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Message |
Sunny Garofalo (Jaguarxj6)
Junior Member
Username: Jaguarxj6
Post Number: 239
Registered: 2-2003
| | Posted on Wednesday, April 02, 2003 - 12:31 am: | |
I remembered this exact same question in Feb on one of the Jag forums I'm a part of, posted by Alex Cannara:
"(blank), here's how it works. In general, the power in an electrical load is current times voltage, so a 10Amp drain on a 12Volt alt or battery is 120 Watts. At 746 Watts/HP, this is a model airplane engine's worth of power output.
Now, the belt from the engine pulley turns the pulley on the alt mechanically, transferring power proportional to pounds times feet per second (tension in the belt times its speed of movement. These units can be converted to torque and rpm, of course, but forget that. The armature of the alt (or a generator) is turning in a magnetic field established by the field coils. The armature has coils too. These coils pass through the magnetic field and because of what we know from physics, the charges (electrons) in these coil wires are deflected by the magnetic field they're moving through. This makes a current flow from the alt/gen to the outside circuit (batt, lights, etc.) and back. The power coming out of the alt/gen is equal to this current times the output voltage, say 12V, across the load.
Now, physics and nature are complex, but the electrons know what they're doing. The very motion of the charges in the coils causes an opposite magnetic field that fights the rotation of the armature in the main field. This appears as a mechanical torque at the alt/gen shaft and therefore tends to pull on the belt against the engine's pull. This is why belts slip when not tight enough.
This reactive torque or belt force is directly due to the current flowing in the output circuit from the alt/gen. So, the more Amps you try to cause to flow in the output circuit (more lights, etc.) the more the alt/gen resists the torque/pull of the engine.
There's self-limiting feature to this because as the alt/gen is slowed by the electrical load on it, it's output voltage decreases, so the power to the electrical load will decrease too -- lights dim if you turn on more of them. The engine also slows, if the belt doesn't slip, because power is being demanded of it that's more than the current fuel input can deliver. So, to get the rpm back up and drive the heavier electrical load, you step on the gas a little. This use of a variable electrical load on a dynamometer is exactly how HP measurements are made (746W = 1HP).
Some engines have automatic ways of compensating for this by adjusting the throttle as rpm drop -- lawnmowers, for instance, have very simple feedback mechanisms. Obviously, when the A/C is turned on, the same thing happens and the engine slows. I've even planned to add a small electric vacuum valve that will let extra air by the throttle when the A/C is turned on, boosting rpm back up. The lights are a smaller load, so don't really make rpm drop much.
However, on our Honda Accords, with lots of brake lights, you can even see the rpm drop when you hit the brakes in neutral! In any case, the linkage from mechanical engine power to electrical power is the magnetic field seen/caused by the armature in an alt/gen. The details are described in Maxwell's equations. A subset appear as Lenz's Law, which describes the reactive behavior of a coil carrying a current in a magnetic field. If nature didn't work this way, we'd all be dead anyway, since it's the Earth's magnetic field that protects us from the nasty charged particles the Sun blasts out!
I hope this helps, but a basic book on motors and generators should describe all you might need. Maybe "How Things Work" too!
--
Alex
79xj6L SII (BRG + wires)
86xj6 SIII (Black)
61 Sprite MkII (Red)
Menlo Park, Calif" |
Randall (Randall)
Junior Member
Username: Randall
Post Number: 241
Registered: 1-2003
| | Posted on Wednesday, April 02, 2003 - 12:12 am: | |
Not to mention the added drag from having the headlights up, if this applies to your car. |
Tim N (Timn88)
Advanced Member
Username: Timn88
Post Number: 2709
Registered: 6-2001
| | Posted on Tuesday, April 01, 2003 - 7:18 pm: | |
It does, you dont get energy to power them for free. If you have a hand generator, you can do a demonstration of this. If you spin it with a lightbulb hooked up, you will feel resistance. if you do it with nothing attasched (wires not connected) you will feel much less resistance beacuse you dont have to "pump" current.
The amount of gas used is probably not very noticeable, especially with xenons. |
James Glickenhaus (Napolis)
Member
Username: Napolis
Post Number: 875
Registered: 10-2002
| | Posted on Tuesday, April 01, 2003 - 6:53 pm: | |
Yes |
Matthew J Germane (Mjgermane)
New member
Username: Mjgermane
Post Number: 25
Registered: 1-2003
| | Posted on Tuesday, April 01, 2003 - 6:45 pm: | |
I have heard that if the headlights are on the alternator has more resistance and thus uses more fuel because the engine is having to work harder to drive the belt on the alternator. Is this true? |
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