Question 35
In a three-phase system, the voltages are
separated by
45°
90°
120°
180°
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Answer:
How can a 2 phase system 90 degrees apart be balanced? It is 90 degrees apart, I think it should be 180 degrees.
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13 Answers

Martin Brilliant, Sc.D. Electrical Engineering & Information Theory, Massachusetts Institute of Technology (1958)
Answered 3 years ago · Author has 9.8K answers and 4.8M answer views
How can a 2 phase system 90 degrees apart be balanced? It is 90 degrees apart, I think it should be 180 degrees.
In a balanced single phase system the two “hot” wires are already 180 degrees apart. When one wire is, for example, +120v, the other is -120v. The power delivery is pulsed, because a quarter of a cycle later, when one wire is a 0v, the other is also at 0v.
A two phase system can deliver constant power if the two pairs are 90 degrees apart. Suppose we label one pair A and B, and the other pair, 90 degrees out of phase with it, as C and D. Snapshots a quarter of a cycle apart would look like:
A = 0v, B = 0v, C = +120v, D = -120v.
A = +120v, B = -120v, C = 0v, D = 0v.
A = 0v, B = 0v, C= -120v, D = +120v.
A = -120v, B = +120v, C = 0v, D = 0v.
A = 0v, B = 0v, C = +120v, D = -120v again.
When pair AB is delivering maximum power, pair CD is delivering none; when CD is delivering maximum power, AB is delivering none. Between the two, they deliver constant power.
P.S. A possible source of confusion is that “two phase” does not mean “two wire.” It means that there are two pairs of wires carrying AC out of phase with one another. I described a two phase system with four wires consisting of two balanced pairs. In a three wire two phase system, the two pairs have one wire in common, the neutral wire, which is always at zero voltage, and the other two wires are like A and C above. When the pair NA is delivering maximum power, the pair NC is delivering none, and so on.
Another possible source of confusion is that there is a three phase configuration, called Delta, that has three wires. It derives from the fact that when three pairs 120 degrees out of phase share a common wire, the common wire carries zero current and can be omitted. The common wire in a two phase system can’t be omitted, so there is no way to carry two phase AC with only two wires.
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