Clifford
I understand that during WWII it was found that airplane skin corrosion was
reduced if the battery polarity was positive ground. therefore auto were
changed to positive ground.
Jerry
Clifford Knight wrote:
> 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
> > > > >
> > > > > /// unsubscribe/change address requests to
> > majordomo@autox.team.net or
> > > try
> > > > > /// http://www.team.net/cgi-bin/majorcool
> > > > > /// http://www.team.net/mailman/listinfo
> > > > > /// Archives at http://www.team.net/archive/mg-mmm
> > > > > /// Send list postings to mg-mmm@autox.team.net
> > > >
> > > > /// unsubscribe/change address requests to majordomo@autox.
> > team.net or
> > > try
> > > > /// http://www.team.net/cgi-bin/majorcool
> > > > /// http://www.team.net/mailman/listinfo
> > > > /// Archives at http://www.team.net/archive/mg-mmm
> > > > /// Send list postings to mg-mmm@autox.team.net
> > >
> > > /// unsubscribe/change address requests to majordomo@autox.
> > team.net or try
> > > /// http://www.team.net/cgi-bin/majorcool
> > > /// http://www.team.net/mailman/listinfo
> > > /// Archives at http://www.team.net/archive/mg-mmm
> > > /// Send list postings to mg-mmm@autox.team.net
> >
> > /// unsubscribe/change address requests to majordomo@autox.team.net or
> try
> > /// http://www.team.net/cgi-bin/majorcool
> > /// http://www.team.net/mailman/listinfo
> > /// Archives at http://www.team.net/archive/mg-mmm
> > /// Send list postings to mg-mmm@autox.team.net
>
> /// unsubscribe/change address requests to majordomo@autox.team.net or try
> /// http://www.team.net/cgi-bin/majorcool
> /// http://www.team.net/mailman/listinfo
> /// Archives at http://www.team.net/archive/mg-mmm
> /// Send list postings to mg-mmm@autox.team.net
/// unsubscribe/change address requests to majordomo@autox.team.net or try
/// http://www.team.net/cgi-bin/majorcool
/// http://www.team.net/mailman/listinfo
/// Archives at http://www.team.net/archive/mg-mmm
/// Send list postings to mg-mmm@autox.team.net
|