DOes an alternator strain the motor the same amount no matter what?
 

When an a battery is low or is being used heavily by lights etc., does the alternator rob more horsepower from the car during that time or is it always the same resistance to the motor?

It takes power to make power i.e. it takes more mechanical energy to generate more electrical energy i.e. if the vehicle's electrical loads are higher it will be harder to turn the alternator.

P.S. it also takes more power to spin an alternator at higher rpms.

Though it will need to do so for a shorter period, resulting in the same work done and the same edges fuel used, right?

Yeah, what Frank sez, yer alternator (and mine and Franks) robs more power at higher loads because you don't get something for nothing, power in equals power out, if the alternator is generating more amps, it needs to take more power from the engine to do so. Simple fysiks. To think otherwise would be an HHO proponent.

What? No, the alternator generates DC power, whether it runs at 1,000 or 8,000 RPM it will deliver the required amps at 14 volts or so to keep your car supplied with electricity. Speed doesn't affect its ability to create power, for the most part, very low rpm's being the exception.

What Frank was so astutely referring to is the fact that the alternator has bearings which are not perfect, the higher speed it runs, the more power it takes to overcome the friction to turn the bearings. As well, since the rotor of the alternator does not exist in a vacuum, air friction plays a role as well, but mostly, there is a cooling fan on the beast, and the faster it spins, the more power it needs.

The cooling fan in fact, being a pump type of thing, actually obeys a cube root function in its power requirements, so an increase from 1,000 to 8,000 RPM results in a 8³ increase in power required or 512 times the power is required at the higher RPMs to run the fan, good thing it starts out as a tiny fraction of a horsepower to turn the fan.

Yes, the field of the stator is adjusted as required for the necessary amperage therefore requiring more torque at higher current..