[TR] [6pack] Alternator Output vs RPMs

[Bob Danielson]

Thanks for all of the responses to this question. You learn something new
everyday.


Bob Danielson
1975 TR6 CF38503U
Running w/ Throttle Body Injection
Toyota 5 Speed & Nissan LSD
http://tr6.danielsonfamily.org 


-----Original Message-----
From: Navarrette, Vance [mailto:vance.navarrette at intel.com] 
Sent: Monday, September 15, 2008 4:06 PM
To: Bob Danielson; 6pack at autox.team.net
Subject: RE: [6pack] Alternator Output vs RPMs

        Bob:

        Yes, it makes sense.

        An alternator with no load on it will only require the frictional
losses + some small amount of work to keep the windings active.
        If you place a load of say, 20 amps on the alternator, you are now
demanding the original amount of work + (13v x 20a) = 260 watts. This is
roughly 1/3 HP of work (~760 W = 1 HP) that now needs to be done. This work
does not appear out of thin air, it is supplied by the motor because now it
is much harder to spin the alternator.
        The regulator is there solely to keep the voltage constant,
otherwise the faster you spin the alternator the higher the voltage would go
(assuming the load is not changing). So you should NOT see your voltmeter
move very much at all as you add or remove loads - although the engine may
speed up or slow down if it is idling.
        The other variable here is the tune of the engine. If the engine has
a hot cam, then it does not have any torque reserve at idle so a small
change in load can really slow down the idle speed. A stock cam delivers a
robust torque at idle, so you may not notice the idle speed change much when
a load is applied.
        An electric fan is the way to go, even though there is a small loss
of efficiency when converting the load to an electrical one. A mechanical
fan will spin up to 5,000 RPM, and so will draw up to two or three HP. An
electric fan will normally spin no faster than about 2,000 HP and so will
draw 1/2HP or less while operating (0 HP when not operating).

    Vance Navarrette
    Cogito Ergo Zoom
    I think, therefore I go fast

-----Original Message-----

I'm having a discussion with a TR6 friend who has a Delco 55 amp alternator
on his car like I do. He says that every time his Perma-Cool electric fan
comes on (he doesn't have a mechanical fan) he sees a 400 RPM drop (stock
mechanical tach) in engine speed. His reasoning is that when an accessory
calls for power the alternator has to work harder and, in turn, the engine
works harder.

<snip>

Does it
make sense that his RPMs would drop when his fan comes on?

Just curious.

Bob
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