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I was going through a physics book which contains a short description of Nikola Tesla . The description mentioned about poly phase circuits (power supply). Out of curiosity I googled it up and I found that in a poly phase supply we use alternating currents of more than one frequency, say, w1, w2,w3,w4...

Can anyone tell me what was the logic behind the invention of polyphase power supply,or what forced Tesla to invent this polyphase power supply system?

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Polyphase power supplies have more than two lines, offset at different phases but all at the same frequency. For a two phase power supply, the voltage between the two lines will be zero twice per cycle, which mean no instantaneous power is being transmitted then. If you have three lines each offset by 120 degrees of phase, then at any instant there will always be a pair which has nonzero voltage difference. Hence a more even delivery of power.

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@user1631- Great, Thanks for showing me the insight. Could you elaborate "what is uncool with having zero voltage difference between the lines once or twice or thrice... in a cycle?" Waiting for your answer. – user16186 Dec 31 '11 at 2:58
Addition: it 3-phase isn't only useful because it provides a more even delivery of power, it's useful because when connected to a balanced load it delivers completely constant power. – Alan Rominger Jan 29 '12 at 23:03

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