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| 10 This formula gives a result in revolutions per second, which can be multiplied by 60 to provide the number of revolutions per minute. As the speed of the motor is related to the frequency of the supply, if a different speed is required, then a variable-frequency drive can be used to alter the supply frequency to attain the necessary speed. AC motors can be used in single- phase or three-phase topology. However, a single-phase design would not have enough poles to generate a rotating field, and extra winding will be required to provide it. The additional winding must be out of phase with the supply to generate the rotating field; therefore, a device, usually a capacitor, is used to change the phase. AC Induction Motors AC induction motors (also known as asynchronous motors) are often chosen for their simplicity. They have only one moving part—the rotor—which makes them robust and reliable. Their simple design also means that they are very cost- effective. Additionally, it is possible to run them directly from the mains, making installation easier. The rotor of an AC induction motor can be constructed in several different ways, including as a metal axle or a loop of wire. Still, it is most frequently a "squirrel cage" design for higher efficiency. This type of motor is made from parallel conductive metal bars, usually copper or iron, connected at each end to a ring manufactured from the same material (Figure 2). The squirrel cage assembly is built around an iron core. The rotating magnetic field induces a current in the rotor bars, which, in turn, generate magnetic fields that interact with the original magnetic field from the stator. This process produces force on the rotor bars, causing the rotor to rotate to "catch up" with the stator electric field. However, if the rotor rotates at the same speed as the magnetic field from the stator, no voltage would be induced in the rotor bars. If a load is applied, it slows the speed of the rotor, and the interaction of the two electric fields generates torque, which drives the load. The larger the load, the more the rotor will slow. The percentage difference between the speed at which the stator field rotates and the rotor speed is called the slip, which can be calculated using the following formula: Figure 1: Almost all AC motors use windings to generate a rotating magnetic field in the st tor. (Source: "Meni-clicks/stock.adobe.com") Figure 2: Squirrel cage rotor assembly. (Source: "Madalin/stock.adobe.com")