The generation of 
AC electric power is commonly 
three phase, in which the waveforms of three supply conduits are offset from one another by 120°.  Standard frequencies are either 50 or 60 
Hz.  The voltage across a pair of these conduits, or between a single conduit and ground is 
single phase electric power.  Single phase power is what is commonly available to residential and light-commercial consumers.  The design of 
induction motors for single-phase power incorporates special design features to attain starting 
torque.
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