[Al Skierkiewicz]

Ok James,
I thought I had it about seven or eight different times then I remembered I had a Johnson version of the FP motor sitting on a shelf so i took it out and looked at and did a little measuring and fudging some numbers and here is what I came up with.
The FP motor has three commutator segments and therefore three armature windings. If you draw out the equivalent circuit you will see that depending on what the brushes are contacting, you can have one of three conditions, 1) two windings in series, 2) one winding is parallel with two windings in series or 3) Two windings in parallel (the third is shorted by one of the brushes) Now doing some math making some assumptions and backing into the calculations, you can solve for L. At no load the specs are CEMF=2.35 volts I=1.1amps speed=15000RPM. Calculating for equivalent impedance from 1.1/(12-2.35)=.1 ohms. At 15000RPM shaft takes .004seconds/revolution or a commutator segment is engaged for 1.33mSec.(less than the "on"time of the speed controller at 50% duty cycle) Now making the assumption that a prudent designer would make the inductive reactance equal the series resistance at the highest frequency (1/1.33mSec=750Hz) then L would be 10 microhenries. The time constant for this inductance would be 0.2mSec. Hence, the current would would have a chance to rise (rather quickly) to full level at duty cycles down into the single digits. (about 2-3% duty cycle) As the motor RPM goes down, the inductance has less of an effect.
In analyzing the drill and chalupa which both have more windings, the inductance is likely far less per winding. (since there are more windings per armature, there is less wire/turns for each coil) In any case it does not seem that the controller switching at 120 Hz would have much effect on speed above 10% duty cycle.
In reading through Greg's linked article, the author referred to series wound DC motors (non permanent magnet) and controllers using kHz switching frequencies. Although a lot of his discussion and drawings are useful, the conclusions cannot fit our motor/controller combination. When looking at the voltage waveforms and sawtooth response, remember his switching frequency is very high and may (likely) be affected by controller output impedance. As the output impedance rises, the time constant stretches out and the resultant sawtooth response can be observed.
Now, if controllers are not introducing the error (deviation from linear) and inductance doesn't appear to have that effect than there is obviously something else. This is what I think it may be. At low RPM say 1000, the controller actually has time to turn on and off several times before a commutator segment will have turned past the brush. Total current will be a function of 1) speed at which the three winding conditions will switch vs. time 2) how many pulses can be delivered to a winding pair on average and 3) load. Since the test in question did have a load (Ok a small one) it is easy to see that these other factors would have an effect on motor performance.
At low speeds the meter should read more accurately (still flawed by rise time and frequency response) but cannot take into account the effect of rapidly changing load conditions as speed increases. The graph of controller output voltage, scope vs. meter should be an interesting thing to look at.