On board alternator???

[Dimitris76]

Hi!

I am wondering what is this:

http://www.rcgroups.com/forums/attachment.php?attachmentid=1668143

I mean the AXi brushless motor that's mounted in line with the clutch bell. Is it a home made alternator? It can't be an on board starter since it's mechanically connected over the clutch bell and not directly on the crankshaft...

What about the electronics that are hidden behind the black shrink tube? Rectifing diodes? Capacitor? Both? Does anybody have more info on such a circuit?
Is it capable to support the R/C electronics AND keep the batteries charged?
Is the extra engine load noticable during flight?

Any info appreciated!

Dimitris

PS the heli on the pics was for sal a while ago but the seller (second owner) doesn't mention anything... DS

[Chippie]

Cant really see what model number the motor is..could be a low rpm/volt one...

I'd say the heatshrink wrapped components are likely to be six diodes, a capacitor of some reasonable value and maybe a regulator or two..I say two, one to run the r/c gear and the second to charge the batteries....

[Dimitris76]

Could you describe the rectifing circuit in more detail?
I thought that 12 diodes (or 3 rectifing bridges x 4 diodes in each) would be needed in order to rectify all three phases that a brushless motor outputs when used as an alternator.

And a sufficient capacitor in parallel of course...

Also, why two regulators? Can't you use justone in parallel with the Rx battery pack? That way the batteries will be constantly kept topped while being able to supply higher currents on demand.

Regards,
Dimitris

[pmackenzie]

Could you describe the rectifing circuit in more detail?



http://www.allaboutcircuits.com/vol_3/chpt_3/4.html

About 1/2 way down the page.

http://sub.allaboutcircuits.com/images/03267.png

Pat MacKenzie

[Dimitris76]

Great!

Thanks pmackezie!

Is a capacitor still needed (or adding up all the rectified waves in time fills in the gaps between the curves and gives us pure DC)?
If yes - are there a current/voltage dependent formula we can use to calculate the capacitor's appropriate capacity/voltage?

Dimitris

[pmackenzie]

The ripple on a 3 phase rectifier is pretty low, but you would probably still want a cap. Most regulators need one for proper stability anyway.

Why not PM Eyefly for some details?

Pat MacKenzie

[Dimitris76]

Good idea I'll ask him to post here so that everybody can read his answer.

Dimitris

[Dimitris76]

PM sent
Hi Eyefly!

I was looking at photos of the heli you had for sale and wondering what is that brushless motor on top and if it's an alternator how you did it.

Here is a thread I started about it:

http://www.rcgroups.com/forums/showthread.php?t=808935#post9025518

Could you be so kind and post there in order to "enlighten" us? Is it a DIY thing or an off the shelve add-on?

How did you link mechanically the electric motor to the clutch bell? What shaft, links, etc? Are they off the shelve or home made on lathe?

More info about the circuit and it's performance?

Thank you in advance!

Best Regards,
Dimitris, Sweden

PS I hope you already sold the heli by now!

[Eyefly]

You got it buddy. It is a small AXI motor mounted on top of the clutch bell and it acts as an alternator. The heli is a gas helicopter and has a pull start on the bottom. So there is no need for a wand and this is where the generator goes. The heli has a relatively small 1100 mah nimh pack to start/store power but the alternator makes all of the electricity for flight. Since this is a gas engine, there is little to no power loss because gas engines produce alot of torque.

As far as what is under the shrink wrap, its a secret. I am not willing to tell anyone what I have done...

Just kidding... ;) I am not an electrical engineer so I actually have no idea what magic lies under the shrink wrap. However a fellow by the name of Raja does. He is the one who makes these and sells them for gasser helies. Here is his web sight. He is on Helifreak and Runryder as well. You can pm him and ask. Sorry I could not be of any more help.

http://mysite.verizon.net/vze49gsg/id2.html

Alex

[XJet]

The only problem with using a brushless motor as an alternator is the fact that even with no load, there's a lot of magnetic drag on the rotor.

If you check out an automotive alternator, they turn very freely when not loaded -- because they have no permanent magnets in them. This also makes it an easy job to regulate the output voltage because you simply vary the exciter current rather than having to put a regulator in series with the feed.

If you're using a brushless motor to keep a battery topped up then it's a rather inefficient way to do it -- however, if you're feeding something that constantly draws a pretty high current you probably won't be so badly affected.

[jeffs555]

Yes, but there are many advantages with a permanent magnet alternator. It is simpler, weighs less, will work without a battery, and is more reliable(no slip rings). In a gas heli, the load from the alternator would be so low compared to load from the rotors that any difference in efficiency between a PM alternator and a field wound alternator would not even be noticed.

[basicmods]

These things work very very well from what i've heard. I have a gasser heli and have been looking at one to supply clean power constantly while the heli is in flight. If anyone gets raj to tell what's in that little magic box, post it here will ya, a schematic and parts list would be awesome.

[Dimitris76]

I have been dreaming forever about a nice big helicopter that I could just fill up with gas, place it on the ground and start it with a flip of a switch on the Tx - without caring about rx batteries, glow plugs and drivers, starters and starter batteries etc...

The alternator combined with one of these:

http://www.hobby-lobby.com/fema.htm

might make it come true!

Dimitris

[basicmods]

well a big gasser with that generator is almost there. That would work as well, but mounting it might be a pain. My gasser is a pull start, so with one of those generators it would be just the TX, the heli, and a gallon of gas for a whole day flying (or until the TX battery died, whichever came first) If you had a couple TX batts you could literally fly until you were tired of it.