Michael A. Terrell wrote:
John Popelish wrote:
RW Salnick wrote:
(snip) You won't need to make any changes to the pulley to
make the
alternator deliver 24 V, but the the diodes and the diode trio will need
to be replaced with 24V versions.
(snip)

Are you seriously suggesting that a 12 volt alternator is
made with a diode trio that cannot handle 24 volts? I was
under the impression that few silicon rectifiers are made
with a breakdown voltage less than 50 volts.


What PIV diodes do you think you need for 24 volts?

Need? 35 volts should be enough to work, but I would like
at least 100 volt.

What PIV diodes do you think they put in the trio?

John Popelish wrote:

Michael A. Terrell wrote:
John Popelish wrote:
RW Salnick wrote:
(snip) You won't need to make any changes to the pulley to
make the
alternator deliver 24 V, but the the diodes and the diode trio will need
to be replaced with 24V versions.
(snip)

Are you seriously suggesting that a 12 volt alternator is
made with a diode trio that cannot handle 24 volts? I was
under the impression that few silicon rectifiers are made
with a breakdown voltage less than 50 volts.


What PIV diodes do you think you need for 24 volts?

Need? 35 volts should be enough to work, but I would like
at least 100 volt.


You are forgetting that when the diode is off, it also has the
battery voltage added to the AC from the alternator, so the bare
minimum would be 24 VDC +(24*1.414=33.936-.6) 33.336, or 57.336 PIV, if
there are no spikes or inductive surges. A load dump can be several
hundred volts, and the diodes have to survive.


What PIV diodes do you think they put in the trio?


The last set I replaced were unmarked. All they had was an OEM part
number. One of the trios opened on the way home from Orlando after
midnight one night. I was laying in wet grass and sand removing the high
output alternator from my stepvan, only to find it was too big to fit my
car. I had to strip both alternators and transplant the diodes to get to
work on time.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Michael A. Terrell wrote:
John Popelish wrote:
Michael A. Terrell wrote:
John Popelish wrote:
RW Salnick wrote:
(snip) You won't need to make any changes to the pulley to
make the
alternator deliver 24 V, but the the diodes and the diode trio will need
to be replaced with 24V versions.
(snip)

Are you seriously suggesting that a 12 volt alternator is
made with a diode trio that cannot handle 24 volts? I was
under the impression that few silicon rectifiers are made
with a breakdown voltage less than 50 volts.

What PIV diodes do you think you need for 24 volts?
Need? 35 volts should be enough to work, but I would like
at least 100 volt.


You are forgetting that when the diode is off, it also has the
battery voltage added to the AC from the alternator, so the bare
minimum would be 24 VDC +(24*1.414=33.936-.6) 33.336, or 57.336 PIV, if
there are no spikes or inductive surges.

I'm not forgetting anything, that is not how it works.
there is a 3 phase bridge rectifier across the AC with the
negative side of the output to ground (so no AC swings can
go more negative than a diode drop below ground). There are
two positive sides to this bridge, connected in parallel to
the 3 phase AC. One set of 3 provides the DC output to the
battery, and the other 3 diodes provide the DC to the field
excitation regulator. But, since the 3 to the battery
prevent the AC from exceeding the battery voltage by more
than a diode drop, None of these 9 diodes sees more reverse
voltage than the battery plus a diode drop.

A load dump can be several
hundred volts, and the diodes have to survive.

This, I agree with, and this is why the main rectifiers as
well as the trio diodes all have PIV ratings in the hundreds
of volts.

What PIV diodes do you think they put in the trio?


The last set I replaced were unmarked. All they had was an OEM part
number. One of the trios opened on the way home from Orlando after
midnight one night. I was laying in wet grass and sand removing the high
output alternator from my stepvan, only to find it was too big to fit my
car. I had to strip both alternators and transplant the diodes to get to
work on time.

What does this failure tell us about the PIV rating of the
trio? Not much, I think. You don't know that thew trio
failed from excessive voltage. Could have been a bond
failure that overheated one of them.

John Popelish wrote:

What does this failure tell us about the PIV rating of the
trio? Not much, I think. You don't know that thew trio
failed from excessive voltage. Could have been a bond
failure that overheated one of them.


All three diodes were open.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Michael A. Terrell wrote:
John Popelish wrote:
What does this failure tell us about the PIV rating of the
trio? Not much, I think. You don't know that thew trio
failed from excessive voltage. Could have been a bond
failure that overheated one of them.


All three diodes were open.

So what do you conclude caused this failure, if anything?

John Popelish wrote:

Michael A. Terrell wrote:
John Popelish wrote:
What does this failure tell us about the PIV rating of the
trio? Not much, I think. You don't know that thew trio
failed from excessive voltage. Could have been a bond
failure that overheated one of them.


All three diodes were open.

So what do you conclude caused this failure, if anything?


I have no idea. All I found was four lugs and burnt plastic ash.
There was nothing left to do failure analysis on.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Michael A. Terrell wrote:
John Popelish wrote:
Michael A. Terrell wrote:
John Popelish wrote:
What does this failure tell us about the PIV rating of the
trio? Not much, I think. You don't know that thew trio
failed from excessive voltage. Could have been a bond
failure that overheated one of them.

All three diodes were open.
So what do you conclude caused this failure, if anything?


I have no idea. All I found was four lugs and burnt plastic ash.
There was nothing left to do failure analysis on.

I would suspect an intermittent short between the rectifier
output and ground. But, I guess, if a high enough voltage
transient occurred, it could have shorted a rectifier and
then that short blew the other two. Of a diode bond failed
enough to overheat one die, causing the same cascade.

But do you agree with my analysis of the normal voltage
applied to these diodes? You clipped it without comment.

Thanks to everyone who's contributed ideas to this thread.

It's my understanding of alternator operation that the voltage is
"built-in" by the number of poles, the number of turns on the rotor/stator,
and least of all by the regulator.
The speed controls current, not voltage.

If the regulator is removed (or the reg.'s ability to reduce the field
current is removed), then the alternator voltage will rise to over 100 V.
(OK for US -
you could get mains power straight from output - either AC or DC, at a high
frequency,
but I need 240VAC in Oz.) I believe a large alternator [200A] might produce
around 7200 Watts when driven this way. It would probably need 2 belts and
12 HP to drive it at this level, assuming bearings, heat, diodes, etc. were
upgraded.

What I want to do is charge a bank of solar batteries at up to a C20 rate
(about 40A in my case), with a 24V alternator driven by a 3- 6HP stationary
engine, with an adjustable regulator circuit that will provide up to 30VDC
and taper the charge as the batteries fill.

Great booklet about alternators :
http://www.1stconnect.com/anozira/Si...alternator.htm

cheers all,

Marcus in Oz @ 28.4VDC (Sunny day!)

On Tue, 24 Apr 2007 15:38:59 +1000, "marcus" wrote:

Thanks to everyone who's contributed ideas to this thread.

It's my understanding of alternator operation that the voltage is
"built-in" by the number of poles, the number of turns on the rotor/stator,
and least of all by the regulator.
The speed controls current, not voltage.

Speed controls voltage, not current - wire size determines current. I
think it was Homer that suggested you try to regulate engine speed for
maximum efficiency from a gas engine.

If the regulator is removed (or the reg.'s ability to reduce the field
current is removed), then the alternator voltage will rise to over 100 V.
(OK for US -
you could get mains power straight from output - either AC or DC, at a high
frequency,
but I need 240VAC in Oz.) I believe a large alternator [200A] might produce
around 7200 Watts when driven this way. It would probably need 2 belts and
12 HP to drive it at this level, assuming bearings, heat, diodes, etc. were
upgraded.

What I want to do is charge a bank of solar batteries at up to a C20 rate
(about 40A in my case), with a 24V alternator driven by a 3- 6HP stationary
engine, with an adjustable regulator circuit that will provide up to 30VDC
and taper the charge as the batteries fill.

Great booklet about alternators :
http://www.1stconnect.com/anozira/Si...alternator.htm

cheers all,

Marcus in Oz @ 28.4VDC (Sunny day!)




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On Mon, 23 Apr 2007 23:10:35 GMT, "Carl Ijames"
wrote:

On these alternators the dash volts light is a 194 and it
goes from battery to the L pin on the 4 pin connector, to provide both
sensing input and idiot light. The internal resistance of this bulb is
crucial, you will blow things up if it is shorted and make things very
unhappy if it is open.

The idiot light supplies the current to magnetize the field until the
alternator is producing power. The light goes from the positive of
the ignition circuit to the field. There's an additional set of three
positive diodes on the alternator that also go to the field - when the
alternator is producing its own power, the light bulb has positive
voltage on both terminals and doesn't light.

Good design to use the alternator itself to supply the field through
isolation diodes - a shorted rotor won't output enough current to
destroy the regulator that way.

Short the lamp and you may destroy the regulator or rotor or both -
too high a lamp resistance and it will change the point where the
alternator begins charging or keep it from charging.

The lamp serves two purposes - tells you the alternator is working and
supplies current to the field to start the alternator working.
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