MadSci Network: Engineering
Query:

Re: WHAT HAPPENS TO POWER GENERATED BY A GENERATOR WITH NO LOAD ATTACHED?

Date: Tue Sep 7 19:09:07 1999
Posted By: Steve Czarnecki, senior technical staff member, Lockheed Martin
Area of science: Engineering
ID: 935516917.Eg
Message:

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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