26 March 07

[Capt Slog]

Following my last rather poor start at testing, I managed to rig up a motor to drive the rotor assembly so that I could do something a little more accurate.  Spinning the thing by hand and trying to count the turns just wasn't the way to go!

A small DC motor run from a Lab power pack gives me adjustable speeds, and seems to be quite repeatable; if I dial up the same voltage I get around about the same RPM.

Here is a picture of the 'scope with a 50 turn coil attached.

This shows about 3.5v at the top of the peak, and the time-base gave about 300 rpm.

(1 volt per division, and 5ms per div)

I get the feeling that this isn't enough voltage for what I want to do, and that I will need some more turns on the coils.  I'm planning on 4 ft diameter blades on this, and from what I can work out, it will be bearly cutting in untill 300rpm to charge 12v.

Just for interest, I spent a little time using Exell spreadsheet, and plotted a sine wave.  I've overlaid this on my scope picture and got a fairly good match.  I can't display it here, but please have a look at my photo uploads for a 'sine.doc'.

Other figures from my test...

440. rpm gave 4v
     
540. rpm gave 5v.
     
     

More to follow when I get my camera recharged.

[Flux]

If that is 1 volt/division I make it 2.5v not 3.5v peak.

If you have 9 coils, one phase will give 7.5v peak. Star connected will give you 12.75 so as you say that is cut in at 12v at 300 rpm.

That seems fine to me for a 4 ft prop but I am not sure what you are using.

Hugh's 4ft prop would be fine for that cut in speed. If you want something slower then you will need more turns.

Flux

[Jerry]

Hi Capt. Slog.

What is the shape of your magnets and coils? I have a hard time captureing pictures from my scope.

                     JK TAS Jerry

[Capt Slog]

Thanks Flux, one of these days I will check my typing,  My excuse is that I was at home on dial-up and so time was precious!

I don't know what speed to expect from a prop, but 300 rpm just sounds a little more exciting than I really wanted.  If that is what they are supposed to spin at, then perhaps it will be ok.

[Capt Slog]

Jerry, these are round magnets 1" diameter by 3/8" thick, 12 pole dual rotor.  The coil is round too, with an internal diameter of 1".  Details are in my last diary entry.

I had no problem getting pictures of the scope, other than holding the camera still enough.  The trick for taking pictures of TV is to get the external light down to force a slow shutter speed.

[Flux]

Hugh's 4 ft prop cuts in about 300 rpm and when sensibly loaded with a matching alternator it is very quiet and peaceful. The same speed on a poor pvc rotor may be frantic and noisy.

Air X is bigger than that and is not even near cut in at 300 rpm. Blades vary so much that it is difficult to give general advice. Build decent blades and 300 rpm will be fine. Much slower will seriously restrict your power output.

Flux