|
|
Gary,
Here is something you can try. Take the
belt off the alternator and using your hand, spin the alternator pulley, Then
turn on the master and spin the pulley. Then turn on the alternator and spin
the pulley. Tell us again what you think happens when that “3 amps” of field
current hits that alternator.
If you can turn that pulley with your hand
with the alternator on, I apologize profusely! Not because I am wrong, but
because you will have to be one Gorilla!
Bill B
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Gary Casey
Sent: Monday, March 23, 2009 8:44
AM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re:
Alternator (Off topic
This discussion on alternators is interesting. First, I'll have
to disagree with Bill B about the alternator "load" with the engine
not running. The voltage capability of the alternator is proportional to
rpm and at cranking speed it will not be able to deliver battery voltage and
therefore its output will be zero and therefore the power to turn it will be no
more than the friction in the bearings. The field will, of course, be
full on, which will result in a battery drain of about 3 amps. Compared
to the starter draw of maybe 200 amps it is trivial. Some people leave
the alternator off to save that current, but there isn't much incentive to do
that. The only real reason to ever turn the master on without the
alternator is if you don't intend to start the engine (maintenance, setting the
radios, or whatever) - then there will be less draw on the battery.
The second post by Tom is partly true - most voltage regulators are
"switching" regulators in that the field is either turned on or off.
But the battery only partially smoothes the output. The main reason
this works is that the alternator field windings (in the armature) have a very
high inductance and a "free-wheeling" or "clamping" diode
is placed across the field. When the regulator turns off current
continues to flow through the field, driven by the inductance of the field itself.
This is easiest to visualize by drawing the field coil with a diode
connecting the top and bottom (ground terminal). Shut off the supply and
pretend that the coil is a big flywheel and the current doesn't want to stop -
it will flow through the diode from the ground to the top (supply) end, which
is now at a slightly negative voltage.. The alternator output will
decrease gradually (well, in milliseconds) until the system voltage drops below
the threshold set in the regulator, whereupon the regulator will turn full on
again, bumping it back up. Imagine a bicycle wheel that you just have to
hit now and then to keep it turning - or read Jim's explanation. The big
advantage of this is that the regulator is either full on or full off and
therefore will dissipate very little heat. The disadvantage is that
electrical noise will generated by the switching, but that noise is generally
not a problem for any other electrical device. There are some regulators
(B&C for example) that are "linear" and they actually regulate a
continuous current in the field and the manufacturers of these claim that their
system is "quieter." They will, however, draw more continuous
current will run hotter, requiring some sort of cooling fins or something.
Are they worth the extra cost? I doubt it, but I have one that was
acquired as part of the system.
I believe Tom's idea of turning on all the electrical loads in a car to
test the battery is that the alternator is typically incapable of supplying
that load at idle speed and therefore the battery voltage will drop enough to
cause the engine electronics to stop working, "killing" the engine.
Problem is that all current automotive systems will continue to work down
to less than 6 volts. The battery would have to be virtually gone to drop
that low. Gone far enough that it probably couldn't crank the engine.
Finally, Mike has a question on alternator "noise."
This is generally caused by the high-frequency pulses of current produced
by the alternator stator windings. All diodes have finite response time
and when "hit" with a sudden pulse there will be a slight delay
before they start conducting. This produces a sharp voltage spike in the
windings and this will propagate throughout the electrical system of the plane.
If you put a scope on the main power line you will see this
characteristic if you look close enough. The voltage spikes will be small
and very short - maybe one microsecond - but they are there. The
repetition rate of these is such that it is in the audible frequency range and
therefore can sometimes be heard in the audio system. (as I recall, a
typical alternator puts out something like 14 pulses per rev and it turns at
about 2.5 times engine speed, which is 583 Hz at 1,000 rpm. The best way
to kill this noise is to use a "feed-thru capacitor" in the
alternator output line. I'll take that back - the BEST way is to buy
components that aren't sensitive to the noise, but how do you know that until
you've already spent the money. Feed-thru caps with the required capacity
are typically metal cylinders about an inch long with a heavy wire going into
each end and a mounting tab on the side. The mouting tab is important -
that's the path that conducts the noise to ground. It's best to mount the
cap directly to the noise source - the alternator - and the next best is to the
engine. Also, you should check all the connections in the high current
path. Maybe run a ground wire from the alternator ground terminal to the
engine (I know, the alternator is already bolted to the engine, but do it anyway).
-----Inline Message Follows-----
This is why you
should not turn your alternator on until after the engine starts. The
alternator loads will almost double the load on the starter. The voltage
regulator will call for maximum output from the alternator while the starter is
cranking. For a 60A alternator this is about 720 watts. Darn near 1
HP! In cars, the alternator is automatically put to sleep during starting
by the ignition switch. In planes, it is up to us.
Bill B
|
The field is either on or off.
The regulator would create too much heat with a linear function, so solution
is just a 'switch' function in the regulator IC.
Battery does the trick in smoothing the
voltage.
George -- simple test is with the engine idling, kick on the lights/fan...
note the rpm drop.
That is the load of the alternator turning on. Newer vehicles will have
additional inputs to the alternator so it knows when the AC is on, or when
you kick WOT (wide open throttle) as it will disconnect the alternator and
A/C off for a brief amount of time. Also a simple battery
test is to (in the car) kick on all loads at idle.... if the engine dies,
battery reserve isn't there and time to replace 'em.
Tom
However, by switching the field on and off at
a fairly fast rate and varying the pulse
width duration of the field current, a linear
approximation can be achieved with a constant voltage as the result, within
the limits of the system of course.
That is in essence how the voltage regulator
achieves its function.
Slightly off topic, but does
anyone have a suggestion for an off the shelf filter to kill/reduce
alternator electrical noise? I have an alternator off of a Geo Metro/Chevy
Sprint with internal regulator. I have a switch on the panel to energize the
field. After the engine is running and I switch on the alternator I get a
noticeable hum in the intercom. This is one of those nagging little issues I
havent gotten around to fixing yet since first flight.
|
Also in a previous thread someone (maybe Al G.?)
mentioned an over-voltage protection device for the alternator output that
would be compatible with the internally regulated alternator. I'd like to
add something like that as well - suggested sources?
----- Original Message -----
Sent: Sunday, March 22,
2009 7:45 AM
Subject: [FlyRotary] Re:
Alternator (Off topic)
You are right, of coarse, about the alternator
field being either on or off.
I may have implied that the field current was a
linear function but I was only trying to
simplify the explanation.
However, by switching the field on and off at
a fairly fast rate and varying the pulse
width duration of the field current, a linear approximation
can be achieved with a constant voltage as the result, within the limits of
the system of course.
That is in essence how the voltage regulator
achieves its function.
As for your "simple test" it may not give
the expected results, as most modern car engines, those that are controlled
by computers, will have the idle speed determined by the computer as well.
So increasing the load at idle, either mechanical or
electrical, will cause the
computer to sense a decrease in RPM and if it goes
below a predetermined value the computer will open the throttle (or idle
speed valve) to increase the RPM to the correct idle speed.
Jim
--- On Sun, 3/22/09, thomas walter <roundrocktom@yahoo.com>
wrote:
From: thomas walter
<roundrocktom@yahoo.com>
Subject: [FlyRotary] Re: Alternator (Off topic)
To: "Rotary motors in aircraft"
<flyrotary@lancaironline.net>
Date: Sunday, March 22, 2009, 10:03 AM
|
Jim,
The field is either on or off.
The regulator would create too much heat with a linear function, so
solution is just a 'switch' function in the regulator IC.
Battery does the trick in smoothing the
voltage.
George -- simple test is with the engine idling, kick on the lights/fan...
note the rpm drop.
That is the load of the alternator turning on. Newer vehicles will have
additional inputs to the alternator so it knows when the AC is on, or when
you kick WOT (wide open throttle) as it will disconnect the alternator and
A/C off for a brief amount of time. Also a simple battery
test is to (in the car) kick on all loads at idle.... if the engine dies,
battery reserve isn't there and time to replace 'em..
Tom
1) Re: Alternator (Off topic)
by James Maher <delta11xd@att.net>
I'm sorry to disagree with you but you are quite
mistaken in your belief about the
The alternator output is mostly a function of load
because of the voltage regulator,
whose function is to keep the voltage constant
with changes in load and RPM.
It performs this function by varying the field
current in the alternator.
If fact this is one of the many advantages of the
alternator over a generator.
The alternator's voltage output is also more or
less constant regardless of engine RPM, also due to the regulator's
function.
You said "The alternator charges the battery
- which in turn runs the accessories".
This is not exactly true. If the battery voltage
is below that of the alternator output
then yes the alternator will function to charge
the battery, however if the battery is fully charged then the alternator
will not be charging the battery but will provide the necessary power to
drive any other loads connected to the system.
So reducing the electrical load on the system will
indeed reduce the mechanical load on
the engine thereby using less overall energy.
Hope this clarifies your misconception.
Don't feel bad, many people do not fully
understand even the most simple of electrical devices and the
alternator/regulator charging system is fairly complex.
Jim
|
|
-----Inline Message Follows-----
With the fuel injected rotaries we are dependent on a good source of power to
run the computer and fuel pumps. I used the information on the
AeroElectric list to design my wiring. http://www.matronics.com/Navigator?AeroElectric-List
They have information on what and where to get over-voltage controllers.
Whey also have several wiring diagrams for "all electric"
aircraft. These include ways to feed the Critical Buss through several different independent paths
so one failure does not turn you into a glider.
Dennis Haverlah
Mike Wills wrote:
Slightly off topic, but does anyone have a suggestion for an
off the shelf filter to kill/reduce alternator electrical noise? I have an
alternator off of a Geo Metro/Chevy Sprint with internal regulator. I have a
switch on the panel to energize the field. After the engine is running and I
switch on the alternator I get a noticeable hum in the intercom. This is one of
those nagging little issues I havent gotten around to fixing yet since first
flight.
Also in a previous thread someone (maybe Al G.?)
mentioned an over-voltage protection device for the alternator output that
would be compatible with the internally regulated alternator. I'd like to
add something like that as well - suggested sources?
----- Original Message -----
Sent: Sunday, March 22,
2009 7:45 AM
Subject: [FlyRotary] Re:
Alternator (Off topic)
You are right, of coarse, about the alternator
field being either on or off.
I may have implied that the field current was a
linear function but I was only trying to
simplify the explanation.
However, by switching the field on and off at
a fairly fast rate and varying the pulse
width duration of the field current, a linear
approximation can be achieved with a constant voltage as the result, within
the limits of the system of course.
That is in essence how the voltage regulator
achieves its function.
As for your "simple test" it may not give
the expected results, as most modern car engines, those that are controlled
by computers, will have the idle speed determined by the computer as well.
So increasing the load at idle, either mechanical or
electrical, will cause the
computer to sense a decrease in RPM and if it goes
below a predetermined value the computer will open the throttle (or idle
speed valve) to increase the RPM to the correct idle speed.
From: thomas walter <roundrocktom@yahoo.com>
Subject: [FlyRotary] Re: Alternator (Off topic)
To: "Rotary motors in aircraft"
<flyrotary@lancaironline.net>
Date: Sunday, March 22, 2009, 10:03 AM
|
Jim,
The field is either on or off.
The regulator would create too much heat with a linear function, so solution
is just a 'switch' function in the regulator IC.
Battery does the trick in smoothing the
voltage.
George -- simple test is with the engine idling, kick on the lights/fan...
note the rpm drop.
That is the load of the alternator turning on. Newer vehicles will have
additional inputs to the alternator so it knows when the AC is on, or when
you kick WOT (wide open throttle) as it will disconnect the alternator and
A/C off for a brief amount of time. Also a simple battery
test is to (in the car) kick on all loads at idle.... if the engine dies,
battery reserve isn't there and time to replace 'em.
Tom
1) Re: Alternator (Off topic)
by James Maher <delta11xd@att.net>
I'm sorry to disagree with you but you are quite
mistaken in your belief about the
The alternator output is mostly a function of load
because of the voltage regulator,
whose function is to keep the voltage constant
with changes in load and RPM.
It performs this function by varying the field
current in the alternator.
If fact this is one of the many advantages of the
alternator over a generator.
The alternator's voltage output is also more or
less constant regardless of engine RPM, also due to the regulator's
function.
You said "The alternator charges the battery
- which in turn runs the accessories".
This is not exactly true. If the battery voltage
is below that of the alternator output
then yes the alternator will function to charge
the battery, however if the battery is fully charged then the alternator
will not be charging the battery but will provide the necessary power to
drive any other loads connected to the system.
So reducing the electrical load on the system will
indeed reduce the mechanical load on
the engine thereby using less overall energy.
Hope this clarifies your misconception.
Don't feel bad, many people do not fully
understand even the most simple of electrical devices and the
alternator/regulator charging system is fairly complex.
Jim
|
|
-----Inline Message Follows-----
Mike;
The first thing to try is a large capacitor at the alternator
between the B terminal (output) and grnd (the case). Try about 10,000
microFarad in an electrolytic type. Be aware of polarity to connect it right.
The over-voltage (crowbar circuit) that I have I got from
Aeroelectric Connection. I don’t think he sells stuff directly anymore
but maybe you can get it from B&C. Connects across the field supply
circuit breaker, and fails to short above something like 17-18 volts (IIRC) –
tripping the breaker.
Don’t know about yours; but normally automotive alternators with
internal regulators feed the current to the field internally directly from the
regulator. The small excitation current required to get things started
comes through the ‘idiot’ light circuit (one of the terminals on the alt)..
Over-voltage protection on that circuit won’t do anything.. On my
Mitsubishi alternator, I took it apart and undid the internal connection.
I learned about this from an article in Contact! years ago (Issue 46,
Sept 1998). Maybe you can find a circuit diagram for your alternator; or maybe
it is made by Mitsubishi or Nippondenso, both of which are covered in the
article. I can provide a copy if that helps.
Best,
Al
-----Original Message-----
From: Rotary
motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Mike Wills
Sent: Sunday, March 22, 2009 7:37
AM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re:
Alternator (Off topic)
Slightly off topic, but does anyone have a suggestion for an
off the shelf filter to kill/reduce alternator electrical noise? I have an
alternator off of a Geo Metro/Chevy Sprint with internal regulator. I have a
switch on the panel to energize the field. After the engine is running and I switch
on the alternator I get a noticeable hum in the intercom. This is one of those
nagging little issues I havent gotten around to fixing yet since first flight.
Also in a previous thread someone (maybe Al G.?)
mentioned an over-voltage protection device for the alternator output that
would be compatible with the internally regulated alternator. I'd like to
add something like that as well - suggested sources?
----- Original Message -----
Sent: Sunday, March 22,
2009 7:45 AM
Subject: [FlyRotary] Re: Alternator
(Off topic)
You are right,
of coarse, about the alternator field being either on or off.
I may have
implied that the field current was a linear function but I was
only trying to
simplify the
explanation.
However, by
switching the field on and off at a fairly fast rate and varying the
pulse
width duration
of the field current, a linear approximation can be achieved with a constant
voltage as the result, within the limits of the system of course.
That is in essence how the voltage regulator
achieves its function.
As for your
"simple test" it may not give the expected results, as most modern
car engines, those that are controlled by computers, will have the idle speed
determined by the computer as well.
So increasing
the load at idle, either mechanical or electrical, will cause the
computer to
sense a decrease in RPM and if it goes below a predetermined value the
computer will open the throttle (or idle speed valve) to increase the RPM to
the correct idle speed.
Jim
--- On Sun, 3/22/09, thomas walter <roundrocktom@yahoo.com>
wrote:
From: thomas
walter <roundrocktom@yahoo.com>
Subject: [FlyRotary] Re: Alternator (Off topic)
To: "Rotary motors in aircraft"
<flyrotary@lancaironline.net>
Date: Sunday, March 22, 2009, 10:03 AM
|
Jim,
The field is either on or off.
The regulator would create too much heat with a linear function, so
solution is just a 'switch' function in the regulator IC.
Battery does the trick in smoothing the
voltage.
George -- simple test is with the engine idling, kick on the lights/fan...
note the rpm drop.
That is the load of the alternator turning on. Newer vehicles will have
additional inputs to the alternator so it knows when the AC is on, or when
you kick WOT (wide open throttle) as it will disconnect the alternator and
A/C off for a brief amount of time. Also a simple battery
test is to (in the car) kick on all loads at idle.... if the engine dies,
battery reserve isn't there and time to replace 'em.
Tom
1) Re: Alternator (Off topic)
by James Maher <delta11xd@att.net>
I'm
sorry to disagree with you but you are quite mistaken in your belief about
the
The
alternator output is mostly a function of load because of the voltage
regulator,
whose
function is to keep the voltage constant with changes in load and RPM.
It
performs this function by varying the field current in the alternator.
If
fact this is one of the many advantages of the alternator over a generator.
The
alternator's voltage output is also more or less constant regardless of
engine RPM, also due to the regulator's function.
You
said "The alternator charges the battery - which in turn runs the
accessories".
This
is not exactly true. If the battery voltage is below that of the alternator
output
then
yes the alternator will function to charge the battery, however if the
battery is fully charged then the alternator will not be charging the
battery but will provide the necessary power to drive any other loads
connected to the system.
So
reducing the electrical load on the system will indeed reduce the
mechanical load on
the
engine thereby using less overall energy.
Hope
this clarifies your misconception.
Don't
feel bad, many people do not fully understand even the most simple of
electrical devices and the alternator/regulator charging system is fairly
complex.
Jim
|
|
-----Inline Message Follows-----
|
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