I just repaired a rather cheap and nasty petrol (gas) generator for a family member.  It's a small 2-stroke engine with a small 850Watt direct drive alternator, a mechanical governor on the engine and absolutely no electronic voltage regulation.  Interestingly, the generator has no electrical connection to the rotor, e.g. slip rings.  Instead the rotor has a single (but multi-turn) coil, the ends of which are connected by a couple of paralleled 1N540x rectifiers.  The stator appears to have two windings, one connected to the output socket and one that provides the field and is simply connected to the output winding via a current limiting capacitor.  I can kind of see how it works, I guess it must rely on residual magnetism to start up though.  I can't imagine efficiency is particularly good though.

I managed to get it working (the rectifiers were toast), but I was interested to know if this is a common or well known alternator configuration?  Does it have a name?  Google isn't telling me anything, though I'm probably not asking the right questions.

FWIW Voltage (and speed) regualtion is predictably poor, though not quite as bad as I thought it would be (20-30volts from zero to full load, and about 5-8Hz change in frequency, nominal 240V 50Hz).  A large DIY chain store were selling these recently for just 99UKP which would explain it's rather minimal design.

Regards

Mike
 

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