Does anyone know why MG and others changed their polarity to +ve earth in
the mid '30s? - and then back again in the '60s?
Cliff
----- Original Message -----
From: <hiro@octagongarage.com>
To: "Mike Hughes" <hughes.c.m@worldnet.att.net>; "Pat Mullen"
<pmullen@telus.net>
Cc: <mg-mmm@autox.team.net>
Sent: 31 October 2002 04:19
Subject: MMMs are Negative Earth
> 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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