DUMP LOADS (wind mills)

[solarnewbee]

Good Day All!

I'm adding a windmill to my system in the Philippines to cover typhoon and monsoon season and it's a 3-phase as unit with rectifiers added just before the MPPT controller to keep the wire size down. this windmill does not have a way to slow it down like feathering or braking.
the manufacturer recommends a dump load that will pull the motor down to safer speeds. I have ordered an ats charge controller for it to transfer automatically. I just don't know what a dump load is, beyond maybe 12v water heating elements in series on a water heater(also mentioned by the mfr). not really any electric water heaters in the Philippines but i can get the elements from the camping/rv supply house.

If someone has an idea and wouldn't mind drawing a picture or detailed idea i would appreciate it.

Best Regards!

SN

[rossw]

You don't specify the capacity of the machine (watts/kilowatts and/or voltage).

My own wind turbine has a 3kW loadbank which is basically just a big metal box with lots of ventilation holes, and a whole lot of thick resistance wire inside. Mine is a 3-phase load, which is used on the input side of the controller (AC side) where it's more effective and less likely to be affected by component failure.

[solarnewbee]

sorry Ross,

it's a 2400 watt 24/48v PGM motor

If your resistance load is there all the time won't that subtract from wattage or am I missing something.

can you draw me a picture (flow diagram)?

thanks, SN

[rossw]

it's a 2400 watt 24/48v PGM motor

OK, so it's a managable size. If it was a 300W toy, or a 45KW+ beast, we might have had to reconsider

If your resistance load is there all the time won't that subtract from wattage or am I missing something.

The controller has a large, double-pole relay that kicks in when the battery voltage is above a set point, somewhere around 55V
When the relay kicks in, it closes the two remaining phases to the two remaining pins on the dump load, in effect putting three resistors across the three phases, and loading the turbine down quite heavily.

Because they're on the AC side of the bridge, it doesn't draw anything from the battery except what is needed to power the relay, possibly.

[solarnewbee]

Thanks Ross! Got it.

Now the Mfr's instructions say that my 11 blade windmill should not be under this kind of braking in extreme winds (typhoons) so I may have to engineer something simple like they use on windmills on farms like grandpa had. Pull a lever and the tail fin folds sideways.

thanks,
SN

[niall]

hi Solarnewbee

its a modified car alternator windmill ......much like ( if not the same as ) sold by companies like " Missouri wind and solar "

 thats not to say it wont work in your situation ..... it has a few fairly large neo magnets  that can burn out a coil in a storm

i dont think anything will protect it , the blades are designed to go into stall or " flutter" mode  ....thats a bit iffy at best

but Grandpa usually knew best ......theres the nub

dont let it run in a storm .......take it down i guess ?

but then ...i,m a wuss  ......

taking it down in stormy weather and living to fight another day may not be a bad option .... and good luck , whatever you decide...........



       

[niall]

sorry 

i should have added ....tail furling ...

it works to protect the mill ....its simple and effective......and will protect the alt

oddly ...its hard to find a good video on utube explaining it ...this is the best i could get

[frackers]

Best description I've seen is here
http://www.thebackshed.com/Windmill/Docs/Furling.asp

[niall]

that about summed it up ......Frackers link 

so a combination of the two then maybe .....

furling and a dump load of whatever you can get your hands on that might work ..... (a custom 12/24/48v water heater element ,an air heater ...even bits of old tumble drier elements rewired if need be ......)

but the resistance of the dump load needs to closely match  ( and ideally exceed )  the mills maximum output......ohms law is a pain , but you want to do right by your mill ....

furling is an interesting concept , i,ll give it that .....but once you try to set it up the penny will drop

this is my mill fully furled .......on the ground .....not ideal.....but it looks furled  ...and somehow that makes me feel better as i,m still trying to give it purpose in life  ...all fun

[rossw]

but the resistance of the dump load needs to closely match  ( and ideally exceed )  the mills maximum output......ohms law is a pain , but you want to do right by your mill ....

I hate to be a nit-picker, but especially for someone who's new to the game, this needs to be clarified.

The RESISTANCE needs to be matched to the turbine. If the resistance is too high, you won't pass enough current to make an effective brake. If the resistance is too low, you will dissipate most of the generated power in the alternator (and potentially burn it up). Once you have the right RESISTANCE established, it must be of a sufficient POWER rating to dissipate all that generated power without itself burning up or making a fire hazard.

Eg, if your alternator has a nominal output voltage of say 26 volts, then a load resistor of about 0.75 ohms will cause 34 amps to flow, and will dissipate 900 watts. We're talking about a resistor like a toaster... it will need to be even bigger if you don't want it glowing red! (or water-cooled or similar)

[niall]

your so cute i love you

[niall]

the  Resistance has to higher ......otherwise how would we stop the thing ... the Resistance has to be higher .....otherwise everything would go to pot

[niall]

ah well ...a melt down ......no one  gives a $#!+ about windmills anymore ....but they have their moments........

[solarnewbee]

Hey all! Good Day!

Just for clarification mine is a Dual PMG, differences pictured below, not an alternator designed for 24v parallel or 48v in series. It's 2400w 11 blade rig and the PMG generator is longer to accommodate dual 3-phase coils.

Niall do I need to build enough resistance to stop it or just braking down to a reasonable speed. I just want some resistance braking until wind speed get up past say 40mph nearly tropical storm levels.

for my own peace of mind I will probably set the pole with a hinge to take it down for typhoons and maybe figure out a simple angled furling hinge in the distant future.

thanks for the great discussion guys,

Sn

[Pete]

Hi Solarnewbee, you will need to match the dump load to the generator size. Your regulator will be a Shunt regulator, which means that when the batteries are charged it sends the output of the generator into the dump load. If you don' t have a dump load the turbine would just unload when the regulator cuts in and destroy itself because of the speed it would be running at. In a solar setup the regulator just open circuits the panels, but doing that with wind or hydro will just destroy the turbine in high winds or just by water pressure .
You will have to find a dump load capable of taking the current your generator will supply. If you are using it at 24 volts you will have to be able to sink 100 amps into it, at 48 volts you will be sinking 50 amps.
Power = Volts X amps    or more useful for you Power = Current Squared x Resistance.
So for a 24 volt load you will need  0.24 ohms as a load.
For 48 volts you will need  a 0.96 ohm load.
Making sure that your load can handle the current will be the challenge.
A water cooled load dump would be the easiest to make.
Pete