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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
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of James Maher
Sent: Saturday, March 21, 2009
9:14 AM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re:
Alternator (Off topic)
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
--- On Sat, 3/21/09, George Lendich <lendich@optusnet.com.au>
wrote:
From: George Lendich
<lendich@optusnet.com.au>
Subject: [FlyRotary] Re: Alternator (Off topic)
To: "Rotary motors in aircraft"
<flyrotary@lancaironline.net>
Date: Saturday, March 21, 2009, 5:07 AM
To my way of thinking, the alternator is running with the
engine RPM so the power generated is relative to the RPM. This can't be
changed unless you disconnect the alternator.
The alternator charges the battery - which in turn runs
the accessories.
So no savings can be made unless you disconnect the
alternator.
So turning off things does little - it only saves the
battery charge which is being recharged whether you like it or not.
Unless you put a clutch on the alternator.
So no savings of HP or fuel.
In some cases, systems like to have a continuous drain on
the battery, like the Harleys. That's why they run around with their lights
on - so they don't overheat the battery. The chaps who select
'lights on' rather than the hard wired factory systems, always have
battery problems.
It doesn't matter what is generating the
electricity, a generator, alternator or battery it takes at least 1 HP to
make 746 watts of electricity.
That is the conversion factor under ideal
conditions with an efficiency of 1.
Of course in the real world there are losses that
reduce the amount of electricity or increase the horsepower necessary.
Alternators are probably more efficient than
generators but then there are losses in the rectifier and regulator
circuits.
The laws of physics dictate that there is no free
lunch.
Energy can neither be created or destroyed, only
converted from one form to another
(mechanical,electrical,chemical etc.), and always
with losses in the conversion process.
So turning off electrical devices will reduce the
load on the engine by whatever amount they draw plus what ever losses are
required in the conversion.
Jim
--- On Thu, 3/19/09, Ben Baltrusaitis <expressben@verizon.net>
wrote:
From: Ben
Baltrusaitis <expressben@verizon.net>
Subject: [FlyRotary] Alternator (Off topic)
To: "Rotary motors in aircraft"
<flyrotary@lancaironline.net>
Date: Thursday, March 19, 2009, 9:08 AM
When I
was a kid a guy at the parts store demonstrated to my Dad that when
electrical power was needed, a generator put a load on the engine. After
that, my Dad was careful not to run lights, radio, heater fan, or other
non-essentials when he was trying to get good gas mileage.
I have
continued that tradition, however, I have seen it stated that electrical
draw on an alternator doesn't increase the mechanical load.
When low
on fuel will it help to turn off electrical components not needed for
flight?
Is it
true of an alternator; an electrical power demand doesn't cause an
increased mechanical load?
Or, does
keeping headlights on during the day decrease gas mileage?
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