Three waves, evenly spaced.
In an ideal balanced three-phase system, the three phase voltages have the same frequency and amplitude. They are separated by 120 electrical degrees: one third of a cycle.
The phases exist at the same time. They do not take turns switching fully on and off.
See the phase relationship.
Follow the peaks: each phase reaches the same point in its cycle at a different time. At 50 Hz, a third of a cycle is approximately 6.67 milliseconds.
Which two points are being compared?
Voltage is always measured between points. Phase-to-neutral voltage and phase-to-phase voltage are different quantities.
For a balanced sinusoidal star supply, the phase-to-phase RMS voltage is √3 times the phase-to-neutral RMS voltage. A nominal 230 V phase-to-neutral example therefore gives approximately 400 V phase-to-phase—not 690 V.
Why it is useful for motors.
Three-phase currents in suitably arranged stator windings create a rotating magnetic field. The motor can use that field to produce torque.
A VFD can generate a controlled three-phase output for a motor. The frequency of that output need not match the incoming mains frequency.
Balance is an ideal, not an assumption.
Real supplies and loads can be unbalanced. Losing a phase or having unequal conditions may affect operation and needs proper engineering investigation.
Understanding three-phase electricity helps explain a fault report. It does not provide instructions for changing connections or testing a live supply.
A closer look
For balanced sinusoidal three-phase operation, real power is √3 × line-to-line RMS voltage × line RMS current × power factor. This assumes the stated balanced conditions; it is not a universal shortcut for distorted or unbalanced measurements.
Check your understanding
Are three phases 120 degrees apart three different frequencies?
Reveal the explanation
No. In the balanced system shown they have the same frequency. The 120-degree separation is a difference in phase position.