Re: car alternators as motors

[Bob MacDowell <bobmacd@netcom.com>]

Robert McDonald <s348607@student.uq.edu.au> writes:

>> AFAIK, alternators have a higher power to weight ratio than DC machines,
>> and can  run  at  much higher speeds, with little maintenance (very robust
>> - no brushes). Combo of light weight+maintenance free. Of course, a
>> rectifier  set would be used to charge the battery, but that is also more
>> robust  than brushes. 

Brushes exist (on commutators) to provide commutation, or switching of 
the motor fields as it rotates.  DC motors and generators have commutators
built in, but AC motors and alternators rely on outside commutation.  
For instance, AC induction motors plugged into the AC power grid are 
commutated naturally by the 60-cycle AC. 

If you're working in DC, then AC devices need to be commutated somehow.
Electronic commutation of AC *motors* is hard because you have to control
things so precisely; but electronic commutation of AC *generators* is
pretty darned easy because you don't care about phase; all you have to
do is rectify it. :-) 

>  So does this mean that APU's for a series hybrid would be 
>better served with an alternator rather than a DC generator?

Yup.   Even locomotives switched to traction alternators decades ago. 
There's one advantage to a DC generator, though, you can connect it to
your batteries as a _motor_ to start the APU.    Until they switched to
alternators, locomotives put their 64V battery supply onto the 600V 
traction generator to start it.  (and if you think the Maniac Mazda has
a voltage drop, I gotta tell you... push that "Start" button and the 
lights go OUT! :-) 

I suppose you could use a traction alternator as a starter motor in that
fashion, but it'd take a lot more electronics to commutate it... so the 
question would be one of cost, weight, and simplicity.  Probably a gear-
reduction DC starter would win...