How do different engine speeds affect cost, durability, and generator output?
In order to get standard 60 Hz (cycles per second) alternating current, the engine must turn a direct-coupled generator at a constant speed which translates to some multiple of the desired cycles per second. For 60 Hz power, this means (commonly), an engine speed of 3600 rpm (e.g., 3600rpm/60sec/min = 60 rps), or 1800 rpm, or 1200 rpm. For 50 Hz power, the necessary engine speeds are 3000 rpm, or 1500 rpm, or 1000 rpm. Since the voltage reverses once per cycle, a 2 pole alternator is necessary at 3600 or 3000 rpm, a 4 pole alternator at 1800 rpm or 1500 rpm, and a 6 pole alternator at 1200 or 1000 rpm. If the generator is not directly driven, then other engine speeds may be used (with a pulley or gear ratio designed to produce the proper alternator rpm). In the US, 3600 rpm is common in small and portable units. Unfortunately, the life of an engine at 3600 rpm is much shorter than its life at 1800 rpm, and this effect is much magnified in practical use by the fact that the design life
