Rules and regulation
Ed Hamley sent an update on his Triumph Thunderbird charging problems. He checked the alternator output as I recommended last month and found the output of the original alternator was 20v at low revs, rising to over 40v with a handful of throttle, so clearly that wasn’t the trouble. The problem turned out to be the regulator/rectifier. Ed borrowed one from another bike and the charging was restored... for a while; then this reg/rec failed. ‘Could it be that these reg/recs can’t handle that extra voltage?’ Ed asks.
Well, I’m not sure what the maximum current from an alternator is, but I don’t think any fault can cause it to exceed original specification because it has a permanent magnet and the only way to vary output is to alter engine speed. Some Japanese alternators (and dynamos) use an electromagnet, enabling the output to be varied by altering the strength of the magnetic field, in fact the old three-brush dynamos used to pick up their field voltage direct from the dynamo output, so without a battery in circuit to offer resistance, output increased field-increasing output until the thing burned out.
I wonder if the regulators got too hot; they regulate by dumping excess current to earth which generates heat, so they need to be in the airflow – not hidden inside a tool box, possibly cuddled up against rags or a tool roll. This can be especially tricky if the bike has a magneto for ignition, meaning that the alternator output is only needed when the lights are on.
The only other thing I have noticed about regulators is that most are instantly destroyed by connecting up to the wrong earth polarity.