A "three-phase system" is a polyphase system having three phases. The term "polyphase system" just means a system having multiple phases. If it is used by itself, "a polyphase system" doesn't mean "a three-phase system".
Q: In a modern industrial plant the most likely polyphase power system in use is a A. one-phase system. C. three-phase system. B. two-phase system. D. four-phase system.?