mkfiddler11 wrote: chalker wrote:
soyuppy wrote:no...V=I*R. If R increase, assuming I remain the same, then V increase. V is the voltage drop. Or you can test it yourself. Just use higher resistance in you setup on the same blade and motor. See what happen.
There's a BIG assumption you made - I remaining the same. In reality, I won't usually remain the same in this circuit. Windu34 is correct in that by putting a higher resistor in the circuit, it makes it harder for the turbine to spin the motor. The reasoning behind this is due to something called EMF (https://en.wikipedia.org/wiki/Electromotive_force
) and Faraday's Law of Induction (https://en.wikipedia.org/wiki/Faraday%2 ... _induction
). The best way to think about this is that the blades generate a force on the shaft. That force has to overcome the 'physical resistance' of the motor to turn the shaft. That 'physical resistance' is due to a combination of the magnetic 'flux' of the motor windings and the electrical resistance of the circuit. Increase the electrical resistance, and the same amount of force will have more 'physical resistance' to overcome, resulting in less output (e.g. Power) from the generator (i.e. less current and voltage).
In many electrical engineering classes, the hydraulic analogy (https://en.wikipedia.org/wiki/Hydraulic_analogy
) is used to help explain things, by comparing electrical circuits to water plumbing. In this situation, think of the generator as a water pump. By increasing the circuit resistance, you are effective making the hose connected to the pump a smaller diameter. If you hook up a smaller diameter hose to a pump, the amount of water you get out of the pump will go down (e.g. the current decreases), but that won't be necessarily offset by the water coming out of the hose being more forceful (e.g. the voltage), hence the net power is reduced.
Thank you all for the responses. Our team experimented with different resistors to check the voltage results. With blades and the stand setup constant, they measured higher voltages for higher resistance in the circuit (10 ohm vs 5 ohms). The voltage output is almost double for both high power and low power of the fan. Whereas, at a tournament their voltage dropped when a 6.8 ohm resistor was used. I feel that the motor power at the tournament must be lower than our setup.
I stand corrected. This is very counterintuitive, but indeed higher resistance in this configuration will generally result in higher measured voltages (versus what would happen in a full scale generator that takes into account active and reactive power - https://en.wikipedia.org/wiki/AC_power#Reactive_power
). I think the reason for this is that I forgot to take into account that these motors have internal resistances that are a few ohms and generate DC power. Hence, we are effectively forming a voltage divider circuit (https://en.wikipedia.org/wiki/Voltage_divider
). By increasing the resistance, we are increasing the proportion of the voltage that goes to the external resistor versus the internal resistor, hence the voltage goes up (to a limit).
This is one of the reasons I love SO... there's always an opportunity to learn something new and be surprised when the practical application of something doesn't align with your theoretical understanding. Thanks for bringing this up and pointing out the results you were seeing!