Re: why does a generator become more difficult to rotate

With the output of a generator in open circuit (no load) there is obviously no current flowing through the windings. When a load is applied a current flows. That current induces a magnetic field. This induced magnetic field will oppose the fields that cause the rotation of the machine. This is known as Lenz's Law.

http://micro.magnet.fsu.edu/electromag/java/lenzlaw/

http://www.launc.tased.edu.au/online/sciences/physics/Lenz's.html

As current increases the induced fields increase and the machine becomes harder to turn.

It is also how back emf in a motor is formed.

http://www.animations.physics.unsw.edu.au/jw/electricmotors.html#back.

This is a consequence of the law of the conservation of energy.