I am doing some benchtesting of a model and came to realize that what I measure is NOT necessarily what I think I am measuring ...or is it?
Basic question: can you define what is meant by generator efficiency of up to 85% ??
Some of the home brew axials I presume are a bit less then those made with commercial tolerances and laminated cores. I am disussing resistance heater load such as a calrod in a hot water tank, not battery charging.
But how is that efficiency really measured or calculated?
I have been going over the aspect of the internal windings being in series with the load resistor. BUT, we measure the voltage across the load resistor to calculate power produced.
As a simplistic example, if the output voltage is 120v for an Rl = 10.6 ohms, then 1.35KW output is recorded P=v^2/Rl ohms
If I choose a 5.5KW 230v calrod element I get 10.6 ohms (convenient, huh?)
BUT, what about the internal windings of the generator ? how does it fit into the equation? There is power consumed by the windings inside the generator, which is why they heat up.... just another resistance heating element (more complicated but to make a point)
In my benchtop experiment with 12inch rotor, I have a 12v PM Pittman DC servo whose Rt of the windings is 2 ohms. I can get from 2500rpm down to near zero = 5v max
I can apply NL, 10 ohms, 2.5, or 0.5 ohms load Rl while the rotor is spinning
When open circuit ie NL, the back EMF of the generator is what is measured and is linear with the rpm.
LINK
note that at any rpm, the voltage measured across Rl is proportional to the factor (Rl/Rl+Rt)
IF load is 2 ohms, and I measured 1v across the Rl, then the voltage divider effect means that 1 v is also dropped across the windings Rt and 1/2 of the turbines power is wasted in the generator and the efficiency is 50% ...but I can use a 0.5 ohm load as well, and now 80% of the power is in the generator, and if Rl=10 ohms then 83% of the power is across the load and 17% inside the generator ( but not providing much load at all).
Total power produce by the turbine is Pload x (Rl+Rt/Rl)
Back to real turbine situation:
Does the 85% efficiency number mean that for a load RL = 10.6 ohms, that the generator must be built with an internal loss of 15% of 1.35KW = 202 watts and the winding Rt is now defined by 202=v^2/Rt and if Rt= 1.0ohm by coil construction parameters ( # windings/wire size), then the internal volage drop is only 14.2v .... Am I missing a basic relationship here?
BTW, w/r to another thread, a simple result of the benchtesting is that full load = 1/2 rpm of NL ...so IF you are at TSR=7 at full load, and the generator circuit gets zapped, that turbine is spinning at 420mph tip speed at 30mph WS ..watch out!
Stew Corman from sunny Endicott
scorman@stny.rr.com
I removed the link you had in the intro. Instructions clearly state "No HTML in intro"