Have you ever cranked a gen

1. May 19, 2006

linux kid

Have you ever cranked a generator? When it's not connected to anything it's easy to turn but when you connect it to a something (like a light bulb) it gets hard to turn. Why is that?

2. May 19, 2006

Hootenanny

Staff Emeritus
I think Lenz's law could explain this one . Unfortunatly, I do not have enough time to post a complete answer, but basically Lenz's law states that the current is induced in a direction such to oppose any chnages in magnetic flux, thus producing a force. Try googling for Lenz's law.

~H

3. May 19, 2006

Integral

Staff Emeritus
The work done by your load must be done on the generator. When there is no load you do no work, therefore it is easy to turn. When a load is attached you must provide the energy which does work at the load therefore it is harder to turn.

Think of it of "pushing" electrons through the load.

4. May 19, 2006

Danger

Can't give you any math on it. We just used to call it 'electromagnetic drag'. The more electrical load there is, the more resistance there is to rotation. In fact, we used to have 'alternator cut-out' switches in our cars so we could drop the EM drag out of the picture for racing. It's worth a few horsepower.

5. May 20, 2006

gnome

Interesting. How long can a car run on battery (without the alternator)?

6. May 20, 2006

Danger

I really don't know, gnome. I made it home from over 20 miles away, with my low-beam lights on, when my alternator packed it up a few years ago. Of course, with the age of my vehicles the only electrical draw when running (except for the lights) is the ignition system. Those new things full of computers and electric fans and such are probably a different story.

7. May 20, 2006

gnome

Oh, I wasn't even thinking about computers & such; just wondering about how long the battery can supply ignition. I'm thinking in terms of a project I'm planning for this summer -- a 1980 TR7.

So, your alternator cut out would be what -- a relay breaking the circuit between the alternator & battery?

8. May 20, 2006

Danger

You could use a relay. We just spliced a regular toggle switch into the wire. I think that the only way you can figure out the life of a battery that's not being charged is to disconnect your alternator and let it idle. I rather suspect that it might run out of gas before it runs out of electricity. If it's a major concern, you could always replace your stock distributor with a magneto. That supplies it's own power from the engine rotation.

9. May 20, 2006

gnome

No - not a major concern. Just curiosity. Thanks.

10. May 20, 2006

brewnog

Gnome, electrics should be the least of your worries with a TR7 project! Start worrying about rust!

My fan belt snapped on my 1600 Seven; I got about 120 miles before I needed a boost. The battery was about 1 year old at the time, and was running no electrical ancilliaries except for the ignition, instrumentation and indicators. (Fan is electric but switchable.)

Gnome, you in the UK?

11. May 20, 2006

Loren Booda

To teach us to appreciate the power of a field generator, our junior high school electronics teacher, Mr. Murphy, invited us to hold the leads while he cranked. Yeow!

Might this thread be referring to reverse EMF? Both mechanical friction and inductive impedances play a part in the initial description.

12. May 24, 2006

gnome

No -- opposite side of the ocean. My TR7 spent most of its life in the southwest, so actually rust isn't an issue. Electrics, on the other hand, what a mess ... It lives up (or down) to all the wonderful things people say about British Leyland electrical systems. We're definitely going to totally re-wire it.