What Do Motors and Generators Have in Common?
Electric motors and generators are electromagnetic devices that are the opposite of each other. Electric motors convert electrical energy into mechanical energy, while electric generators convert mechanical energy into electrical energy.
Both motors and generators have current-carrying loops that rotate in magnetic fields. They operate on the principle of Faraday's Law of Induction, where a rapidly changing magnetic field produces an induced electromotive force (emf).
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Electromagnetic Induction
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Wires that carry current produce magnetic fields. Michael Faraday discovered that when a wire is in a changing magnetic field, a current is induced, just as if there were a source of emf. This source is called an induced emf or voltage, and the more rapid the changing field, the greater the emf that is produced.
Construction
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The current-carrying loops common to both motors and generators that continuously rotate in a magnetic field are wrapped around an iron core called an armature that makes the magnetic field inside of them stronger. The armature and hence the loops constantly rotate because the loop current in the loops reverses direction. The changing direction of the loops causes an induced emf to be generated.
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AC Motors and Generators
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Alternating Current (AC) generators are the opposite of motors, because they convert mechanical energy into electrical. The loops in the magnetic field are rotated by mechanical energy sources. The generated emf is a sine wave that varies in time. Steam made from burning fossil fuels such as coal, oil and natural gas is a common source in countries like the United States. In Europe, the steam is created with the use of nuclear fission. Hydroelectric plants, such as those found at Niagara Falls, use water pressure to rotate the turbines, which are rotors with vanes or blades. Wind and water are not commonly used as mechanical energy sources because they are not as efficient and are more costly.
AC motors are used to convert electrical into mechanical energy. The loops in the magnetic field are rotated with the use of alternating current. This current is usually produced via induction; an electromagnet causes the magnetic field and uses the same voltage as the coils.
DC Motors and Generators
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Direct Current (DC) motors and generators are similar to their AC counterparts, save that they have a split ring called a commutator. The commutator is attached to electrical contacts called brushes. The changing direction of the current through the commutator causes the armature and thus the loops to rotate. The magnetic field in which the armature turns may be a permanent magnet or an electromagnet.
DC generators have a generated emf that is direct current. These generators have largely been supplanted by AC ones. DC motors are preferred over AC for devices where a variable speed and torque control relationship is primary, such as in drills.
Motors v. Generators
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Basically motors are generators, and one can be converted into the other. When motors operate they produce an emf, just like generators do. It is called a back emf, and does not immediately appear when they are initially turned on. The back emf works to reduce the current in the loop, thus slowing the motor down, and gets larger as the speed of the motor increases. This causes the power requirements of the motor to also increase, especially under loads that are very large. This contributes to energy waste and inefficiency in their operation.
Uses
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AC generators are used as a source of alternating current in buildings, electronic devices, emergency backup generators and circuits. In cars, AC generators are used to charge the battery in the engine.
AC motors are found in refrigerators, trains, computers, industrial machinery and household appliances, and many other electronic devices. They are used in some engines in electric cars.
DC generators are used to charge the battery in very old cars. DC motors are found in hobby motors, machine tools where the speed needs to be changed on the fly, portable electronics and some industrial machinery.
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References
Resources
- Photo Credit Michael180
