Electrical Motors and Meters
Electrical motors are devices that convert electrical energy into mechanical energy. Electric meters use electric motors and counting devices to translate current flow into a measured use of electrical energy. Electrical motors perform the inverse transformations performed by electrical generators (i.e., the transformation of mechanical energy into electrical energy). To a limited extent, all electrical motors perform these inverse transformations as they operate to produce a counter or back electromotive force.
The modern form of the electrical generator evolved from the research and engineering work of nineteenth-century Danish scientist Hans Christian Ørsted and English physicist Michael Faraday who developed a rotator (later to be known as an electric motor) containing a conductive (i.e., electrical current carrying) wire rotating around a magnet. Working independently, American inventor Joseph Henry also developed an early model of the electric motor. Although it was another half century before widespread production, the first practical electric motors were all designed according to principles first documented by Faraday and Henry. Moreover, Faraday's work with electromagnetic induction in 1831 formed the basis of a collection of papers eventually published as Experimental Researches in Electricity, a publication that became the standard authoritative reference on electricity and magnetism for nineteenth-century scientists and inventors. During the last decades of the nineteenth century, to serve the needs of the European and American industrial revolutions, electric motors drove an increasing number of time and labor saving machines. Safer and more productive than steam or combustion engines, electrical motors took on an important role in the creation of modern technological society.
Electrical motors contain a set of magnets upon which the forces created by an electrical current act. An electrical motor contains a spinning electromagnetic armature that interacts with a set of stationary field magnets. All of the magnets contain north and south poles and these poles are mutually repulsive to one another (i.e., like poles repel and opposite poles attract). As electrical current flows through a conducting coil with the proper orientation to the magnets, it also produces a magnetic field that likewise contains north and south poles. The interaction of the armature's north and south poles with the north and south poles of the field magnets and the magnetic field induced by the electrical current produces a torqueforce on the armature.
As a result of the application of torque to the armature, the armature spins. DC motors (drawing DC or direct current) utilize split-ring commutator devices containing conducting brushes that allow current to flow to the armature to regulate the reversal of magnetic poles. As the armature rotates, its reversing magnetic poles interact with the poles of the permanent field magnets to produce continuing torque. The armature can be connected to a shaft that, as armature rotates, is capable of doing work (e.g., to drive such labor-saving devices as power tools, fans, etc.).
When electrical motors first start, they require a greater quantity of electrical current to spool up to operating speed than they do to maintain their operating speed. For example, when large motors are turned on they often dim lights hooked to the same power source as they require this extra current. The extra current requires greater electromotive force (emf). In fact, because all electrical motors have current-carrying coils that turn in an electric field, all electrical motors also generate emf. In electrical motors this emf is termed back emf, and it acts against the applied emf driving the motor. Because of increased friction during motor start-up, the back emf can significantly reduce the efficiency of the applied emf. This requires the device to draw significant amounts of extra current until the device reaches efficient operating speed.
Electric motors can range in size and power from those able to drive large ships to microscopic devices incorporated into computer hardware.
Most electrical meters utilize electric motor-driven shafts and a series of gears to measure the use of electrical energy on an analog counter consisting of a series of rotating disks. A known and regulated fraction of the total current flowing through a circuit is utilized to operate the meter, and the speed of the motor is directly proportional to the current flow. Accordingly, as the current flows faster, the motor spins faster and the counter reflects the increased usage. The gear-driven rotating disks are usually marked in increasing powers kilowatt-hours of (1,000 watt-hours).
Induction-type meters measure alternating current circuit usage, mercury- based and commutator-type meters are used to measure usage in DC circuits.
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