Since the drive pulley at the crankshaft is usually the biggest one in the system, all the accessories are turning faster than crankshaft speed. That can lead to problems such as water pump cavitation and overspeeding of a generator if the pulley ratios are dramatically altered from the original design. Fortunately, alternators are designed to run at several times engine speed, so they don't pose much of a concern when they replace a generator. That's why an alternator is able to charge a battery at idle peed, something most generators can't do.
