I made a comment on this topic a couple of days ago:http://forums.mikeholt.com/showpost.php?p=701799&postcount=33
Start with a hypothetical 100% efficient motor. The electrical power input exactly equals the mechanical power output. Now reduce the input voltage. If the mechanical output were to remain constant, and the efficiency were to remain constant, the current drawn would need to increase.
_Real_ motors are not 100% efficient. Change the voltage and you change the efficiency.
Real motors draw reactive power; change the voltage and you change the reactive current flow.
Real motors will change speed when the supply voltage changes, possibly reducing the mechanical load.
Real motors can _stall_, ending up right at that starting current situation described above.
Depending upon the specifics of the situation, a reduction in supply voltage can do anything from slightly reduce the current drawn by the motor, all the way up to causing the motor to stall and draw many times more current.