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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).
Gary Casey
-----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. Jim
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.
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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?
Thanks,
Mike Wills
RV-4 N144MW
----- Original Message -----
Sent: Sunday, March 22, 2009 7:45
AM
Subject: [FlyRotary] Re: Alternator (Off
topic)
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Tom,
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>
George,
I'm sorry to disagree with you but you are quite mistaken
in your belief about the
alternator function.
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
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-----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?
Thanks,
Mike Wills
RV-4 N144MW
-----
Original Message -----
Sent:
Sunday, March 22, 2009 7:45 AM
Subject:
[FlyRotary] Re: Alternator (Off topic)
|
Tom,
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>
George,
I'm sorry to disagree with you but you are
quite mistaken in your belief about the
alternator function.
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
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-----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
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-----Inline Message Follows-----
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