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## Re: Electric Cooling Fans

 To: "Bob Kinderlehrer's" , list Re: Electric Cooling Fans David Massey <105671.471@compuserve.com> Thu, 14 Aug 1997 19:06:31 -0400
 ```kinderlehrer@mindspring.com (Bob Kinderlehrer's) writes: >Ok, now I'm very confused. The output of the alternator is dependent on the >rotation speed of the armature, ie RPM of the engine. Unlike an air >conditioner, there is no clutch mechanism that engages and disenges, so if >I put an electric fan in the car, then I have to go faster to get more >horsepower to the fan motor?? >Does this mean I will get better gas mileage if I turn the radio off? :) Ina word, yes. But a radio uses so little power that you will never know it. >I guess I'm lucky because I have a TR3 with a generator which is far less >than 100% efficient. It keeps on turning even after the battery is charged >up and it isn't really needed, so I can use those wasted revs to drive my >electric fan (when I get one). >BobK OK Bob, are you ready for a primer on generators? Here goes. The production of electricity by moving a wire is a magnetic field is described by the following equation: E = Blv Where E = Voltage (in this case 13.8 to 14.4 Volts) B = magnetic field strength (which is generated by the field voltage controlled by the voltage regulator) l = length of the wire in the armature (generator) or stator (in the case of an alternator) v = velocity of the wire We want to maintain a constant output voltage, E. But since the velocity changes with engine speed then something else has to change in the opposite direction to maintain the same output. Well we could change the length of wire in the armature when we change speeds but that would be very inconvenient . But we could change the magnetic field strength rather easily. In fact that's exactly what is done. Every charging system on every automobile has a voltage regulator who's purpose in life is to maintain a constant voltage at the output of the generator by constantly adjusting the field voltage and consequently the magnetic field strength. Here's something you can try: (this works best on generators since the Lucas alternators have an internal voltage regulator and it is hard to monitor the field voltage) Acquire a moving needle volt meter (buy, borrow, bribe someone you know with beer, etc) Connect the leads between engine and the field terminal on the generator (that's the smaller terminal). Start the engine. Observe the field voltage. Depending on the load, it will be somewhere between 9 to 12 volts. Now increase the engine speed and see how the voltage drops. That's because the faster speed enables the generator to generator the voltage with less magnetic field. Join us next week on Beakman's world: The difference between a generator and an alternator. Dave (Mr Wizard) Massey ```
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