Thanks everyone,
Yea, obviously I know next to nothing about electromagnetics, but I
appreciate those of you who took the time to reply.
Let's see if I've got this correct:The force of the magnet inside the
motor is strong enough so that when power is drawn from the motor to
charge the battery, the momentum of the vehicle (which is turning the
drivetrain) -- that motion is not significant enough to fight against
the magnet, which in turn "brakes" the vehicle?
This probably isn't the right way to think about what is going on.
In a permanent magnet motor, the drive coils see a changing magnetic
field as the motor shaft rotates.
This generates a voltage - the "back EMF" - between the ends of the
coil. If you apply a higher voltage to drive current through the coil
against this "back EMF" you produce a torque roughly proportional to
this current which tends to make the shaft spin even faster, and you
are using the device as a motor to accelerate your vehicle.
If you let the "back EMF" drive current through the coil in the
opposite direction, into a battery, capacitor or resistor, you are
using the device as a generator to decelerate your vehicle. Again, the
decelerating torque produced is roughly proportional to current
circulating through the motor coil.
Any current circulating through the coils also generates a voltage
across the resistance of the coil, which generates heat - if you allow
the coils to get too hot they can heat the permanent magnets in the
motor above their Curie point, and they will cease to be permanent
magnets, so you do have to pay attention to the amount of current that
ends up circulating through the coils. The heating effet is
proportional to the current squared, so the polarity of the current
doesn't matter when you are calculating the heat being dissipated
In fact the voltage across any individual coil is an alternating
voltage that changes polarity a number of times as the shaft rotates
through 360 degrees. Permanent magnet DC motors include a mechanical
switching arrangement - the "commutator" - that rectifies this
alternating voltage into a direct voltage roughly proportional to the
speed at which the motor shaft rotates, which reverses polarity when
the direction of rotation is reversed.
If you want to follow the behaviour of the motor in more detail, you
have to start worrying about the inductance of the individaul motor
coils, which affects the rate at which the current through the
individual coils can change, and - even later - you can start worrying
about the extent to which the current through the coils induces a
magnetic field which can add to or subtract from the magnetic field
being produced by the permanent magnets.
When you get to this level, permanent magnet electric motors start
looking a lot like brushless DC motors (which use built-in electronic
swiches rather than mechanical commutators) and stepping motors (where
you are expected to supply the electronic switches).
Hope this helps.