What is "Phase Rotation" or "Phase Sequence" in a 3 phase system, and why is it important for connection of 3-Phase motors?

In a standard 3-Phase AC system (400V/50Hz, or 210V/60Hz), the sinus waveforms of voltages per phases are shifted between each other, with an angle of 120 degrees between each phase, in order to form a symmetrical system. This means that the peak voltage of each phase doesn’t occur at same time, but it is shifted by 120 degrees (which means that difference between phases in time is 1/150s for a 50Hz system and 1/180s for a 60Hz system).

Because this system is symmetrical it is possible to have only two different sequences of phases. We call these sequences (rotation directions) “LEFT” and “RIGHT”, or “Clockwise” and “Counterclockwise”

Why is this important concerning the connection of 3 phase motors?

A three phase motor generally has fixed windings in multiples of 3, which depends on the motor construction (3, 6, 9 …). And every winding is connected to one of the 3 phases (every third winding is connected to same phase). Depending on the phase sequence we connect to these windings, we can have different rotation direction of the motor shaft. If the phases are connected in the wrong direction, the motor will rotate in the opposite direction that we wanted. This is why it is important to check phase sequence in the outlet before connection to a 3-phase motor, and also to check the sequence of windings in the 3-Phase motor, in order to obtain the expected direction of the rotation of the motor shaft.

Example:

If we have a water pump (with a 3-phase motor) which pumps water itno the wrong direction, it is enough to change any 2 phases between each other, at the connection of the motor or in the outlet itself, in order to reverse the direction of the water pump. (Of course, don’t connect the water pump yourself in the first place, call some authorized technician).