Modeling motor control

[kamocat]

I'm trying to calculate the torque of a motor based on the voltage, speed, and current.
I'm actually interested in the torque at the output of the transmission, not simply at the output of the motor.
This means I need to know the no-load current of the motor.
I was thinking no-load current was proportional to voltage. Apparently it's not.

Here's a graph:


This is measured with a Jaguar over CAN, though I've confirmed the approximate current trend with a Fluke 179. (I didn't care to take 10,000 samples by hand)

Some curious things about this:
There is a sudden drop in current at 10v (about 16800 RPM on the motor shaft)
At this drop, the sound of the motor changes. Before, it is somewhat buzzing, and afterwards it is quiet and smooth.
After the sudden drop, current continues to decrease.
Speed remains proportional to Voltage, regardless of current.

Some more about my setup:
I'm using an rs550 motor with a Banebots 20:1 planetary transmission. This is attached via chain and sprockets (at a 2:1 ratio) to a small wooden wheel. (The wheel serves as a temporary until I get around to making mecanums for my half-size testing 'bot.)


I'm guessing this particular point is specific to this motor, but has to do with the speed of the motor (as opposed to the duty cycle of the Jaguar).


Have you ever seen this before?
What do you think is happening?