Re: Fan Resistors

[rawaudi@frugal.com (RWheeldon)]

John wrote:

>Continuing along this line, won't 6 volts at 40 amps gives a resistance of
>0.15 ohms (6/40) for the fan motor.  If switched to the high speed position
>(i.e., no other resistance), wouldn't current would jump to over 90 amps
>(14/.15) and power used to over 1200 watts (14*90)?
>
>Is it possible to measure the voltage drop across the resistor instead of
>the fan?
>
>Am I missing something? It just seems like a great deal of current and power
>to run a fan. Thanks.

1200 watts, that about 2 hp.  What load does a mechanically driven fan create?

You are missing something, though.  The fan motor cannot be modeled as a
resistor.  A MUCH better model is a resistor in series with a voltage source
(battery) that is proportional to the speed of the motor.  I would expect
the real motor resistance to be much less that .15 ohm since most of the
power supplied to the motor should go toward moving air rather than heating
the motor.

Regards,
Bob Wheeldon
'90 V8Q
Principal Engineer
Boeing Commercial Airplanes
robert a wheeldon@boeing.com
rawaudi@frugal.com