Dear Pat,
Thank you for your great advise. I feel I basically understand
the mechanism of three brush dynamo, despite my poor English.
Actually I built my engine by myself and have confident of its
performance, so enjoy driving MMMs every weekend to cover at
least 5000miles per year. Good use in far east country, right?
(Remember we have terribly hot and humid Summer for
4-5months).
Anyway, I still don't like electric! It is invisible!
Best regards,
Hiro
>
> From: Pat Mullen <pmullen@telus.net>
> Date: 2002/10/31 木 午前 06:02:42 GMT+09:00
> To: hiro@octagongarage.com
> CC: mg-mmm@autox.team.net
> 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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