MadSci Network: Engineering |
I'm reminded of a discussion I had with a colleague from another part of the company where I reached a particular conclusion about a problem we were working on. As he eloquently said, "the problem with your ipso facto conclusion is that there's no 'facto'". Your question asks what happens to the power generated by a generator when there's no load. The faulty 'facto' here is that power is being generated absent a load -- it's not. It's much like asking how much power a battery produces when nothing is connected to its terminals. A slightly different question to answer is how much shaft power does it take to rotate a generator when nothing is connnected to its terminals. There's a couple of things to consider. Suppose that the generator is being driven by a constant-speed source (e.g., a turbine with a speed governor). The generator would produce a voltage (specifically, it's "open circuit voltage" for that speed, but no current would flow, and no electrical power would be produced. Only enough torque is being transmitted down the generator's input shaft to overcome various parasitic losses such as bearing friction, windage, and eddy current losses. Suppose that instead of a constant-speed source, we insist on a constant-power source. What would happen is that the generator's speed of rotation increases until these parasitic losses grow large enough to absorb all of the applied shaft power. Again, no electrical power is being produced. This is much like pressing the gas pedal in your car when the transmission is in neutral -- even a small amount of gas causes the engine to race madly, yet no useful power is being produced. All of the input power (i.e., the expansion of burning gasoline vapor in the cylinders) is being absorbed by internal friction losses (pistons scraping the cylinder walls, bearing friction, windage losses in the crankcase), and external losses (fan blades, power steering pump, A/C compressor). By the way, my explanation above is a simple example of what's known as an "energy balance": all the power that goes into the generator (as torque and rotation of the shaft" must be accounted for somewhere (friction loss, electrical power output, etc.). Further explanation on the principles of electrical generators can be found in introductory engineering textbooks with the words "electrical machinery" in the title. One interesting book that caught my eye doing a quick inquiry of an online bookseller is "Dynamic Simulation of Electric Machinery : Using Matlab/Simulink". I'm not familiar with the book but it apparently provides mathematical modelling that one can use to simulate questions such as yours to find out "where the power goes". Steve Czarnecki
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