Electric Power Principles, 2nd Edition

4Polyphase Systems

Most electric power applications employ three phases, that is, three‐separate power‐carrying circuits, with voltages and currents staggered symmetrically in time are used. Two major reasons for the use of three‐phase power are economical use of conductors and nearly constant power flow.

Systems with more than one phase are generally termed polyphase. Three‐phase systems are the most common, but there are situations in which a different number of phases may be used. Two‐phase systems have a simplicity that makes them useful for teaching vehicles and for certain servomechanisms. This is why two‐phase machines show up in laboratories and textbooks. Systems with a relatively large number of phases are used for certain specialized applications such as controlled rectifiers for aluminum smelters. Six‐phase systems have been proposed for very high power transmission applications. Five‐phase systems are sometimes used for motor drives with high reliability requirements.

Polyphase systems are qualitatively different from single‐phase systems. In some sense, polyphase systems are more complex, but often much easier to analyze. This little paradox will become obvious during the discussion of electric machines. It is interesting to note that physical conversion between polyphase systems of different phase number is always possible.

4.1 Two‐phase Systems

The two‐phase system is the simplest of all polyphase systems to describe. Consider a pair of voltage sources sitting ...