let the resistor get red hot anyways, so use whatever you've got. there is utterly no reason to buy large resistors in my mind unless they are really cheap.

the most expensive component is the ceramic tube the wire is wound on... i mean there are legit applications when you need a 500 watt resistor that doesn't change but .1% across 300C temperature rise, but i can't think of many...

also, if you put the resistor coils at the bottom of a 4+ foot long pipe you'll create an enhanced natural draft going on and they will run much cooler

gizmo

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Re: DIY Air resistive loads?
« Reply #3 on: December 08, 2013, 11:40:06 PM »
Something like an old fashioned cooker ring maybe

Yep, dead easy to use, and you can buy cooker elements for next to nothing from any recycling centre. There's an article on my site about using cooker elements ( encapsulated resistors ) to build dump loads. See http://www.thebackshed.com/Windmill/articles/BuildingEncapsulatedResistors.asp

Glenn

Vortechs

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Re: DIY Air resistive loads?
« Reply #4 on: December 09, 2013, 11:25:58 AM »
 Thanks guys, that's excellent info! Having 2 or more coils in parallel would also be safer, if one fails at least there's another to take the load. I'll get building  :)

birdhouse

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Re: DIY Air resistive loads?
« Reply #5 on: December 09, 2013, 06:02:56 PM »
Quote
I have no idea how you work out what the correct resistance should be  ::)

you don't have to do it with math.   i used nichrome wire out of electric wall heaters (cadet).  i hooked them up to a 28v source and varied the length until they dumped some heat but never got orange or even red.  then with an ammeter in series, i was able to figure how many watts at 28v were being dumped.  i then paralleled  as many as needed to dump the required watts without overloading my controller. 

adam

SparWeb

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Re: DIY Air resistive loads?
« Reply #6 on: December 10, 2013, 08:25:09 PM »
I suggest the element from a clothes dryer.  2400W - 3000W at 240VAC will give you 24-30W at only 24V, but with a bit of re-wiring you can pick any resistance you want.
They are usually in an easily removable module too.
Dryers are being thrown away all the time, so visit your neighbourhood appliance stores, park around back, bring a screwdriver...
No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

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XeonPony

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Re: DIY Air resistive loads?
« Reply #7 on: December 21, 2013, 10:44:56 AM »
yup driers or hair driers are my faverit source as they provide allot of easily workable nichrom at a good thick guage.

And I do allot of perrallel circuits with a potential relay as a fail safe to a secondary bank.

Make them wide and roomy for max disapation
Ignorance is not bliss, You may not know there is a semie behind you but you'll still be a hood ornimant!

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Vortechs

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Re: DIY Air resistive loads?
« Reply #8 on: December 23, 2013, 03:50:45 AM »

SparWeb

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Re: DIY Air resistive loads?
« Reply #9 on: December 23, 2013, 07:25:07 PM »
Sure,
It's cheap.
But it's also conveniently located in the back of clothes dryers in a convenient box that requires 2 screws to remove.  Dryers are disposed of by everybody, everywhere.  The waste annoys me, but I can't fix stupid.  Scavenging a few parts makes me feel better sometimes.  There were to complete dryers in the dumpster last weekend when I went to the waste disposal site in my town.  Even though they have an area set aside for appliances!!!!!!
No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

System spec: 135w BP multicrystalline panels, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

Vortechs

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Re: DIY Air resistive loads?
« Reply #10 on: December 24, 2013, 05:10:31 AM »
In Europe electrical equipment has to be recycled by law but nearly every recycling centre I know of will not allow you to 'scavenge' or take anything away, my local is the one exception I know of so I'm lucky but not everyone is.

Vortechs

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Re: DIY Air resistive loads?
« Reply #11 on: January 11, 2014, 10:48:16 AM »

I ended up buying a large power resistor to use as my dump load, as suggested by Huge Piggott here:
http://scoraigwind.co.uk/using-a-high-power-resistor-as-a-dump-load/ I want something that can safely handle 500w continuously and that looks tidy, so it seemed ideal.

It finally arrived yesterday and so today I connected it up.... The noise is unbearable, it's like having a giant mosquito in the room, I have to keep re-boiling the kettle to make it shut up. Oh well, back to the drawing board.  ::)

DamonHD

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Re: DIY Air resistive loads?
« Reply #12 on: January 11, 2014, 11:42:40 AM »
Depending on your dump controller / BMS you can sometimes alter the dump PWM frequency, eg from mosquito to a few Hz.

Rgds

Damon

OperaHouse

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Re: DIY Air resistive loads?
« Reply #13 on: January 11, 2014, 02:07:45 PM »
What is making the noise?  Any time you have enough current it will make a magnetic field.  A steel cabinet used to protect the resistors will vibrate as will adjacent cables.  I built a machine that would make welding cables jump a half inch.  Using paired cables to each resistor might help.  This can reduce individual strength and reverse flow can cancel the field.  Paired conductor insulation will also deaden the vibration.

birdhouse

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Re: DIY Air resistive loads?
« Reply #14 on: January 11, 2014, 03:23:03 PM »
is there a charge controller involved? 

the morningstar ts-60 will buzz at high frequencies while dumping as part of it's PWM.  mine is in a shed where i rarely have to hear it, so i think of it as minimal, but could see how one in the home could really start to bug you! 

adam

SparWeb

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Re: DIY Air resistive loads?
« Reply #15 on: January 11, 2014, 04:27:41 PM »
Much the same experience.  The dump load can be just as loud as the inverter.  When both are on it's quite a lot of "music".  I wouldn't want that noise in my house, either.
Perhaps water heating elements (in a water tank) would be quieter.
No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

System spec: 135w BP multicrystalline panels, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

birdhouse

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Re: DIY Air resistive loads?
« Reply #16 on: January 11, 2014, 04:43:54 PM »
Quote
Perhaps water heating elements (in a water tank) would be quieter.

i feel like the noise comes from the CC, not the dump load itself,  but i could be wrong.  for people who have never heard it PWM, it's very odd.  for me, its more like:

"the batts are full, and i love that crazy squelly noise!"

adam

Vortechs

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Re: DIY Air resistive loads?
« Reply #17 on: January 20, 2014, 03:08:20 PM »
The noise is definitely from the resistor as the CC is outside in a shed. The CC is a Trace / Xantrex C40 which is PWM'd,  it's what's causing the electrical noise coming from the resistor, is this adjustable as DamonHD suggests? There's nothing I can find about it in the manual.

Using paired cables to each resistor might help.  This can reduce individual strength and reverse flow can cancel the field.  Paired conductor insulation will also deaden the vibration.

Sorry, what do you mean by 'Paired',  2 cables in parallel or cables stripped of their insulation? I have the insulation stripped back about 6 inches.

I dont have any hot water system in my cabin so a water heating element is no use to me unfortunately.

I'm going to try building a resistor from thin Nichrome wire and see if that eliminates the noise, hopefully it wont be able to resonate as easily as the thicker stuff...

SparWeb

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Re: DIY Air resistive loads?
« Reply #18 on: January 20, 2014, 11:33:11 PM »
Quote
... The CC is a Trace / Xantrex C40 which is PWM'd,  it's what's causing the electrical noise coming from the resistor, is this adjustable as DamonHD suggests? There's nothing I can find about it in the manual...

I just checked the manual and I can't find a feature to stop that either.  Not a surprise, really, since the PWM cycle is exactly what made it such a big step forward when it came out in the '80s. 
The Tristar TS60 can be forced to regulate solar PV as simple on-off charging.  If you want to know if the Tristar could do this for the hydro diversion load, you should call Morningstar.  That would discard much of the voltage regulation capability, however, so instead the voltage would rise and fall as the diversion load cycled on and off periodically.  I don't know how often.

Something tells me the nichrome will buzz too. 

But feel free to prove me wrong!  :)
No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

System spec: 135w BP multicrystalline panels, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

XeonPony

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Re: DIY Air resistive loads?
« Reply #19 on: January 21, 2014, 05:18:11 AM »
Or good time to get your self a small water heater going, easy to get 600w dc eliments.
Ignorance is not bliss, You may not know there is a semie behind you but you'll still be a hood ornimant!

Nothing fails like prayer, Two hands clasped in work will achieve more in a minute then a billion will in a melenia in prayer. In other words go out and do some real good by helping!

OperaHouse

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Re: DIY Air resistive loads?
« Reply #20 on: January 21, 2014, 08:57:21 AM »
A paired cable would be lamp cord or better still vacuum cleaner power cord with just two conductors bound together in rubber. 

I have a secondary system in my garage with a morningstar 10A PWM.  Relatively low current, epoxied in an aluminum case and no inductors.  All reason indicates it shouldn't make noise but it does.

Vortechs

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Re: DIY Air resistive loads?
« Reply #21 on: January 21, 2014, 12:34:09 PM »
What makes you think paired cables might quieten it down?

Or good time to get your self a small water heater going, easy to get 600w dc eliments.

I have a large kettle that always sits on top of the stove and an instantaneous gas water heater for the shower, so no hot water tank. I wonder though if I could convert one of those old oil filled radiator type heaters to a 24V 500W element? They're designed to do 3KW on a thermostat so they should be able to cope with 500W continuous...


gizmo

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Re: DIY Air resistive loads?
« Reply #22 on: January 21, 2014, 03:06:55 PM »
What about adding a few caps across the dump load? It would take the edge off the PMW.

I would still prefer to use encapsulated heating elements over bare nichrome wire, simply because the outside of the element is insulated from the internal wire, so it can be mounded directly onto a head sink.

Glenn

OperaHouse

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Re: DIY Air resistive loads?
« Reply #23 on: January 22, 2014, 12:41:22 AM »
What about adding a few caps across the dump load? It would take the edge off the PMW.


Glenn

That would be a fast way to blow up the controller.  The cap would look like a dead short.

Vortechs

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Re: DIY Air resistive loads?
« Reply #24 on: January 22, 2014, 10:55:45 AM »
The first thing I tried was a large cap across it, it lasted about 45 minutes before it went bang - did quieten  it down a lot though.

mab

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Re: DIY Air resistive loads?
« Reply #25 on: January 22, 2014, 05:14:25 PM »
how about a choke coil in series with the load - and a freewheel diode across the load and choke of course. A few turns of wire on a transformer core should do it.

SparWeb

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Re: DIY Air resistive loads?
« Reply #26 on: January 22, 2014, 10:09:14 PM »
I think the manufacturer of the charge controller would disagree.  Before suggesting "anything", better to find out what they recommend for your problem.
Fairly sure that CC's are designed to use purely resistive loads as the diversion.  Even if they make some allowance, I'd still aim to minimize capacitance/inductance on that line.
No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

System spec: 135w BP multicrystalline panels, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024