3 phase AC dump load

[Rob Littiken]

Ok if you've been keeping up with the threads you probably know that I'm build a 6000 W 500 Vac maximum output on my grid tied wind turbine. I'm installing a dump load to keep the turbine in control if the grid goes down or at extreme winds.

I've figure with Ohms law that I need a 40 Ohms resistive heat element to use as a dumload. I have purchased 6- 1000w Ohmite elements that are 6.8 Ohms each. If I wire them in series I should get my 40 Ohms and 6000W. I was planning on hooking this up to the DC leads after the rectifier ( + to one side of series of resistors and - to the other end of the series of resistors). Relays would control the switching of the dump load.

Now I am wanting to hook the dump load directly to the 3 phase AC legs and eleminate having the extra rectifier (one less thing to go wrong). With three legs of power how would I wire the resistors to get the same effect of 40 Ohms 6000W.

Hope this isn't too stupid of a question. Thanks for the help.

Rob L

[Rob Littiken]

Thought I would post a drawing of what I'm talking about.

Besides the increase in voltage in the DC after the rectifier. Will these two dump loads work the same?

Thanks,
Rob L

[Stewart Corman]

Rob,
AC or DC, you have the same number of electrons moving around, so in answer to your question ...it doesn't matter if you have DC and a string of 6 resistors, or 3 phase AC and have two resistors across each leg as in your diagram ....


now back to your calculations:
I'm build a 6000 W 500 Vac maximum output on my grid tied wind turbineif you have 500v AC 3 phase per leg, then you have 700v DC
6KW = 700v x 700v / R ...so R = 81.66 ohms
where did you get 40 ohms? by me, 40 ohms is 12.25KW

maybe I have simplified it too much??
Stew Corman from sunny Endicott

[Rob Littiken]

I guess this is where I'm confused. I'm planning on hooking the dump load to the 3 phase AC before the rectifier, so the max Volts AC it will see is 500V. If you put 500 V and 6000 Watts into the Ohms law calculation you get 41 Ohms.

My original question rephrased is if I put 13.6 Ohms of resistance between each leg of the 3 phase, will this get the 41 Ohms of resistance that I need.

I think I'm thinking this through correctly? I just wanted to make sure.

Thanks for the help.

Rob L

[Stewart Corman]

I guess this is where I'm confused. I'm planning on hooking the dump load to the 3 phase AC before the rectifier, so the max Volts AC it will see is 500V

I guess both of us are confused then??

How do you measure 500V AC except by measuring across any two legs???
you can't measure all three legs simultaneously

SO, IF, if you have a generator rated at 500V AC at x rpm, then they are talking about across each leg is 500V.
Many use the 1.4x ratio of AC to converted DC for a three phase bridge.
Rob Becker had measured 1.33x if my memory serves me.

So, what is the source of your confusion?
My numbers appear to be correct.

Stew

[Rob Littiken]

I didn't say your #'s were wrong. I'm just trying to figure out how to wire my resistive dump load.

I'm not using the dump load after the rectifier, so I don't think the DC conversion of 1.33x has anything to do with what I'm trying to do.

What I'm trying to understand is with a 6000W generator and each leg makes 500V to get the resistive load do I use these figures in Ohms law to get 41 Ohms. Then do I divide by three for the load for each leg of 13.67 Ohms?

My other thought is I need to take 1/3 of the 6000 Watts, 2000 watts and input this into Ohms law with 500 V to get the Ohms for each leg?

As you can tell I'm new to this and am trying to learn. Thanks for any help you can give.

Rob L

[Stewart Corman]

Rob,
My other thought is I need to take 1/3 of the 6000 Watts, 2000 watts and input this into Ohms law with 500 V to get the Ohms for each leg?
exactly!

OK, we'll do it this way for each leg:
P= V^2/R 2000watts= 500x500/R R= 125ohms

Then do I divide by three for the load for each leg of 13.67 Ohms?

If you had a single 6000watt load across a single AC 500v leg then you get 41.66ohms
looks like you multiply the ohms not divide when you split it three ways
ie 41.66ohms x 3 = 125ohms


Stew

[Rob Littiken]

Thanks

Rob L

[Steven Fahey]

This might help you check your work.

http://www.allaboutcircuits.com/vol_2/chpt_10/1.html
Chapter 1 is single-phase as you'll see, but the next couple of chapters show how to do the calculation for 3-phase.

Ohm's law in 3-phase circuits is not intiutively obvious to me either. :unsure: I check this when I can.

[Cor van Houtum]

hi guy's

we have tested this and the results are that using the same resistors
3x parallel on the dc site
or 3x over the phases AC will give a big difference in brake power

the dc coupled resistors will brake the engine mutch harder as the ac ones
this is due the higher voltage on dc site


kind regards

Cor

[Dale Sheler]

Edited because of a reading comprehension problem.

[Cor van Houtum]

Hi guys,
I have tested the dc variant of the dumpload
it works great but we forgot some basic thing

SPARKS

The dumpload relais burn in on the contact points mutch faster as we should want.
when dumping it make a electric flame bow twice as mutch as with AC coupling

just practice

[Rob Littiken]

Thanks for the info Cor. I was wondering about that. Are you going to go with an AC dump then, or find a DC contactor?

Rob L

[Cor van Houtum]

The only good dc contactor is a gas loaded contactor from omron
but this one is only in NO contact to get (normaly open)
to use as a dumpload contactor you need a NC contactor (normaly closed)

so we go to AC

[Rob Littiken]

Cor,
What NC AC Contactor are you using? (Brand and Model #)

Thanks for the info.

Rob L

[Cor van Houtum]

We use these contactors at the moment

[Rob Littiken]

I think that's a Telemecanique LC1D25G7 ? I have looked at these and they have 3 NO contacts and 1 NC Aux contact that is used. Are you using the the 3 NO contacts to disconnect the inverter from the generator and how are you using the 1 NC Aux contact? I was hoping to find a contactor with 3 NC contacts, so that when the power went off the dump load would come on. I'm not sure if your setup is doing that and if so how?

Thanks for the help,
Rob L

[Cor van Houtum]

These are NC contact relais
when the power is off then the dumpload is on
it has 2 contacts so you can use it with 3 phase dumpload

just connect 1 lead direct to the dumpload and switch the other 2 true this relais

[Rob Littiken]

Is LC1D25G7 the correct part # ?

If so it seems to be listed as having 3 NO contacts, 1 aux NO, & 1 aux NC
See attachment.

Thanks for the help.
Rob L

[Cor van Houtum]

the relais type is LC1D258 230 volts coil

[Rob Littiken]

Thanks. That makes more sense. I couldn't read the original picture clearly.

Rob L

[Jonathan Kraidin]

Hi guys,
I have tested the dc variant of the dumpload
it works great but we forgot some basic thing

SPARKS

The dumpload relais burn in on the contact points mutch faster as we should want.
when dumping it make a electric flame bow twice as mutch as with AC coupling

just practice

You need to be aware that if your turbine is putting out >300v and have +10A going through it you are going to seriously stress your relay over time. During one storm, my shunt control was going on and off. This is why I went with a solid-state relay system. It has worked very reliably.

Here is the link http://www.mindchallenger.com/wind/wgpage16.html

The entire wind tutorial covering F&P and Axial Flux turbines is here http://www.mindchallenger.com/wind

It uses an IGBT. There is built in hysteresis to prevent high frequency oscillations. It will trigger when a defined threshold voltage is exceeded. It does not require any batteries or AC power in order to monitor the system. It is self contained.

With regards to a dump load, I have a non-inductive resistive load across the DC output of my turbine.

[Robert Mann]

Hello In testing of my Tree Top Wind Mill with my new Mann Smart Drive witch is a VAWT
I too came to what if I lose the grit and the turbine runs wild.
See The night be for Huricanne Irene I had desighed to let it run and make power in the
80+ MPH winds on my water tower test site. I buyed 2 MCG defnite pupose contactors
MCDP403 G6 for $27 USD each 3 phase in and out with a 110 coil to pull down contact bar. I cut the top off a little and cut the other to be mount on top of the one that was open one. set the springs in side on the high side to push up when the 110 coil lost power from the grid it would snap up and make contact in the other relay body. I split the 3 phase coming from the turbine to both relays in side . On the top relay on the out side of the relay I ran it into two 500 watt flood lights out side pointing to the VAWT.
It worked like a dream.
Here is the turbine at work
http://youtu.be/EyhnGlPnwsM
Bob Mann