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电机技术（电机技术（Electric Motor Technology）Lessons In Electric Circuits,Volume II-ACBy Tony R.Kuphaldt Sixth Edition,last update March 07,2007Theory, Construction, And Requirements T The principles behind electric motors have been around for almost two centuries， but the technology has come a long way during the past few years. Although Michael Faraday first showed the relationship between current flow and magnetic fields in 1821，it wasnt until 11 years later that these principles were used to demonstrate the first electric motor. Actually， it was the invention of the commutator that allowed electromagnetic forces to be coaxed into rotary motion. Once electrical power could be turned into mechanical power， it was easy to show how the reverse was possible. In need of a source for his electric light，Thomas Edison proposed generating DC power to customers in 1882. Meanwhile， Nikola Tesla investigated brushless AC induction motors. By 1888， he was ready to oppose Edison by proposing the use of AC for power distribution. Despite some dirty campaign tactics by Edison，we all know how that contest turned out.Because of the importance of electric motors，I will spend some time this month examining their construction， theory，and what's required to use them. What began as a real distinction between DC and AC motors has become muddled due to the introduction of electronics to provide smart control. But lets start off with a simple look at electromagnetics.ELECTROMAGNETIC FORCEAt the center of our solar system，the sun goes through a sunspot cycle every 11 years. Sunspots are areas of intense magnetic fields that poke through the photosphere and are eventually drawn toward its attracting magnetic pole. When this collected opposition overpowers the suns dipole state，a flux reversal flips the dipole state. This is consistent with maximum sunspot activity and the 11-year cycle.The Earth has a record of flip-flopping magnetic poles， but the period is much longer. The last one occurred about 740，000 years ago. Needless to say， we have never experienced one， and we dont know how it might affect life as we know it. Theories suggest that the Earth's moving magnetic field may be due to the uneven flow of the outer liquid core deep in the Earths layers. Plate tectonics show us how irregular and continuously changing the surface is， so it's easy to see that were not dealing with smooth surfaces and ideal spheres rotating in perfect balance on axis. However life will be affected by this drifting magnetic field （and internal currents） ， we can find consolation in the relationship between a magnetic field and an electric current.Some materials that are largely made up of iron ore （magnetite） can be converted into a lodestone （magnetized mineral） by having the normally random magnetic fields of its electrons aligned. This can come from a large current passing through the conductive material. Extreme currents bring extreme magnetic fields that can permanently affect electron spin. When the external force has subsided， the magnetic alignment can remain， giving the material a permanent magnetic field. The first compass was made by floating this permanently magnetized material on water. With the water acting as a frictionless bearing，the lodestone can rotate and align itself with the magnetic field of the Earth （north/south）.The relationship between magnetic fields and current was discovered by Hans Christian sted in 1820 when he noticed a compass needle deflecting from magnetic north when he applied electric current to a wire running close to the compass. The righthand grip rule demonstrates the direction of a magnetic field around a wire when the electric current is flowing in the direction of the thumb（see Figure 1）.In 1825， British electrician William Sturgeon showed how the magnetic field produced from a current in a wire could be increased without additional current by stacking the wire in parallel。 His electromagnet was a horseshoe-shaped piece of iron that was wrapped with several turns of the wire. The iron core acts as a concentrator for the magnetic field produced by current flow in the wire. You can cause the north/south orientation of an electromagnetic core to swap polarities if the coil is wound in the opposite direction or if the direction of the current through the wires is reversed. Figure 2 shows a representation of the attraction of opposing magnetic poles and the repulsion of like magnetic poles. By controlling the direction of current flow in the coil of an electromagnet，you define its pole orientation. The right-hand grip rule can also show the north magnetic orientation（thumb） when the fingers simulate the current flow direction in the wires of a coil.THE DC SOURCEN Not counting Leyden jar capacitors（circa 1745） ， early scientists were restricted to one source of power， the electric pile， or battery. Alessandro Volta experiments with the rtificial electrical organ named