Simple direct-current motors consist of a magnet or electromagnet (the stator), and a coil (the rotor) which turns when a current is passed through it because of the force between the current and the stator field. So that the force keeps the same sense as the rotor turns, the current to the rotor is supplied via a commutator – a slip-ring broken into two semicircular parts, to each of which one end of the coil is connected, so that the current direction is reversed twice each revolution.
For use with alternating-current supplies, small DC motors are often still suitable, but induction motors are preferred for heavier duty. In the simplest of these, there is no electrical contact with the rotor, which consists of a cylindrical array of copper bars welded to end rings. The stator field, generated by more than one set of coils, is made to rotate at the supply frequency, inducing (see electromagnetic induction) currents in the rotor when (under load) it rotates more slowly, these in turn producing a force accelerating the rotor. Greater control of the motor speed and torque can be obtained in "wound rotor" types in which the currents induced in coils wound on the rotor are controlled by external resistances connected via slip-ring contacts.
In applications such as electric clocks, synchronous motors, which rotate exactly in step with the supply frequency, are used. In these the rotor is usually a permanent magnet dragged round by the rotating stator field, the induction-motor principle being used to start the motor.
The above designs can all be opened out to form linear motors producing a lateral rather than rotational drive. The induction type is the most suitable, a plate analogous to the rotor being driven with respect to a stator generating a laterally moving field. Such motors have a wide range of possible applications, from operating sliding doors to driving trains, being much more robust than rotational drive systems, and offering no resistance to manual operation in the event of power cuts. A form of DC linear motor can be used to pump conducting liquids such as molten metals, the force being generated between a current passed through the liquid and a static magnetic field around it.
Related categories• ELECTRICITY AND MAGNETISM
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