My DC motor experience comes from 20+ yrs in the lift truck industry. We constantly had problems w/ battery maintence/charging issues, affecting the life of the DC motors. [Hyd pumps and drive motors.]
Our engineering deptgave us a good explanation of what goes on...
Load requires a certain amt of HP to get the job done. [Expressed in watts].
Volts x amps = watts.
Working an example: 36V x 100A =3600 watts to get the work done. Given that the load stays the same, and the volts available drop, due to batt charge level, bad wiring, etc, the equation now looks like this:
Say 30V are available. 3600/30 = 120A. This motor is now drawing 120A, which equals more heat.
In many cases, we saw brush holders, brushes, contactor tips, wire ends, com bars, melted, if the situation got bad enuf.
I see no reason that this scenario would not apply to a DC fuel pump motor??? [Btw, I ran this very scenario past my bud, an EE, w/ many yrs of experience. He agreed]
Since our pump output is directly related to VDC input, [the very reason we use "hotwire kits, and good grounding],
a well established supply of volts, and a corresponding good ground system, would lead me to believe many failures are going to be found elsewhere.. Pump components, level of heat tolerence, tank/sump/return design
i too am an electiical engineer-----------and have a lot of experience with dc motors----------i too own lots of electric forklifts and thousands of dc motors that operate in my manufacturing plants----------and while your math is right it is not correct in the application------with a forklift when the voltage drops you simply increase the throttle/lever to make up for the lower battery voltage------the voltage at the motor actually increases until it can no longer do the work needed at which time you recharge it-------very poor analogy that doesn't apply to this application---------a simple dc motor on a non regulated power supply will not increase current draw if the voltage drops----------the motor will run slower and do LESS work meaning it will take more time to do the task at hand--------the entire equation of ohms law tells us that for a given voltage a permanent magnet brush dc motor will draw a given amount of energy to do work------usually expressed as watts as you have stated-------lets look at your formula--------36 volts at 100 amps indeed means that the motor is consuming 3600 watts-------it also means that the motors resistance to the flow of electricity is .36 ohms---------this also means that if the voltage drops to 30 volts the current will DROP to 83 amps and the wattage drawn by the motor will drop to 2490 watts and it will run slower and COOLER--------UNLESS you have a variable power source to increase the VOLTAGE there is NO WAY to get more than 83 amps to be drawn by a resistance of .36 ohms---------there are some issues that complicate these simple assumptions that are related to EMF from the moving armature etc but the bottom line is that a dc motor on an unregulated supply is not going to suffer damage from low voltage unless it has a locked rotor and if the rotor is locked that is the problem not low voltage---------in a fuel pump application whare a liquid is being pumped into a fuel line with a return path the mechanical impedance is so mismatched that EMF is really a non issue..............RC