hi r chung,
 I think the best way to consider the issue of fan motors is to consider the simple circuit which would exist if you had a resistor or no resistor to the motor.
 With no resistor then you have applied the maximum available voltage to the motor so the motor will draw the maximum current and spin the fastest.
[before any one says it I assume aSIMPLE circuit with no locked rotors water elaks or dead bugs stuck  under the brushes]
 Now lets add a resistor in series. The motor & resistor circuit draws some current so a voltage appears across the resistor. (mr ohm said Volts = amps x resistor)
if there is a part of the battery voltage "used" on the resistor then there is less at the motor.
The circuit (& the motor ) will draw less current and the motor will spin slower.

Now there are some very fancy ways of controling the motor electronically but I think this is a fair description of the simplest (and common) way of fan control

regards Don

Thank you for reply Don.  Your explanation good and I understand.  I guess resistance of motor constant and not depend on current or voltage.  So if voltage across motor 3 v and current through motor 6 amp, resitance of motor = .5 ohm and not change?  Is this typical resistance for motor?  Thank you kindly for your time.

R. Chung