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C900 Bulletin Board
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Re: I see
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Fri, 25 Jul 2003 05:44:34 Share Post by Email
In Reply to: I see, JJ, Thu, 24 Jul 2003 15:00:03
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An alternator uses electromagnets to create an electric field for the armature to rotate in. If the magnetic field is constant, the voltage of an alternator (or generator) will increase as the speed increases. If you don't do something to regulate the output voltage, you'd get 12 volts at idle, and at 4000 RPM, it would be 20 or 30 volts, and all the electics would blow up.

Also, for a fixed rpm and magnetic field strength, the output voltage will drop as you pull more current out. That's pure physics - you don't get something for nothing. If you were carrying a cinderblock up stairs, you'd do a certain amount of work. If you carry two cinderblocks, you'd work harder.

The voltage regulator adjusts the strength of the magnetic field (field coils) to control the output voltage of the alternator to compensate for differences in engine speed and electrical load. It isn't exactly perfect, so there are variations in the voltage, but it tends to run between the high 12's to low 14 volt range. Let's not discuss temperature compensation for battery charging.

But the RPM drop is pure energy transfer. If you turn on an electrical load, say the brake lights, that takes power. On a typical 900, brake lights will pull about 60-100 watts. That power has to come from somewhere, and that somewhere is the engine. The alternator is the middleman.

If you want an interesting science fair trick, take a cheap DC electric motor - not an AC motor like a drill, but one of those little 3V jobs you can get at Radio Shack for running toys. Spin the armature. It spins fine. Now, short the leads together and try and spin the motor. It will be very hard to spin. That short circuit is a big electrical load (for a small motor).

This is called dynamic braking. Electric cars use it for 'regenerative' braking - if you want to slow down, you use the electric motor as a generator, and it takes the forward motion energy of the car and turns it into electricity to recharge the battery. This causes the motor to act as a brake. It's also used as a quick way to stop an electric motor. If you just remove the power from an electric motor, it'll spin down. If you put a large electric load across it, it'll come to a stop quickly - you're turning the spinning energy of the motor into work.



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