Re: Mini Alternator

Are you seriously asking about putting an alternator on an electric motor?

Generating electricity from the rotation of a fuel-powered engine makes sense, because you are starting with a non-electric energy source (the fuel). You lose energy at each step because no energy conversion process is 100% efficient, but you still end up ahead in the end because batteries don't store as much energy weight-for-weight as fuel does.

But generating electricity from an electric powered motor is a lot less efficient than just tapping power off of your main batteries, perhaps with a regulator.

I'm doing something similar to the last post on a somewhat larger scale. I am very interested in attaching an alternator to an electric motor in order to add a charge to the batteries running it. I don't intend to get "free" energy out of it, just slow the battery drain. The setup I'm working on is all electric running off the NiCd battery pack to the motor. The whole system is running off the custom battery pack and was hoping to find a way of prolonging the run time a bit. Anyone know if it's possible and how to hook up the alternator without running the risk of damaging the battery pack(if the alternator had a higher output than the battery capacity for example)? Any information/ideas would be useful, thanks.

It won't work. You will be increasing the battery drain. The alternator is less than 100% effective, which means the extra energy required from the battery to drive it, will be only partly returned to the battery.

In principle, there are only two solutions to your problem:

1. Reduce motor current draw (using a switching speed regulator). That will, of course, reduce the motor's power output. In other words: If you go slower, the battery will last longer.
2. A bigger battery. The disadvantage here is that the bigger battery is heavier, so a 20% bigger battery will yield less than 20% increase in flight time.

In your case, since you are running ancient battery technology, there is a third, and much better option: Replace the NiCd battery with a more modern LiPo battery. That should give you at least 3 times the flight time with similar battery weight.

If you have a brushed DC motor, you may also want to replace it with a brushless AC motor (and a suitable speed controller). They are lighter and slightly more efficient. Also, they will drive bigger props without needing gearing, which further improves efficiency.

Exactly. High school physics really should be mandatory. Let's put it this way "You can't get sum'tin for nuh'tin".

I would only add the helpful quibble that if you go to a hobby shop they'll sell you a "brushless" motor, and may well get confused at the term "brushless AC" -- particularly since some folks know _that same motor_ as a "brushless DC motor". Just ask for a brushless motor, a LiPo pack, a suitable speed controller and a suitable charger -- they'll set you up.

celg2000 wrote in news: snipped-for-privacy@rcgroups.com:

"Ye canna change the laws of physics, Cap'n!" - Scotty, Enterprise engineer.

Nevertheless, you are attempting to break the laws of physics.

Look up the second law of thermodynamics.

Look up "entropy".

Brian

About 12 years ago or so a team of folks and I were looking at doing some LD

fights. There was a discussion with someone at AMA HQ about how Manyard

Hill had used a bicycle generator belted off the front end of his engine for

EMF to his RX and servos. The impression I was given was that the 'technology' was being kept secret. However, it should be fairly simple.

Get one and try it. Probably use a Zener diode for regulation and a small

battery pack for float control.

That being said, I think an alternator is a better idea because it is a rotating magnet rather than a rotating comutator which means true DC rather

than alternating DC (0 to +) and less chance of RFI splatter. There are a

couple of problems there, like how to wind the coils and how to balance the

magnet (IIRC old alternators [first generation] were prone to coming apart).

Good luck, and please keep us appraised of your efforts. I'm still be interested in playing part of that game. My fuels guy had a mix that (he

claimed but I never actually tested) would run 40 minutes to an ounce of

fuel. Figure fuel at about 6.6 pounds per galleon and you know how to build

your aircraft and stay within the 11 LB limit.

Jim Branaum AMA 1428 Six_O'clock_High Target snipped-for-privacy@guns.com

An alternator puts out AC, there are diodes on the alternator to change it to DC. It does have less "noise" than on a commutator/brush system. There was a Popular mechanics article on wiring directly to an auto alt to get

117vac to run drills and ac appliances. They wired in ahead of the diodes of course. I'm trying to solve the same problem. Lots of luck,

Lee

You could probably use a brushless DC motor as the alternator. A bridge rectifier like a car configuration would give you DC. I would use a better regulator than a Zener. The Zener will turn all of your excess voltage into heat at what ever current the alternator will supply. The result is a lot of heat to get rid of and a lot of wasted engine power that burns fuel. A simple buck voltage regulator would give good efficiency and adjustable output voltage to set at the float charge value for the battery chemistry you choose. Nicads or Nickel Metal Hydride would be a better (safer) choice than Lithium Ion unless you want to get serious on charge control.

Good Luck, BobH

| About 12 years ago or so a team of folks and I were looking at doing | some LD fights. There was a discussion with someone at AMA HQ about | how Manyard Hill had used a bicycle generator belted off the front | end of his engine for EMF to his RX and servos. The impression I | was given was that the 'technology' was being kept secret. However, | it should be fairly simple. Get one and try it. Probably use a | Zener diode for regulation and a small battery pack for float | control.

It's available and has been for a long time --