[UK-CONTEST] Portable generators

[Steve Thompson]

Not a great deal to tell, at least within the limits of a 
reflector posting, Dale. A lot of the incidental info. works 
better with a pen and paper for sketches.

It's very instructive to use a low voltage transformer to feed a 
'scope and look at the output waveform from a basic genny, with no 
load, resistive load and high power tx. It's rarely anything like 
a sine wave. Typically the field is fed with a munged version of 
the output voltage, so you get something that's sort of 
triangular, or sin^2ish at best. A capacitor input PSU usually 
puts a 'notch' in the rising edge of the waveform. You'll soon see 
why a 3kVA genny struggles to run a 400W amp.

We had a genny where the field supply circuit failed and 
replacements were a decade or more obsolete so we thought we'd 
experiment. From memory the field needed something like 35V off 
load and 47V at 2-3A at full load so we built a linear PSU style 
dc supply based on a 741 feeding a big darlington. The output was 
monitored from a low voltage transformer, rectified and fed into a 
RC network so the peak voltage was averaged over a few cycles to 
provide the feedback signal. Residual magnetisation made it self 
starting but it could be powered from a battery for start-up if 
needed.

Dive into the business end of the alternator - hopefully you'll 
find some slip rings supplying field current into the rotor. I'm 
told some building site gennys have the entire field system 
embedded within the rotor, in which case you're stuck with it. 
After that it's down to experimentation - you'll probably need 
lower voltage and current than our example, that was for 15kVA. 
Take care, and don't blame me if you end up making smoke rather 
than mains!

Steve

> Hi steve. Could you tell us more about supplying the field from
> an external source? This sounds like a really good fix.
> Incidently I made a high power linear a few years ago with a
> choke input filter in the EHT PSU which performed well on a
> genny so I agree withhg you, the main culprit is the capacitor
> input filter. Dale G3XBY