A single-phase power supply provides alternating current electrical power in a system in which all voltages vary in the same pattern. This sort of distribution is most common when the power load is primarily used for lighting and heating with only a few larger motors to power. Single-phase power supplies that connect to an AC electric motor do not generate revolving magnetic fields. Instead, they need more circuits for starting and generally have ratings at or below 20 kW.
In the United States and some other countries, single-phase power supplies are often divided in half to generate split-phase electricity, powering lighting and household appliances. The standard frequencies for power systems in single-phase systems are usually 50 or 60 Hz, although special traction networks sometimes operate at 16.67 Hz or other levels specific to electric railways.
Three-phase power supply systems work somewhat differently. Within each conductor, the current reaches its top instantaneous value in order instead of all at once. Frequently, the waveforms of these conductors hit their peak at a schedule so that each one has gone through a third of its period when one of the other two hits its peak, which keeps power generation on a consistent schedule while avoiding instability.