Why Are Three

Three-phase alternating current (AC) induction motors are more common for commercial and industrial pumping applications than single-phase motors. Some of the reasons are:

Image 1 shows how the maximum current is passed from Phase A to Phase B to Phase C in a three-phase motor. After this sequence, the maximum current follows the same pattern but with reversed polarity.

Image 2 shows how the three-phase currents create a magnetic field that smoothly rotates inside the motor. The magnetic poles are color-coded to match the background color of the graphs of the current in each phase. The large letters, N and S, next to poles in Image 2 indicate the location of the peak magnetic field. The small letters, N and S, show the locations of weaker magnetic fields.

Notice how the magnetic field moves between poles and forces the rotor to spin in a specific direction. Because the AC is sinusoidal, the magnetic field of each phase is also sinusoidal. This results in a net magnetic field that rotates smoothly around the inside of the motor.

For more information on electric motors, refer to the Hydraulic Institute’s “Drivers Application Guidebook: Electric Motors” at www.pumps.org/guidebooks.

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