Hi.
Well it is not that simple but this is the general idea that a brushed dc motor will work best if you don't exceed max operating voltage.
I was at work writing this post and i didn't had any time so i was very brief.
A brushed dc motor has a rotor with a few coils (electromagnets), a collector which has a set of contacts on it and some permanent magnets.
Let's say that the motors windings have an ohmic resistance of 0,1 ohm and the voltage applied is 6V.
The inductance charging is done very rapidly in low rpm because the inductance is very small so we can omit this aspect.
Initially when the motor is at rest and we apply the voltage one winding gets all the power instantly and it is immediately subjected to
6/0,1=60Amps which generates a lot of temperature in the collector with the possibility of scorching the usually copper made collector contacts.
After the rotor acquire some speed (rpm) the coils move close around the permanent magnets acting exactly like a generator
and this creates an EMF (electromagnetic force) of opposite polarity to the winding's voltage which is rpm dependent.
The current starts to go down until it reaches the value of (6v-back EMF)/0,1 Ohm = say (6-5,9)/0,1=1A (this is the idle current).
Obviously the only load to the motor is the rotor friction.
Now when a load is presented to the motor's rotor obviously the rotor will loose rpm and this will lower the back EMF to say 5,5v.
Then we have (6-5,5)/0,1 = 0,5/0,1=5A and the motor will draw 5 amps.
If we completely stop the rotor the current will be 60A!!!
This explains why the motors draw much more current when they start or when the rotor completely stops under excessive load (stall current),
it is because the back emf is zero.
Now if we increase the voltage to 7 volts and keep the same RPM
(the pump for example will need to have the same rpm in order to pump the same amount of fuel)
the motor will draw (7-5,5)/0,1=15A! Remember that the back EMF is only rpm dependent and the Current is the peak current.
The motor speed controller will reduce the time that this voltage is applied by reducing the duty cycle and the average current will be lower and in fact almost the same as with 6 volts but the peak current has jumped from 5A to 15A!
Obviously this is not good for the brushes and the collector thus the words you might have heard "welded or melted brushes"
Not to mention that this amount of overcurrent will probably drive the winding's core in to saturation heating the core and lower efficiency by much
thus crate more heat.
Of course there are more things involved but this is the basic theory as i know it.
The best motor controller would be a true variable voltage source like a buck converter or better a true variable current controller instead of using PWM but it would be rather heavy compared to the present brushed speed controllers.
I have burned the starter motor and spoiled a pump by using an 8 cell NiMh pack with the JJ1400.
The starter motor only lasted a couple starts.
I am not trying to play the expert here i am just sharing what i know and have experienced.
Please excuse me for the long post but now that i found some time i think someone might find the post useful.
Also i apologize for any generalities and errors in my post.
Chris