Re: [xj-s] electric fan -windmill

[Walter Petermann <corsaro@brokersys.com>]

Juliansean@aol.com wrote:
> 
> I was wondering what happens when an electric cooling fan (aftermarket
> installation) freewheels.  For example at 60 mph, that fan will be spinning
> pretty furiously because of the air flowing past it, even though it may not be
> electrically powered at that moment.
> 
> As a mechanical engineer I'm off home turf, but nevertheless at the risk of
> sounding ludicrously ignorant in front of the entire list I would like to ask
> this question:  would the fan act as a generator?  Or, would it create any
> kind of electrical surge in the car's elecrical system?
> 
> To go further,
> a) what happens when the fan is not powered but spinning fast and THEN it is
> switched on by the thermostat control?
> b) If the fan can't freewheel easily because it is acting like a generator
> (windings are creating resistance to rotation) then wouldn't it act as an
> impediment to freeflow of the air thru the radiator?
> 
> Julian Mullaney

Speculations on a)

  I doubt that the air through the fan at high speeds can make it turn 
  faster than when it is powered up. Assuming that this is correct,
  DC motor theory has it that, if you spin the motor externally, the
  back emf (voltage generated by the motor) is the same as the voltage
  that would need to be applied to the motor to make it turn at the
  same speed. Applying power to the motor while it's spinning would
  reduce the transient current in the electrical system since the current
  would be (12v - back emf)/winding resistance. Therefore it's good
  for the (DC) motor and electrical system to apply power while it's 
  spinning! 

b)The motor will only generate torque against rotation if the power
  leads are tied together. Since torque=constant*current and the current
  will not flow if the fan circuit is open by relay, thermal switch etc.

   Of course, if the motor is the brushless type I don't have a clue on
   a), b)... z).

   Walter

   Walter

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References:

• [xj-s] electric fan -windmill
• From: Juliansean@aol.com

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