Mike,
All MMMs are negative earth. Not like '50 - '60 British car.
So you can use modern CD Player in your L type!
Good luck,
Hiro
>
> From: "Mike Hughes" <hughes.c.m@worldnet.att.net>
> Date: 2002/10/31 木 午前 10:40:56 GMT+09:00
> To: "Pat Mullen" <pmullen@telus.net>
> CC: <mg-mmm@autox.team.net>
> Subject: Re: 3 brush dynamo
>
> Did I read that right? Our MMM cars are NEGATIVE earth?
This is important
> for me to know as I am in the midst of doing the wiring on my
L-1 Salonette
> and don't really care to be frying things!
>
> - Mike Hughes -t?t-
>
> ----- Original Message -----
> From: Pat Mullen <pmullen@telus.net>
> To: <hiro@octagongarage.com>
> Cc: <mg-mmm@autox.team.net>
> Sent: Wednesday, October 30, 2002 4:02 PM
> Subject: Re: 3 brush dynamo
>
>
> > 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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