Most electric motors operate by turning an armature, which is a cylindrical metal core (made of coils of wire) in a magnetic field set up by several permanent magnets attached to the periphery of a rotor. The armature turns a shaft in the center of the permanent magnets. Electric motors come in several variations, including induction motors, synchronous motors, and stepping motors.
Induction motors make a variety of noise, including the whine of the magnetic field in the motor, and the squeal of the bearings. The noise is especially noticeable when the motor is running near full load. Induction motors are widely used for low-speed, low-torque power transmission, such as the electric motors in small appliances and electric cars. Because of the low cost of production and widespread use, induction motors are a good choice for most applications, in particular where size is not an issue.
Another type of electric motor is the synchronous motor. Synchronous motors avoid the problems associated with induction motors by having a nonmagnetic shaft. A synchronous motor has a rotor and armature in which the armature is the stator, and the rotor is the electromagnet (field winding) that opposes the armature current. The armature and rotor are aligned and in a fixed relationship with each other. The rotor is given a permanent magnet, and the armature is given a winding. When the rotor is turned by an electric current, the armature poles turn in the same direction. The armature poles are aligned with the rotor poles, and a torque is produced.
Induction motors, by far the most common type, use a rotating magnetic field to create a force on the armature. An electric current in an armature winding creates a magnetic field in the rotor, and the magnetic field interacts with a current in the armature winding to create an opposing force. This turns the armature, which turns the shaft. The armature and the shaft are usually mounted on bearings, and the shaft is connected to an external load.
Stator windings are the part of the stator that is connected to the mains. The load can be attached directly to the stator windings, or indirectly through mechanical linkage. In motors of this type, the stator and rotor have a permanent magnet arrangement, with a magnetic flux that passes through the armature. There are no field windings on the stator. The armature is free to rotate, and supports the load. The armature can be made of an electromagnetic material, such as laminated silicon steel. The armature can be constrained magnetically, or mechanically. The flux in the armature can be either closed or open. In the open flux armature, the flux may pass through the armature or it may be distributed in the copper. Closed flux armature motors have the flux concentrated inside the armature. In general, the flux is closed in the stator and open in the armature, as in most electrical motors. Motors with closed flux in the armature are also referred to as internally wound motors. In many cases, the permanent magnet is the rotor, which is free to rotate; this is known as an IPM motor. An IPM motor has a rotor and a stator that are completely separated. Several different types of permanent magnet motors exist, including direct current (DC) motors, induction motors, synchronous motors, and switched reluctance motors. These differ in the method of magnetization and flux distribution. In switching reluctance motors, the stator is divided into sections, and the rotor has a number of salient poles that are lined up with the flux in the stator. 827ec27edc