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Re: Electric Cooling Fans

To: "Bob Kinderlehrer's" <>, list <triumphs@Autox.Team.Net>
Subject: Re: Electric Cooling Fans
From: David Massey <>
Date: Thu, 14 Aug 1997 19:06:31 -0400 (Bob Kinderlehrer's) writes:

>Ok, now I'm very confused. The output of the alternator is dependent on
>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

>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).


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

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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