As I understand it, most three prong electrical chords are have a hot wire, ground, and neutral terminals (for extra precautions). My swamp cooler motor cord has four prongs as shown here:
What is the need for the fourth terminal? It seems to me that most, if not all, cords for swamp cooler motors have three circular terminals and one flat terminal as shown above. Why are these terminals shaped as such? Is there any advantage to having their shapes this way?