Automotive alternator windings & rectifier

[Spehro Pefhany]

On Wed, 21 Aug 2013 02:15:27 +0100, the renowned Mike Perkins
<spam@spam.com> wrote:

On 21/08/2013 00:47, Cydrome Leader wrote:
In sci.electronics.equipment Mike Perkins <spam@spam.com> wrote:
On 20/08/2013 08:04, Mike Cook wrote:
Not the alternators I've taken apart. The star point is typically
a crimp, and serves no other useful purpose.

The field, and regulator power, are normally taken from an
additional set of diodes, usually 3, ie one per phase, such that
the regulator is effectively isolated from the battery when the
engine is not turning.

But THAT'S why I'm asking here: this Denso alternator (used in MANY
Hondas, Toyotas, and myriad other makes) does have a common terminal
connection to the rectifier. The rectifier has EIGHT diodes.

Yes I agree that common configuration is 6 diodes. That's why I'm
confused. The system is quite populous and apparently functions. I
just don't understand how.

I can find very little on the web regarding neutral point rectification
for car (Lundell) alternators, but there is a lot about 3rd harmonic
neutral currents. An example how these manifest in electrical systems
for non-linear loads.

http://static.schneider-electric.us/docs/Electrical%20Distribution/Low%20Voltage%20Transformers/Harmonic%20Mitigating/0104ED9501R896.pdf

A conventional automotive alternator uses six diodes to rectify
three-phase AC (Alternating Current) into DC (Direct Current). The
average voltage of the neutral point is 1/2 of the output DC voltage.
While a low output current flows, the voltage at the neutral point is
mostly DC, but it also includes an AC portion. As current output
increases, the AC portion increases.

When the alternator speed exceeds 2,000 to 3,000 rpm, the peak value of
this AC portion exceeds the DC output voltage.

This means that, compared with the output characteristics of the
alternator without neutral-point diodes, the output gradually increases
from midway by 10 to 15% at a normal rated alternator speed of approx.
5,000 rpm.

http://youronlinemechanic.com/alternator-with-neutral-point-voltage/

Here's a diagram of what the above link failed to clearly explain

http://autonopedia.org/renewable-energy/generators/alternator-secrets/

so somehow tapping neutral is good. I guess those two diodes bypass
current flowing int coils the wrong way or something? As to how speed
matters, I'm not following that.


If you consider that at any time 2 windings will be supplying one
polarity of current and the remaining winding the other polarity, you
can see the neutral point is going to move in the direction dictated by
the two windings. At higher currents, and higher frequencies, the
inductance of the winding causes an increasing voltage to appear at the
neutral point.

Some more info:-

http://tinyurl.com/kdcsayk

http://ece.ubm.ro/cjece/web/CJECE_VOL5_2012/3_Barz.pdf



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Don Kelly]

On 20/08/2013 8:33 PM, Spehro Pefhany wrote:

On Wed, 21 Aug 2013 02:15:27 +0100, the renowned Mike Perkins
spam@spam.com> wrote:

On 21/08/2013 00:47, Cydrome Leader wrote:
In sci.electronics.equipment Mike Perkins <spam@spam.com> wrote:
On 20/08/2013 08:04, Mike Cook wrote:
Not the alternators I've taken apart. The star point is typically
a crimp, and serves no other useful purpose.

The field, and regulator power, are normally taken from an
additional set of diodes, usually 3, ie one per phase, such that
the regulator is effectively isolated from the battery when the
engine is not turning.

But THAT'S why I'm asking here: this Denso alternator (used in MANY
Hondas, Toyotas, and myriad other makes) does have a common terminal
connection to the rectifier. The rectifier has EIGHT diodes.

Yes I agree that common configuration is 6 diodes. That's why I'm
confused. The system is quite populous and apparently functions. I
just don't understand how.

I can find very little on the web regarding neutral point rectification
for car (Lundell) alternators, but there is a lot about 3rd harmonic
neutral currents. An example how these manifest in electrical systems
for non-linear loads.

http://static.schneider-electric.us/docs/Electrical%20Distribution/Low%20Voltage%20Transformers/Harmonic%20Mitigating/0104ED9501R896.pdf

A conventional automotive alternator uses six diodes to rectify
three-phase AC (Alternating Current) into DC (Direct Current). The
average voltage of the neutral point is 1/2 of the output DC voltage.
While a low output current flows, the voltage at the neutral point is
mostly DC, but it also includes an AC portion. As current output
increases, the AC portion increases.

When the alternator speed exceeds 2,000 to 3,000 rpm, the peak value of
this AC portion exceeds the DC output voltage.

This means that, compared with the output characteristics of the
alternator without neutral-point diodes, the output gradually increases
from midway by 10 to 15% at a normal rated alternator speed of approx.
5,000 rpm.

http://youronlinemechanic.com/alternator-with-neutral-point-voltage/

Here's a diagram of what the above link failed to clearly explain

http://autonopedia.org/renewable-energy/generators/alternator-secrets/

so somehow tapping neutral is good. I guess those two diodes bypass
current flowing int coils the wrong way or something? As to how speed
matters, I'm not following that.


If you consider that at any time 2 windings will be supplying one
polarity of current and the remaining winding the other polarity, you
can see the neutral point is going to move in the direction dictated by
the two windings. At higher currents, and higher frequencies, the
inductance of the winding causes an increasing voltage to appear at the
neutral point.

Some more info:-

http://tinyurl.com/kdcsayk

http://ece.ubm.ro/cjece/web/CJECE_VOL5_2012/3_Barz.pdf



Best regards,
Spehro Pefhany

Thank you for the information- also thanks to Mike Perkins-The
references were useful my background is in power systems so I have a
different view on 3rd harmonics.

Don Kelly
remove the cross to reply

 

[Paul Hovnanian P.E.]

Mike Cook wrote:

http://www.autoshop101.com/forms/alt_bwoh.pdf

has photos of an alternator with 4 connections for windings (each of 3
phases + common), and the 8 diode rectifier. But wiring diagrams show only
3 connections and 6 diodes.

You are referring to the diagram on page 3 of the above document?

Those are not the individual windings brought out of the alternator. The 6
winding leads are connected internally through the diode bridge and brought
out as two connections to the outside world. The B+ terminal and the
alternator frame ground. This is shown on pages 23 and 24.

Where does the common winding terminal connect (there is a connection on
the rectifier plate)? Where do the other 2 diodes connect?

What isn't made clear in this document are the connections between the
windings, field and internal regulator. The regulator connections are
brought out as the IG, S, and L terminals (some alternators may have more
or fewer of these connections).

I handled one of these disassembled units but didn't have time to ohm out
the connections.

Thanks.

--
Paul Hovnanian mailtoaul@Hovnanian.com
------------------------------------------------------------------
Failure is not an option. It's standard equipment on this model.