Hi Hiro,
How good to hear of a fellow MMM enthusiast who REALLY drives his car!
(I used to own a 1933 J2 and then a 1934 J2 in 1952-54 when I was a
student in England - I am now a Canadian with a 1936 NB nearing the end
of a complete restoration).
For some 30 years, between about 1936 to 1965 prior to the advent of
alternators, and after the MMM cars had finished production, British
cars controlled the VOLTAGE output from their dynamos with a
"Compensated Voltage Control" (CVC) device. This measured the dynamo
output voltage; the CVC control mechanism was placed in series with the
dynamo field coil, between the dynamo + and - terminals. There was no
"third brush". It worked quite well.
Prior to this, in the days of our MMM cars in the early 1930s and
before, the CURRENT was controlled by the third brush method in such a
way that changes in engine speed made comparatively little change to the
current supplied from the dynamo.
The only ways to control the current on an MMM car were by
(a) moving the third brush while doing maintenance on the car
(b) switching resistance and or out of the field coil circuit, usually
using the lighting switch.
Here is an explanation of how method (a) worked on a negative ground
("earth") car like our MMMs:
1. On ANY dynamo, third brush or CVC, there is a magnetic field whose
lines of force pass from one field coil to another, because one field
coil is wound to produce a North pole on its inside face and the other
field coil is wound to produce a South pole on ITS inside face. Such a
dynamo at rest (but with the field coils' slight magnetism remaining
from the last time they were energised) has straight lines of magnetic
force.
(The same principle applies to dynamos with four field coils - like my
NB's dynamo. I'm unsure if your J2 has two or four field coils)
2. As the dynamo starts to revolve, it generates voltage; as this
voltage increases, current flows in the armature wires; this in turn
creates another, revolving, magnetic field which distorts the field
produced by the field coils.
3. This distortion does not seriously affect the output of a dynamo
designed to work with CVC, because the CVC is in control.
4. This type of distortion can be made to control the dynamo CURRENT
output if it can be made to REDUCE THE STRENGTH OF THE EFFECTIVE
MAGNETIC FIELD in the same proportion as the SPEED OF ARMATURE ROTATION
INCREASES.
5. This can be accomplished by connecting one end of the field coil
circuit to the positive output brush (as with the CVC dynamo) and the
other end to a third, movable brush connected near the negative output
brush in such a manner that any one segment of the armature passes the
third brush just before it passes the negative output brush.
6. As you might expect, the nearer together the two brushes are, the
higher the current output of the dynamo.
7. Note that the third brush method of control is much better than NO
control but it is inferior to CVC.
This is because whether the (battery plus lights etc) needs more or
less
current to maintain the correct 13.5 volts, the dynamo doesn't know or
care.
If the dynamo is set to produce about 8 amps at engine speeds above
about 1500rpm, and your ignition coil takes 2 of these amps, AND you
have no lights or any other electrical load, then the dynamo forces the
remaining 6 amps through the ammeter and into your battery.
This is bad if you drive 1000km at 100kmph because your battery gets
overcharged, hence overheated, and may well get bent plates inside as
well as boiling off of much of the acid.
It is also bad if you set the third brush to produce about 4 amps AND
you have your side & headlights on, thus taking 8 amps in addition to
the ignition coil's 2 amps. You will then be discharging the battery at
about 6 amps - let's hope you don't get caught in heavy traffic!
I haven't tried this, but I imagine that if you install a voltmeter in
the car, and switch resistance in and out of the field coil circuit so
as to keep the voltmeter indicating as near 13.5 volts as possible, you
would have most of the advantages of a CVC dynamo, (but with one serious
disadvantage if you are forgetful like me).
Hope this helps.
Cheers ...... Pat Mullen (1936 MG 2-seater NA0895)
hiro@octagongarage.com wrote:
>
> Hi,
>
> We had largest MG event in Japan on Saturday 26th, and I
> participated with my J2. The return trip was just 1000km
> (625miles), my J2 happyly cruised around 60mph - just one thing
> I noticed, maybe oil pump relief valve was sticking since
> pressure when cold become over 160LB!
>
> On the way to the site the third brush of my J2 dymano was
> broken up in pieces (as you know it is very thin), then I had
> to purchase a spare battery.
>
> Does someone explain me what is the mechanism of this type of
> dynamo?
>
> Hiro
> Kobe-Japan
>
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