||We use two different types of magnetiser for
charging the magnets of rotating-coil magnetos: an internal magnetiser and
an external one. The internal magnetiser can be used for most jobs, and we
prefer it because it is so efficient, effective and quick and easy to use.
||The internal magnetiser is based on a design
by Allan Hurst which was published many years ago in the Velocette Owners'
Club's Fishtail magazine. Our version of it has a cylindrical core
containing an iron cross-piece around which several hundred turns of
lacquered copper wire are wound. The cross-piece and winding are
encapsulated in car body-filler between two aluminium end pieces. The ends
of the winding are connected to terminals mounted on one of the end pieces.
In use, the core is inserted into the magneto body so that the iron
cross-piece extends between the magnetic pole pieces inside the magneto
body. We have two cores of different diameters (just under 2" and just
which fit very snugly into the majority of magneto bodies.
||The core is plugged into a mains-powered box
containing a large capacitor, a rectifier circuit, a voltmeter, a
push-button switch and some ancillary bits and pieces. When the button is
pressed, the capacitor is charged from the mains through the rectifier to a
voltage shown on the voltmeter which levels out at just under 340 V. When
the button is subsequently released, the capacitor is discharged through the
core winding. That causes a very strong magnetic field to flow in a loop
through the core's cross-piece, one of the magneto's pole pieces, the
magneto's magnet, the other pole piece and back to the cross-piece. The
magnetic field is a pulse of very short duration while the
capacitor discharges. When 'topping-up' a magnet only one pulse is required.
If charging a magnet that is completely demagnetised, or when flipping the
polarity of a magnet, we usually use two or three pulses.
|If a magneto's magnet is driven fully into
saturation so that it achieves its full strength, its strength will drop,
soon after it is put to use in the magneto, to a maximum operating level.
Most Lucas magnetos are designed so that if the magnet does not have some
sort of keeper (such as the magneto armature), its magnet strength does not
fall significantly below the maximum operating level. So, for these Lucas
magnetos, after we have charged the magnet, we can simply remove the magneto
body from the internal magnetiser's core, and then fit the magneto's
||With BTH magnetos, on the other hand, to prevent a significant
drop in the magnetic operating level below the maximum, the magneto does
require some sort of keeper to be in place all the time between charging the
magnet and getting the armature back inside the magneto. Conveniently, the
magnetic pole pieces of BTH magnetos are directly accessible from the
outside. Therefore, for BTH magnetos, we fit an external keeper to the
magneto while we are removing the internal magnetiser's core and replacing
it with the magneto's armature. The external keeper is conveniently and
simply provided by the large vice in the workshop.
||With all credit to Allan Hurst (assuming he
was the first to conceive of the 'internal' magnetiser for rotating-coil
magnetos), its great advantages are: (1) you can get direct contact (bar a
few thou) between the magnetiser and the pole pieces of the magneto, (2) the
magnetic path length through the magnetiser is very short (a couple of
inches or so), and (3) as a result, the amp-turns of magnetising force that
are necessary to drive the magnet fully into saturation are relatively
||The external magnetiser in our workshop has a frame and
coils of conventional design. In the photo above, it is shown with an RB M1
magneto that has an armature diameter of just under 2", slightly too small
for our smaller internal magnetiser core.
|By comparison with the internal magnetiser,
the magnetic path length through the external magnetiser externally of the
magneto from one side of the magneto to the other is many times longer.
Also, with some designs of magneto (such as the Lucas K1F, K2F, KVF, MO1 and
MN2) the magnetic poles pieces of the magneto are not directly accessible
externally. With the K2FC shown above, the magnetic flux needs to pass
through a total of 1/4" of aluminium (equivalent to an air gap of 1/4") to
get to the pole pieces; with an MO1, the aluminium/air gap is more like
1/2". All of this means that a far greater magnetising force is needed to
achieve the required magnetic flux density at the magnet. Moreover, the
sides of magnetos like the Lucas K2F are convex, and so the magnetiser needs
concave jaws in order to avoid more air gap. The dynamo saddle of the MO1 is
concave, and so the magnetiser needs a convex jaw in order to fit it
properly. Nowadays we only use the external magnetiser if we can't use the
Magneto/dynamo test rig
Pick-up and slip ring tester
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