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Poles are the number of sets of three-way electromagnetic windings that a motor has.
In the simplest three-phase motor, there are 3 separate electromagnets formed by the single set of three-way windings. Thus, there is a set of North-South electromagnetic poles formed. This motor is said to have "2 poles" (although strictly speaking, the motor has 6 electromagnetic poles).
As the three voltage phases, each 120 degrees off from one another, gradually rise and fall, the strength of each electromagnetic winding set rises and falls in relation to the frequency of the voltage changes. This causes the rotor to rotate once per voltage cycle. In a 60 Hertz system, this results in a base speed of 60 Hertz, or 3600 rpm. A 50 Hertz system has a base speed of 3000 rpm.
The next most complex motor has two sets of three phase windings, and is called a "4-pole motor". It is the most common motor produced, and has a 60 Hertz base speed of 1800 rpm (the 50 Hertz speed is 1500 rpm).
6-pole motors operate at 1200 rpm (1000 rpm at 50 Hertz). 8-pole motors operate at 900 rpm (750 rpm at 50 Hertz).