Mailing List flyrotary@lancaironline.net Message #45489
From: Gary Casey <casey.gary@yahoo.com>
Subject: Re: Alternator (Off topic)
Date: Sun, 22 Mar 2009 04:15:21 -0700 (PDT)
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
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Just to put what Jim said in context:  With a 12-volt system a set of nav lights might draw 5 amps or so.  5 times 14 (the operating voltage is closer to 14 than 12) gives 70 watts of power, and the alternator will typically be 70 percent efficient, so the load on the engine will be 0.13 horsepower.  The aircraft might be using 100 hp, so saving 0.1 hp will be real, but not very significant.  In an efficient car it can make a measurable difference.  The headlamps might draw 15 amps, creating a load of 0.4 hp.  The power required to drive the car might be as little as 10 hp, so the savings is definitely measurable.  The numbers I presented are not exact, partly because the alternator is more efficient at full load, so an incremental increase in load will increase the "mechanical load" by less than what I presented.
Gary Casey
Lancair ES, no rotary, but I thought about it (does that count?)

From: Rotary motors in aircraft <flyrotary@lancaironline.net>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Sent: Sunday, March 22, 2009 3:01:36 AM
Subject: flyrotary Digest #2409

        Rotary motors in aircraft Digest #2409

1) Re: Alternator (Off topic)
    by James Maher <delta11xd@att.net>
2) Re: Alternator (Off topic)
    by "Bill Bradburry" <bbradburry@bellsouth.net>

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-----Inline Message Follows-----

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

--- 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

 
 Jim,
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. 
George ( down under)
 
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

Since it's quiet:
 
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?
 
Thanks!
Ben


-----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

 

 

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)

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

--- 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

 

 Jim,

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. 

George ( down under)

 

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

Since it's quiet:

 

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?

 

Thanks!

Ben

 


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