Three-Phase Current and the Rotating Magnetic Field
What do three pulsating fields add up to?
Three-phase currents shifted 120° apart in time flow in three windings set 120° apart in space, and each winding only makes the flux oscillate along its own axis. Move the phase ωt with the slider and the three colored vectors grow and shrink while the gold resultant vector moves; how that resultant's size and direction change is the question of this lesson.
A single phase only pulsates
When AC flows in a single-phase winding, the flux only oscillates along one axis, from maximum through 0 to the opposite maximum, without turning. This is an alternating field, and it can be split into two rotating fields of equal size turning in opposite directions. That is why a single-phase induction motor at standstill has no reason to pick a direction to turn, and it needs a separate starting device such as an auxiliary winding and capacitor.
Three together make rotation
Pass three-phase currents shifted 120° apart in time through three windings set 120° apart in space and add the three flux vectors at each instant, and the resultant vector always has a size of 1.5 times one phase's maximum and only its direction turns. When the currents advance one step in time, the resultant field turns by the same amount in space, so the time phase of the current becomes the spatial angle of the field. Swapping the connections of two phases (changing the phase sequence) reverses the direction of rotation, and if the three phase currents differ in size (unbalance), the tip traces an ellipse instead of a circle and an oscillating part remains.
The rotation speed is the synchronous speed
During one cycle of the current, in the simplest 2-pole winding the rotating field turns exactly once. Raising the number of poles P packs the field pattern more tightly, reducing the angle turned per cycle and lowering the speed by the same factor. In revolutions per minute, the synchronous speed is Ns = 120f / P [rpm], and the electrical and mechanical angles are related by θ_e = (P/2) θ_m. At 60 Hz, 2 poles turn at 3600, 4 poles at 1800, and 6 poles at 1200 rpm.
Back to the first screen
The sum of the three oscillating fluxes was a single magnetic field turning at constant size. Each phase only oscillates on its own axis, but add three of them shifted 120° in time and spread 120° in space, and the oscillating parts cancel while only the rotating part remains, turning at 1.5 times one phase's maximum. With not a single moving part, the time phase of the current becomes the spatial angle of the field. Its speed is the synchronous speed Ns = 120f/P, and swapping two phases reverses the direction. In the next lesson, this rotating field drags the rotor along and becomes the principle of the induction machine.
The rotating field is the engine of the induction machine. Put a rotor in this rotating field made by the stator, and the rotor turns along with it but never quite reaches synchronous speed; that lag is the slip. Only with slip do the rotor conductors cut flux, so that current is induced and torque arises. The next lesson draws the principle of the induction machine and the slip from the speed difference between the rotating field and the rotor (MC-C2).