Running resistive load ? Help.

[Harri Piltz]

Hi there !
I have:
- 2 kW , 9m/s windmill, 3-phase, y-connection, 250 Volts @ 250 rpm max.
- 3-phase AC then converted into DC
- 2,5 kW resistor in a water tank (the resistor in general purpose "stuff" for 230V, 50 Hz grid)

Is it possible to connect the windmill directly to the resistor ?

No need for kind of "cut-in" devices ?

Does the direct connection stall the mill ?

Experience ?

I hear someone saying, that when the wind is slow the voltage is low and the resistance also very low, so that the mill may start without stalling.

So ... how is it ?

Best regards.
Harri

[Rob Beckers]

Hi Harri,

Hopefully someone who has tried this will chime in (I have not).

Short of trying it is hard to say if this will work or not. As you correctly noted, the issue is that you are tying a 21 Ohm resistor to the turbine (slightly less in fact, as resistance goes down a little when the heating element is cold).

On the other side, when the Voltage is low (low RPM of the turbine) the power dissipated by that resistor is also very low. The equation is:

Power = Voltage * Voltage / Resistance

So, at 50 Volt DC (probably about cut-in for your turbine) it's dissipating just over 100 Watt. Maybe that's small enough to allow the turbine to spin up and get out of stall.

If need be it shouldn't be too hard to design a little circuit with an SCR or TRIAC that switches the resistor on at a specific Voltage (for example by using a DIAC to trigger the gate of the SCR), a three-component design would likely suffice.

More in general, optimal load of a turbine goes with the cube of the Voltage for a permanent magnet alternator (which you very likely have). The resistor loads up the turbine with the square of the Voltage, as the above equation shows. So, if the 2.5kW matches full load, the resistor will overload the turbine somewhat for all the other wind speeds. It depends on the turbine/blade design if this is an issue or not (the problem being that the turbine could stall at lower wind speeds).

-RoB-

[Harri Piltz]

Hi Rob !
Many thanks.
Yes, I have a permanet magnet gen.

I also have a power supply (xpower DNR30US24); 30W, 24 V DC out. This controls a 3-way contactor (all 3 contacts in series, because of the DC current).

Because my electronics skills are close to zero, I needed to buy the expertse.

The operating "range" of the power supply starts from 120 V DC.

The idea was; when the voltage from the gen reaches about 120 V DC, the contactor switches the load/resistor on.

Now I'm wondering, if the power supply and the contactor could be omitted and have a "direct coupling" to the resistor.

The system is in a remote place and the maintenance aspects were very important (less stuff, less trouble).

Another thing about chinese mills.
I bought about 2 years back a turbine + wings, rated 3 kW, from China (Qingdao Windwings Wind Turbine Co., Ltd.) or actually two; first one and last one.
I wanted to have windmill, bought chinese because of the price, good learning in any sense.

After a couple of turbulent, stormy days (not more than between 20-30 m/s wind)
the tail fell off, hit the turbine wings. Now I have zillion small bits and pieces, waiting to be transported to waste collection facility.
The whole turbine is full, please allow, crap.

The mast is made here. (I did not trust them in the first place).

Controls are more or less DIY from trusted manufacturers.

Many thanks for your prompt reply.

All the best.
Harri Piltz

[Harri Piltz]

I got the windmill working with the resistive load as descibed earlier.
So far it seems to be working OK.
BR
HarriP