Magnetism

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Magnetism
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Electricity and magnetism are two phenomena which occur hand in hand; we cannot have one without the other. This was demonstrated by Hans Christian Oersted (1777-1851), who showed that a compass needle brought near a wire carrying electrical current deflects. Similarly, if an electric current is passed through a looped wire (called an electromagnet), a magnetic field is created around the wire (Figure 13-2). The greater the current, the stronger the magnetic field will be. Electromagnets are used in a number of devices, including simple on/off switches and practically all electromechanical devices, from small toys and hair dryers to large electric motors in elevators and cable car trolleys.
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Motors and Generators
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Electric motors are devices that convert electrical energy into useful mechanical energy. The principle of operation of an electric motor is simple. If the coil of an electromagnet is placed between the poles of a magnet, the positive side of the wire and the north pole of the magnet are of the same charge and repel each other. The same is true of the negative side of the wire and the south pole of the magnet. The result is that the magnetic field exerts torque on the wire, which is deflected by half a turn and stops (Figure 13-3a). The electromagnet can now be rotated only if the direction of the current in the loop reverses, i.e. the polarity of the battery is changed. This is commonly accomplished by placing a commutator between the battery and the loop (Figure 13-3b). A commutator is a ring split in half – each side is in contact with a brush. As the wire loop continues to spin the polarity of the brushes reverse to assure that the loop and magnet remain of the same polarity. As long as a current is applied, the loop spins continuously. The rotors of practical motors are made of not one, but thousands of loops wound around a soft iron armature.
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Figure 13-2
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Electromagnet
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Figure 13-3
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Electrical motors.
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(a) Without a commutator, the loop is rotated by only half a turn and stops; (b) The loop can be made to spin continuously with a commutator.
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(a)
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(b)
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1R11R1R2V2R 110280.67R = 80.67 W= + = 0.0124P = = = 150 W
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308
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Electric motors can be driven by either direct current (DC) or alternating current (AC). The difference between DC and AC motors is in the way the magnetic fields are created. In DC motors, this is done by means of an electromagnet or a permanent magnet. In AC motors, the magnetic field is created by passing an alternating current through a stator such that the polarity changes just when the armature is lined up with the poles of similar polarity. Because AC motors do not use brushes, they are simpler to construct and require little maintenance.
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One of the main features of electric motors is their ability to produce torque as soon as they start. This is in contrast to internal combustion engines, where no torque is delivered until the engine attains a certain speed. In fact, this is why all cars running on petroleum require starter motors to operate. Furthermore, with only one moving part, electric motors are much simpler and have a much longer lifetime than internal combustion engines. Because of their ability to deliver peak torque at or near stall, electric motors are widespread in trolley cars, elevators, cranes, forklifts, and electric railroad locomotives. We will discuss the application of electric motors in electric and hybrid vehicles in the next chapter.
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Electric generators are devices that convert the rotational energy of turbines or spinning shafts into electrical energy. Electric generators work opposite to motors - a magnet is turned by some external means to induce current through a wire. In a typical power plant, a turbine shaft is directly connected to a magnet that spins inside wire coils.
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Depending on their application, there are different types of generators on the market. In synchronous generators, the rotor turns at exactly the same frequency as the electric grid; for a 60 Hz grid, it makes 60 revolutions per second, or 3,600 rpm (50-Hz or 3,000 rpm in Europe). These generators are often used in coal, oil, or nuclear power plants where fuels can be burned at a controlled rate, making the turbine rotate at a fixed velocity. In asynchronous generators, the magnetic field of the rotor and the electric field of the stator do not synchronize, but the rotor falls behind (slips). Wind turbines take advantage of variable slip generators by adjusting the slip (adjusting the resistance in the rotor winding) to allow the turbine to run faster as wind speed increases. Pole changing generators allow operators to choose the number of stator poles in order to change rotational speed and power of the turbine.
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==References==
==References==

Revision as of 18:16, 30 June 2010

Magnetism Electricity and magnetism are two phenomena which occur hand in hand; we cannot have one without the other. This was demonstrated by Hans Christian Oersted (1777-1851), who showed that a compass needle brought near a wire carrying electrical current deflects. Similarly, if an electric current is passed through a looped wire (called an electromagnet), a magnetic field is created around the wire (Figure 13-2). The greater the current, the stronger the magnetic field will be. Electromagnets are used in a number of devices, including simple on/off switches and practically all electromechanical devices, from small toys and hair dryers to large electric motors in elevators and cable car trolleys. Motors and Generators Electric motors are devices that convert electrical energy into useful mechanical energy. The principle of operation of an electric motor is simple. If the coil of an electromagnet is placed between the poles of a magnet, the positive side of the wire and the north pole of the magnet are of the same charge and repel each other. The same is true of the negative side of the wire and the south pole of the magnet. The result is that the magnetic field exerts torque on the wire, which is deflected by half a turn and stops (Figure 13-3a). The electromagnet can now be rotated only if the direction of the current in the loop reverses, i.e. the polarity of the battery is changed. This is commonly accomplished by placing a commutator between the battery and the loop (Figure 13-3b). A commutator is a ring split in half – each side is in contact with a brush. As the wire loop continues to spin the polarity of the brushes reverse to assure that the loop and magnet remain of the same polarity. As long as a current is applied, the loop spins continuously. The rotors of practical motors are made of not one, but thousands of loops wound around a soft iron armature. Figure 13-2 Electromagnet Figure 13-3 Electrical motors. (a) Without a commutator, the loop is rotated by only half a turn and stops; (b) The loop can be made to spin continuously with a commutator. (a) (b) 1R11R1R2V2R 110280.67R = 80.67 W= + = 0.0124P = = = 150 W 308 Electric motors can be driven by either direct current (DC) or alternating current (AC). The difference between DC and AC motors is in the way the magnetic fields are created. In DC motors, this is done by means of an electromagnet or a permanent magnet. In AC motors, the magnetic field is created by passing an alternating current through a stator such that the polarity changes just when the armature is lined up with the poles of similar polarity. Because AC motors do not use brushes, they are simpler to construct and require little maintenance. One of the main features of electric motors is their ability to produce torque as soon as they start. This is in contrast to internal combustion engines, where no torque is delivered until the engine attains a certain speed. In fact, this is why all cars running on petroleum require starter motors to operate. Furthermore, with only one moving part, electric motors are much simpler and have a much longer lifetime than internal combustion engines. Because of their ability to deliver peak torque at or near stall, electric motors are widespread in trolley cars, elevators, cranes, forklifts, and electric railroad locomotives. We will discuss the application of electric motors in electric and hybrid vehicles in the next chapter. Electric generators are devices that convert the rotational energy of turbines or spinning shafts into electrical energy. Electric generators work opposite to motors - a magnet is turned by some external means to induce current through a wire. In a typical power plant, a turbine shaft is directly connected to a magnet that spins inside wire coils. Depending on their application, there are different types of generators on the market. In synchronous generators, the rotor turns at exactly the same frequency as the electric grid; for a 60 Hz grid, it makes 60 revolutions per second, or 3,600 rpm (50-Hz or 3,000 rpm in Europe). These generators are often used in coal, oil, or nuclear power plants where fuels can be burned at a controlled rate, making the turbine rotate at a fixed velocity. In asynchronous generators, the magnetic field of the rotor and the electric field of the stator do not synchronize, but the rotor falls behind (slips). Wind turbines take advantage of variable slip generators by adjusting the slip (adjusting the resistance in the rotor winding) to allow the turbine to run faster as wind speed increases. Pole changing generators allow operators to choose the number of stator poles in order to change rotational speed and power of the turbine.

References

Further Reading

External Links