Hi Greg,

On 15-Oct-03, Greg Mayman wrote to George White:



 GW>> It's definitely not the diodes. It's to do with the internal
 GW>> design. However, it is one of the factors that influenced the
 GW>> change to alternators. The main factors were:

 GW>> a) Higher output in the same frame size
 GW>> b) Simpler regulators
 GW>> c) Wider working speed range.

 GM> I think a) and c) were mainly due to the smaller pulley size on
 GM> alternators that makes them turn at about twice as fact as DC
 GM> generators for the same engine speed.

No.
a) was due to the better magnetic efficiency and smaller brushgear.

 GM> They couldn't do that with generators because of the greatly
 GM> increased wear of the commutator and its brushes with increased
 GM> speed.

c) was indeed due to the use of sliprings instead of the commutator
allowing higher max rpm, however the inherent balance of the axial
field windings and the cast poles allowed better balancing of the
rotor by just drilling a few holes in the pole castings which allowed
higher speed still. This then allowed the higher pully ratio which
gives better low speed (engine idle) output.

 GM> But the main advantage of alternators over DC generators is the
 GM> stationary generating windings, because it eliminates the need for
 GM> making a heavy current connection in or out of the rotor.

That problem, the commutator. Which, of course, is why a dynamo is
such a rarity these days, sold state rectifiers on alternators are so
much better all round...

 GM> I remember this from my studies of nearly 50 years ago.

:-)

 GW>> In a dynamo the field is static, and only two pole, and the
 GW>> output is rectified by the commutator off the rotor.

 GW>> In an alternator the field rotates, and is usually 6, 8 or 10
 GW>> pole. The output is a 3 phase static winding, can be either star
 GW>> or delta wound (usually delta, from memory) that feeds the
 GW>> rectifier bridge.

 GM> I would have expected star winding to be used as it can have a
 GM> lesser number of turns. OTOH the slightly thinner wire used for
 GM> delta winding would make for easier winding of the coils.

I can't think of any designs that were star, all the ones I came
across were 3 phase delta, 6 diode bridge for the main output and a
smaller 3 diode bridge in parallel with half the main bridge for the
field excitation.

                ___________________Main
                Ý       Ý       Ý
                -       -       -
                ^       ^       ^
                Ý-------+-------+---->Ý---Ý
                A       Ý-------+---->Ý---ÝField
                Ý       B       Ý---->Ý---Ý
                Ý       Ý       C
                -       -       -
                ^       ^       ^
                Ý       Ý       Ý
                -------------------Ground

Where A, B and C are the three points of the delta.
I suspect a star winding is better where there is a potential
inbalance between the phases (say the power grid) and a delta winding
where there is balance between the phases (as in a vehicle
alternator).

 GW>> I don't know the electro-magnetic design stuff involved, other
 GW>> engineers did that, but it's to do with the fact the extra poles
 GW>> and different magnetic circuit mean that the field reaches a
 GW>> limiting value (field full on) and with appropriate design the
 GW>> output current reaches a limiting value which can be handled by
 GW>> the stator windings.

 GM> According to one of my reference books, the current flowing in the
 GM> generating winding causes a partial cancellation of the field,
 GM> which automatically limits the output of the device. This is a
 GM> principle that operates in all generators and alternators.

Thats it!

 GM> OTOH, the current level at which this occurs in agenerator might
 GM> well be beyond the level of current that the commutator and
 GM> brushes can handle for any length of time. They are the weakest
 GM> points in a DC generator. So some form of external current
 GM> limiting is usually needed on a DC generator.

True.

George

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