First, lets talk about it as a motor....
a) Speed is proportional to Voltage. You will get X rpm per volt applied. If the motor-constant is 21 rpm/volt, then 10 volts will give you 210rpm. As load is applied, current-draw increases dramatically, and inefficiencies cause the speed to drop somewhat.
b) Current is proportional to Torque. For every X ft-lbs of torque-load you put on the shaft, the motor will draw Y amps. This too is fairly linearly proportional.
Now, turn it around and look at it as a generator...
a) The output voltage is proportional to speed.
b) The output current is proportional to -load- (i.e. resistance).
c) Obviously, you must have -some- voltage output in order to develop any current at all; but even at a voltage less than the machine's rated voltage/speed as a motor, it will still deliver a current which is more related to the load-resistance than it is to voltage.
To prove this, drive it with a large diesel engine at 1,000 rpm....put a dead short across it, with an ammeter in series.
Even with only a miniscule voltage at it's output terminals, you will certainly find a very high current flowing!
And of course, lots and lots of smoke coming out... <g>
