Why does a power system use three alternating phases?

Three-phase electricity uses three alternating voltages at the same frequency, spaced one third of a cycle apart. In a generator, appropriately arranged windings encounter a rotating magnetic field at successive positions, producing sinusoidal voltages whose peaks arrive at different times. One cycle is 360 electrical degrees, so their separation is 120 degrees. These are coordinated waveforms, not three kinds of electricity.

For a balanced resistive load, each phase's current follows its voltage, and instantaneous power is their product. Individual powers rise and fall at different times, but their variations compensate. If the voltage and current peaks are V and I, each phase contributes VI times its squared sine wave. Three squared sines separated by 120 degrees sum to 3/2, so total power is the constant 3VI/2. This also equals three times the per-phase RMS voltage times RMS current. RMS, or root-mean-square, gives the value equivalent to steady DC for the same average heating in a resistor.

The three currents also sum to zero at every instant in this ideal balanced case. A star-connected system's shared neutral then carries no current. Unequal loads or distorted waveforms can change that result, so real neutrals may carry substantial current. Balance is a condition behind the calculation, not an assumption that every operating network automatically satisfies.

In a three-phase motor, currents in windings arranged around the machine produce magnetic fields whose combined direction rotates. That field interacts with the rotor to produce torque. Electrical timing and physical winding position work together: the same coordinated phases that provide smooth total power can create the rotating field that drives machinery.

Voltage between two phases is different from voltage between one phase and neutral. For a balanced star system, the line-to-line RMS voltage is √3 times the phase-to-neutral RMS voltage, because the two shifted waveforms are subtracted rather than their peaks simply added. Keeping all three waves on one time axis makes these relationships visible. Their individual rises and falls are precisely what combine into a steadier transfer of energy under the stated balanced conditions.