Voltage Regulated by Resistor in Generator Circuit
Great job on the experiment!

The motor that you are using as a generator has a high internal impedance largely due to the windings and the internal magnetics structure. You can think of this as a resistor inside of the motor that is in series with its output. Of course this is not a real resistor, just a way of modeling it.

You didn't mention it, but when you are experimenting consider that the load placed on your motor acting as the generator may cause the driving motor to speed up or slow down. This will of course affect the results of your experiment.

In electronics, we know that maximum power will be transferred to a load when the load impedance matches the source impedance. You may find it interesting to add a column to your chart that shows power in the load (= V 2/R) to see if you can find the point of maximum power transfer. You will need to extend your experiment with higher values of resistance most likely.

Once you have determined the maximum power you can obtain from your generator, you can then see if it is suitable to power your target device. If it does have enough power, then the most likely the solution will require a buck regulator to efficiently step down the higher voltage.

Keep up the good work.