Re: car alternators as motors

[XURQ03A@prodigy.com ( LEE A HART)]

The alternator comments reveal lots of interesting misconceptions. Rather than
quote them all, I'll just add a few comments (in no particular order).

Alternators are LESS efficient that generators. Brushes were not particularly
troublesome on old cars; they normally lasted for the life of the car with no
maintenance. They switched to alternators because they are CHEAPER and smaller.

Auto alternators have a wound field. The spinning rotor is an electromagnet,
powered thru slip rings. By varying the current and voltage in the rotor, you
vary the output voltage. The voltage regulator (sometimes inside, sometimes
outside the alternator), controls the field current to regulate voltage at the
desired level. If you change this regulator, you can get any voltage from 0 to
well over 100 volts, and anything from 0 to maximum rated current.

Some cheap alternators (motorcycles, garden tractors, etc.) have permanent
magnet rotors. Since you can't adjust the field strength, you can't regulate
the charging current. Given the chance, they'll happily fry the battery (which
is why they have to be replaced every season or two).

One reason alternators are so inefficient is that they have two diodes in
series with the 12v output. Each diode has a 1v drop (at full current). That
takes you down to 85% right there. Then, the field draws about 5% of the
output current, so you're down to 80% efficiency. We haven't even covered
bearing losses (sleeve bearings in the back), slip ring brush drag, winding
resistance (Detroit skimps on wire gauge), wind resistance (their low
efficiency requires a lossy fan), and core losses (high because they use far
too thick a lamination for the high operating frequencies involved). So
overall, you're only around 60% efficiency.

All motors need two magnetic fields to work; one in the stator (fixed part),
and one in the rotor or armature (moving part). Both can be wound with wire,
or either one of them can be a permanent magnet. Here is a chart to identify
what type motor you have:

                      ------------rotor or armature construction----------
                      permanent     lump of iron   coils and    coils and
stator construction   magnets       copper, alum   slip rings   commutator
-------------------   ---------     ------------   ----------   ----------
permanent magnets     won't work    hysteresis     inside-out   DC PM
                                    brake          AC synchronous

wound coils           AC            AC induction,  AC wound     DC compound,
                      synchronous   hysteresis     rotor,       shunt, series,
                                                   selsyn       sep.excited,
                                                                AC universal

coils and commutator  DC brushless  switched       special      Rube
(usually electronic)                reluctance     servomotor   Goldberg

Lee Hart                     If you would not be forgotten
4209 France Ave. N.          Soon as you are dead and rotten
Robbinsdale, MN 55422 USA    Either write things worth the reading
phone (612) 533-3226         Or do things worthy of the writing
e-mail XURQ03A@prodigy.com   (Ben Franklin, Poor Richard's Almanac)