Testing wind/water turbine performance

[van4]

I'm required to test a harizontal axis turbine's performance and plot a RPM vs Power curve. The turbine is driving a small DC generator that produces 5V at the turbine's maximum RPM with no other load on the generator.

I am planning on installing different sized resistors in parallel with the generator and then measuring the voltage out. This will effectively act as a load which will decrease the RPM, and in turn decrease the voltage produced by the generator, which can then give me a plot. My question is - what size resistors do I need?

Through my study of wind/water turbines, I've calculated that the maximum power that this turbine can extract from it's environment is ~20W ideally. My attempt at answering my question was that I use P=V^2/R to rearrange for R=V^2/P=5^2/20=1.25Ω. So would I need resistors from 0Ω to 1.25Ω?

 

[Tom.G]

Well, a 0Ω load will be a short circuit, which means you will have zero voltage across it; tough to get any power without any voltage .

The 1.25Ω load will be around the maximum power point so you will need resistors above and below that value. In addition to a 1.25Ω load, three values below and three values above should give you a decent curve. The lowest and highest values values could be a factor of four or five from the 1.25Ω, or about 0.25Ω to 6.25Ω.

Since you are working with rather low resistances here, be sure to account for any significant resistance in the wiring. That means you should measure both the resistance and the voltage at the point they connect to the generator terminals.

Have fun!... and let us know your results.

 

[van4]

Well, a 0Ω load will be a short circuit, which means you will have zero voltage across it; tough to get any power without any voltage .

The 1.25Ω load will be around the maximum power point so you will need resistors above and below that value. In addition to a 1.25Ω load, three values below and three values above should give you a decent curve. The lowest and highest values values could be a factor of four or five from the 1.25Ω, or about 0.25Ω to 6.25Ω.

Since you are working with rather low resistances here, be sure to account for any significant resistance in the wiring. That means you should measure both the resistance and the voltage at the point they connect to the generator terminals.

Have fun!... and let us know your results.

Great, thanks a lot.

I mentioned 0Ω, but what I meant was a very very small Ω :)

I was worried that resistors below the 1.25Ω value would cause some sort of problem because if you put the resistance into P=V^2/R, you get a higher power out than you have available. But your comment does make sense. Also, I assume I'll need resistors which are rated to ~20W? (My knowledge of electronics is very basic)

Thanks

 

[Tom.G]

Also, I assume I'll need resistors which are rated to ~20W?

Fortunately one of the standard power ratings for resistors is 25W. We normally use a safety factor of two for power ratings which means a 50W resistor would be specified; that's another standard power rating but a bit hard to find from the usual electronic suppliers. The reason for the 2X safety factor is to keep the temperature down, prolonging the life, and so you don't get burned when you touch them!

In your specific case only for low voltage and brief laboratory use, you could use 25W or even 10W resistors if they are immersed in a water bath to keep them cool.

Actually since the peak power will be with the 1.25Ω resistor, the other values will dissipate less power. For instance both the 0.25Ω and the 6.25Ω load will dissipate somewhere between 4W and 16W, depending on the characteristics of the generator.

To cut down on the costs, remember that resistance adds for series connected resistors. And for the lower values you can parallel several resistors. For instance four 5Ω resistors in parallel equals 1.25Ω. If the resistors in a series or a parallel combination are all the same value then the power dissipation is evenly distributed among them. Thus four 5W resistors would be rated at 20W provided there is air/coolant flow around and between them... that means don't mount them into a tight bundle.