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9 | Demystifying Motor Selection In the first article of this series on electrical motors, we looked at AC and DC motors and gave an overview of their operation. Since those motor designs were first introduced, they have been improved and now feature several subcategories. This article will go into more depth on the most important subcategories and look at the potential applications for each type. AC Motors Two main types of AC motors are used extensively today: induction and synchronous. All AC motors discussed in this article rely on the stator to create a rotating magnetic field, which interacts with the rotor's magnetic field to produce torque. The main difference between the designs is how the rotor generates that magnetic field. The motors are built with a casing that is hollow in the center. In all examples here, the stator comprises a ring of electromagnets situated around the inside of the casing (Figure 1). The electromagnets are arranged in pairs of poles—with at least one pair for every phase of the supply— and wound so that opposite poles are directly across the ring from each other. A motor with one pair of poles per phase is referred to as a two-pole configuration, and one with two pairs of poles per phase would be a four-pole configuration. The number of poles affects the speed of the rotor and the torque. When an AC supply is connected, current flows in the coils to produce a magnetic field. The sinusoidal nature of an AC supply means that the magnetic field rotates around the rotor. A motor with a single pair of poles per phase would rotate once per cycle of the supply, two pairs once every two cycles, and three pairs once every three cycles. The magnetic field's rotation speed can be calculated using the following formula: Image Source: "Surasak / stock.adobe.com"