Electric machine

In electrical engineering, electric machine is a general term for electric motors, electric generators and other electromagnetic machines. They are electromechanical energy converters: an electric motor converts electricity to mechanical power while an electric generator converts mechanical power to electricity. The moving parts in a machine can be rotating (rotating machines) or linear (linear machines). Besides motors and generators, a third category often included is transformers, which although they do not have any moving parts are also energy converters, changing the voltage level of an alternating current.

Electric machines, in the form of generators, produce virtually all electric power on Earth, and in the form of electric motors consume approximately 60% of all electric power produced. Electric machines were developed beginning in the mid 19th century and since that time have been a ubiquitous component of the infrastructure. Developing more efficient electric machine technology is crucial to any global conservation, green energy, or alternative energy strategy.

When classifying electric machines (motors and generators) it is reasonable to start with physical principle for converting electric energy to mechanical energy. If the controller is included as a part of the machine all machines can be powered by either alternating or direct current, although some machines will need a more advanced controller than others. Classification is complicated by the possibilities of combining physical principles when constructing an electrical machine. It can, for example, be possible to run a brushed machine as a reluctance machine (without using the rotor coils) if the rotor iron has the correct shape.

Generally all electric machines can be turned inside out, so rotor and stator exchange places. All rotating electric machines have an equivalent linear electric machine where stator moves along a straight line instead of rotating. The opposite—linear to rotary dual—is not always the case. Motors and generators can be designed with or without iron to improve the path of the magnetic field (teeth to reduce the air gap is a common example) and with and without permanent magnets (PM), with different pole number etc., but still belong to different classes of machines. Electric machines can be synchronous meaning that the magnetic field set up by the stator coils rotates with the same speed as the rotor; or asynchronous, meaning that there is a speed difference. PM machines and reluctance machines are always synchronous. Brushed machines with rotor windings can be synchronous when the rotor is supplied with DC or AC with same frequency as stator or asynchronous when stator and rotor are supplied with AC with different frequencies. Induction machines are usually asynchronous, but can be synchronous, if there are superconductors in the rotor windings.

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