[Al Skierkiewicz]

Chris,
Do not start checking the menu just yet. The back EMF is present all the time the motor is turning. It varies with angular velocity, the number of poles in the armature and the number of poles in the magnet structure. It is a critical component in the no load speed of the motor. At this speed, the back EMF and the supply voltage cancel and no additional current flows in the motor windings. Look at the curves for any of the kit motors, and you will see that free speed occurs at current minimum. Now, if you drive a motor to free speed and open the supply voltage, the back EMF is capable of generating current if and only if there is a circuit path in which it can flow. In any of our motors, if the controller is put in the coast mode and the drive is removed, the circuit exists through the motor, wiring, FET diodes and battery. Since the diodes have a finite voltage drop in the forward conducting state, 1.25 volts per diode, current will only flow when the EMF is higher than the battery voltage by 2.5 volts (two FET strings in series). This is only possible if the motor is driven from an external source. At full throttle, the motor is spinning at a rate that is determined by the 12 volt source. When that is removed, the EMF will be around 12 volts and not enough to turn on the diodes. So what happens? The motor slows down due to frictional losses and magnetic interaction.
In the brake mode, the FETs are turned on to produce a closed circuit with the motor series resistance, the wiring resistance and series resistance of the two FET strings. Since Rds ON is 5.5 mohms and there are three FETs per string, the series resistance is 5.5/3=1.8mohm times two strings in series is 3.6 mohm. Total circuit resistance is likely in the neighborhood of 20-30 mohms.
However, as you may have pointed out, at less than full throttle, the controller is essentially in coast mode when a FET string is not turned on during the PWM off time. Again, it is unlikely that the diodes in the FETs are turned on (forward biased) during this time. Should this occur, once the EMF in the motor falls below 12 volts+2.5 volts=14.5 volts, the diodes would turn off. This becomes even more complex because of the dual winding caused by the brush contacting more that one commutator and the fact that the two windings are not rotating through the magnetic field at the same angular phase. i.e. the two will not reach maximum EMF at the same time.
So think about this, Stall current on the CIM is specified at 133 amps. Let us assume that is for two windings since it is labeled maximum current. If the supply is 12 volts at the motor terminals, then using Ohm's Law, the motor series resistance works out to .09 ohms or .18 ohms per winding. The free load specification is 2.7 amps, again at max, so making a calculation, 0.243 volts would drop across the series resistance. Therefore at free speed, the back EMF would be 12+.243. An open circuit at that point would still not produce the required 2.5 volts to turn on the diodes. Now we are dealing with averages here so the inductance is not yet present in the calculations. Most certainly, the inductive spike is present for short periods of time. A scope would prove that during those times, the diodes do conduct and clamp the spike at 12 volts plus the diode forward voltage as you would expect.
So where does this all leave us? A quick check of the Jaguar manual says that the controller is designed for a very specific motor, i.e. the Mabuchi RS555 or the CIM. My suspicion is the motors were modeled and switching parameters set to optimize those motors. The manual goes on to say that other motor types may not behave the same and recommend the reader check the Brushed DC Motor Control Reference Design Kit (RDK) User’s Manual for more information on motor selection and use. If you have not downloaded that yet, it might prove to be very interesting.