MrWizard wrote:
how much HP does an automotive A/C system use ? 2 maybe 3 HP ?
your engine just idling is producing way more than that
its not about engine HP
its all about pulley ratio and 'alternator RPMs'
the 'current design' alternators in production need more RPMS to produce max output
but it doesn't have to be that way
OK, lets do some basic electrical/mechanical analysis.
Lets assume a 300 amp output to the electrical system when spinning at 1000 RPM at the rotor.
Power will be 300 amps x 13.7 volts or about 4110 watts.
If we assume a 90% efficiency at the lower RPM range then the input mechanical power to the alternator is about 4567 watts - we are neglecting other losses and loads.
The input mechanical power divided by 746W/1HP means that approximately 6.12 HP is needed.
Since HP and Torque are not mutually exclusive, the following equation can be used to figure out how much rotational force is being developed at the pulley.
HP = Torque x RPM / 5252 or rewritten, Torque = HP x 5252 / RPM -> 6.12 x 5252 / 1000 = 32.14 ft-lb of rotational force.
If we assume an alternator pulley diameter of say 6”, which is pretty big, then the radius becomes 3” or 1/4 of a foot.
This means the tangential force applied to the alternator pulley must be about 32.14 ft-lb / 1/4 ft which equals 128.6 lbs of force for a rotor rotation at 1000 RPM.
The tension and friction values required on the belt and pulley will have to be considerably higher than this value or the pulley won't turn or slipping will occur.
Since this tension force is being applied to bearings through shaft deflection, and a whole new can of worms open up.
Now couple all of the other loads being operated at the same time to the drive belt being operated at the desired RPM and tension increases even more.
So the main issue with low RPM, high current, alternators is of coarse loading of the motor, belts, pulley system, bearings, etc...
But hey, if it was easy, cheap, and small enough, then they would be standard on everything from the Prius up...
:)
