"Fred Abse" wrote in message
news
On Thu, 08 Dec 2011 18:55:41 +0000, Ian Field wrote:


To my mind the inevitable clearance gap between the armature poles and
the
magnet/field winding poles must make some difference to a transformer
with
its alternating magnetic field in a closed core.

It will have the effect of increasing the reluctance, hence reducing the
flux in the core, hence the EMF per rad sec ^-1.

I recall, on a trip to the Hoover Dam power station, being told that by
reducing the alternator air gaps from (IIRC) 3/4" to 3/8", facilitated by
better bearings, they had got another 20% capacity.


Regarding the Sturmey Archer 'dynamo', the magnet is pretty crappy which
probably contributes to even more difference.

For 1936 magnet technology, it was pretty good.

My googling efforts have turned up some snippets of info, such as the SA
'dynamo' magnet demagnetises almost instantly if the armature is removed
without shuffling in a keeper (guess how I already knew this!) and that the
magnet also gradually loses its strength over time even if left undisturbed.

The wheel I found at the back of the garage had a 1975 stamped hub which
showed fairly strong cogging when rotated, whether this means they adopted
beter magnet alloys as they became available I don't know.

It keeps the 1.2mAh SLA for my front & rear LED lighting set charged OK.