An electric motor is a machine that converts electrical energy into mechanical energy, or torque. This rotatory mechanical energy is harnessed to perform a task, such as to lift and lower an elevator. The electric motor is a basic type of motor used in industry, transportation, homes, and elsewhere. Like a generator, it also consists of stator and a rotor. However, it works in an inverse fashion; in an electric motor, it is the stator windings electromagnetic fields that make the rotor turn around and produce mechanical energy, whereas in a generator, it is the rotating rotor winding and electromagnet that induces (produces) AC current on the stator winding; in other words, in a generator, electricity is produced by the mechanical energy of the rotating rotor.
Electric motors can be classified by the kind of current used for their drive. The DC motors have the advantage of an economical and smooth regulation of their rotational speed (rpm). The AC motors include synchronous and asynchronous electric motors. In a synchronous motor the rotational speed (rpm) is rigidly dependent on the frequency of the feeder current. In an asynchronous motor the rotational speed decreases as the load increases. A third type of alternating current motor is the commutator motor, which permits a smooth regulation of rotational speed within wide limits.
The asynchronous motor is the most widely used, because it is simple to manufacture and is reliable in operation (particularly the squirrel-cage motors). In the industry, three-phase alternating current (AC) is used to power a heavy-duty electric motor. Their main disadvantages are a considerable consumption of reactive power and the lack of a smooth (gradual) speed regulation. This is the reason why in many high-power electric drives, synchronous electric motors are being used instead of the asynchronous one. However, DC motors are utilized if speed regulation is of paramount importance; the more expensive and less reliable AC commutator motors are also very occasionally used in these cases.
The power rating of electric motors ranges from a fraction of a watt to dozens of megawatts. Electric motors have various forms of frame construction: open frame, in which the rotating and current-carrying parts are protected against accidental touching and foreign objects; protected frame (including drop-proof and spray-proof designs); closed frame (dust-proof and moisture-proof); hermetic frame; and explosion-proof frame (in case of an explosion of gases inside the motor, any flame is confined to the interior of the motor housing).
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| Above, a three-phase asynchronous electric motor. |
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| The stator and stator winding of an electric motor. |