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airlincoln(at)sbcglobal.n
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 Posted: Tue Nov 23, 2010 4:52 pm Post subject: peak loads for motors/pumps |
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Say you're doing your load analysis and you have a flap motor (running all of 30 seconds during a flight) that when turned on briefly draws 14 amps and then quickly settles down to 3.5 amps while running -- do you consider the "peak load" 14 amps or 3.5 amps as far as determining wire size and fuse size? Same question with a high pressure electric fuel pump for fuel injected engines.
Do not archive
Lincoln Keill
RV-7A
[quote][b]
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nuckolls.bob(at)aeroelect
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| Posted: Wed Nov 24, 2010 7:34 am Post subject: peak loads for motors/pumps |
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At 06:49 PM 11/23/2010, you wrote:
| Quote: | Say you're doing your load analysis and you have a flap motor
(running all of 30 seconds during a flight) that when turned on
briefly draws 14 amps and then quickly settles down to 3.5 amps
while running -- do you consider the "peak load" 14 amps or 3.5 amps
as far as determining wire size and fuse size? Same question with a
high pressure electric fuel pump for fuel injected engines.
|
Transient peak current demands are generally not part of
a load analysis because they are not high energy events.
By high energy, we're considering watt-seconds of demand
during a transient event is tiny when compared with the
sum total of all other loads.
Another example: The cold resistance of an 28v incandescent
landing light bulb can easily be under 1/2 ohm. 1/2 ohm
across 28v is 56 amps . . . except that this current is so
large compared to normal running current of say 4 amps,
the resistance of wring sized to OPERATED the lamp contributes
greatly to the mitigation of inrush current.
The energy demands of a start up transient (say 10A average
at 28v for .2 seconds) would be 56 watt-seconds. The energy
demands for the next ten minutes for approach to landing and
taxi in is (4A at 28v for 600 seconds) or 67200 watt-seconds.
A load analysis focuses on a flight-cycle. From master
switch ON to master switch OFF with one question in mind.
Does the power generation system produce enough energy to
recharge a battery used for cranking and to run all necessary
accessories for all phases of flight.
See the exemplar load analysis aids and samples at:
http://www.aeroelectric.com/PPS/Load_Analysis/
This does not mean that inrush currents are not an important
part of the system's performance analysis. Inrush currents
represent a potential for antagonizing other systems. I.e.,
as far as your GPS, EFIS, radio, or MP3 player is concerned,
turning on that landing light or pump motor is an influence
upon the quality of incoming power. A noise.
A low internal impedance of the battery goes to mitigation
of bus voltage excursions during the inrush event. Sizes of bus
feeders has a smaller but significant influence too. But as
a general rule, the only design consideration for wiring up
devices that produce large transient events is to make sure their
circuit protection doesn't nuisance trip.
Bob . . .
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