Running engine with alternator not connected electrically

[VicS]

Right chaps,

The only odd thing is that if I switch off the feed to the alternator, the voltage stays at 14.6. Does this suggest a degree of self-excitation after all? (Again, this is only for short periods of 10 seconds or so and under no load).

Yes once the alternator is generating it supplies its own field current, usually via the field diode trio.

 

[lw395]

Right chaps,

Just an update on this. It all seems to work now (at least under no load). I'm using the boat battery as it's winter and I have it lying around. Before I start the engine, the battery voltage is about 12.7. I switch on the little illuminated switch that puts a positive feed to the alternator via it's lamp, and pull the string. The engine fires up, the lamp goes out and the voltage goes up to about 14.6. That's as much as I can do until I finish paying for Christmas and can afford the inverter! The only odd thing is that if I switch off the feed to the alternator, the voltage stays at 14.6. Does this suggest a degree of self-excitation after all? (Again, this is only for short periods of 10 seconds or so and under no load).

Yes, the alternator will keep charging once it's been started.
Also, it can be deceptive reading the voltage on a battery that's just been charged, it will store a bit of charge like a capacitor, sometimes it can take a long time to come down to its 'resting' voltage if there is zero load on it, just a high impedance meter.

 

[Beyondhelp]

No, it would be to see how the belt runs between the engine an alternator, and whether the tensioner is all running true, as well as whether I can pull start it easily enough when it's also turning the belt and alternator. (This is me trying to build a generator, it's not actually a boat engine). I've pretty much made the decision to at least connect the alternator to the battery (perhaps without any field coil excitation) just to try all that though. It won't take long to just run those wires.

To move on from what has been repeated 100x already, since you have the luxury to have a play here, it would not be hard to modify the alternator to put a switch in line with the field windings, the current here is quite low in the order of a couple of amps. Then if you want to run with no load will be totally fine.

Have a play with putting field voltage on and off, it makes quite a difference to how hard the alternator is to turn - even with no load attached. Projects like yours are fun.
My dad helped me build one when I was about 12 years old! We used an old Atco lawnmower engine, probably about 50-100cc and it was entirely inadequate to drive a small alternator (no idea at all what that was from) You could run the engine at max speed. Connect the field winding and it would stall the motor in moments. In the end I used a long wire to the battery to drop the current. Sadly I had no idea what kind of charge current I got, and those old engines were really noisy!

Oh and I could pull start this, so you should be fine!