[ausev] dc motors vs dc generators

[Christopher Robison]

On Mon, 2007-04-23 at 15:08 -0500, Donovan Becker wrote:
> I know that there are a bunch of electrical engineers within this group, so 
> I wanted ask if any of ya'll could tell me the difference between dc motors 
> and dc generators. I read recently that they're the same, you just spin the 
> shaft faster than the rated output to create energy. Is this true? In case 
> your wondering this does involve ev's, just in an indirect manner. Thanks.
> 
> Donovan
> 

There's more than one way to generate DC, but yes, a DC motor will
generate DC electricity. This is automatic with permanent magnet and
shunt-wound DC motors, and more difficult with the series-wound DC
motors that are usually used in EVs.

The concept of spinning the shaft faster than the motor's particular RPM
per volt for a given input voltage applies to shunt and permanent magnet
motors. AC motors have a similar feature, except you're spinning the
rotor faster than the input frequency spins the field.

For series-wound motors, I believe you have to reverse the polarity of
the field with respect to the armature, and you have to provide a seed
current to produce enough magnetism to get the process started. For
these reasons, series-wound DC motors are almost never used to generate
power, and only one series DC EV controller that I know of is capable of
regenerative braking.

  --chris