Magnetic Machines Help (page 2)
A solenoid, having a movable ferromagnetic core, can do various things. Electrical relays, bell ringers, electric “hammers,” and other mechanical devices make use of the principle of the solenoid. More sophisticated electromagnets, sometimes in conjunction with permanent magnets, can be used to build motors, meters, generators, and other devices.
A Ringer Device
Figure 14-6 is a simplified diagram of a bell ringer. Its solenoid is an electromagnet. The core has a hollow region in the center, along its axis, through which a steel rod passes. The coil has many turns of wire, so the electromagnet is powerful if a substantial current passes through the coil.
Fig. 14-6 . A bell ringer using a solenoid.
When there is no current flowing in the coil, the rod is held down by the force of gravity. When a pulse of current passes through the coil, the rod is pulled forcibly upward. The magnetic force “wants” the ends of the rod, which is the same length as the core, to be aligned with the ends of the core. However, the pulse is brief, and the upward momentum is such that the rod passes all the way through the core and strikes the ringer plate. Then the steel rod falls back down again to its resting position, allowing the plate to reverberate. Some office telephones are equipped with ringers that produce this noise rather than conventional ringing, buzzing, beeping, or chirping emitted by most phone sets. The “gong” sound is less irritating to some people than other attention-demanding signals.
In some electronic devices, it is inconvenient to place a switch exactly where it should be. For example, you might want to switch a communications line from one branch to another from a long distance away. In wireless transmitters, some of the wiring carries high-frequency alternating currents that must be kept within certain parts of the circuit and not routed out to the front panel for switching. A relay makes use of a solenoid to allow remote-control switching.
A drawing and a diagram of a relay are shown in Fig. 14-7. The movable lever, called the armature , is held to one side by a spring when there is no current flowing through the electromagnet. Under these conditions, terminal X is connected to terminal Y but not to terminal Z. When a sufficient current is applied, the armature is pulled over to the other side. This disconnects terminal X from terminal Y and connects X to Z.
There are numerous types of relays, each used for a different purpose. Some are meant for use with dc, and others are for ac; some will work with either ac or dc. A normally closed relay completes a circuit when there is no current flowing in its electromagnet and breaks the circuit when current flows. A normally open relay is just the opposite. (Normal in this sense means “no current in the coil.”) The relay shown in Fig. 14-7 can be used either as a normally open or normally closed relay depending on which contacts are selected. It also can be used to switch a line between two different circuits.
These days, relays are used only in circuits and systems carrying extreme currents or voltages. In most ordinary applications, electronic semiconductor switches, which have no moving parts and can last far longer than relays, are preferred.
The Dc Motor
Magnetic fields can produce considerable mechanical forces. These forces can be harnessed to do work. The device that converts dc energy into rotating mechanical energy is a dc motor . In this sense, a dc motor is a form of transducer . Motors can be microscopic in size or as big as a house. Some tiny motors are being considered for use in medical devices that actually can circulate in the bloodstream or be installed in body organs. Others can pull a train at freeway speeds.
In a dc motor, the source of electricity is connected to a set of coils producing magnetic fields. The attraction of opposite poles, and the repulsion of like poles, is switched in such a way that a constant torque, or rotational force, results. The greater the current that flows in the coils, the stronger is the torque, and the more electrical energy is needed. One set of coils, called the armature coil , goes around with the motor shaft. The other set of coils, called the field coil , is stationary (Fig. 14-8). In some motors, the field coils are replaced by a pair of permanent magnets. The current direction in the armature coil is reversed every half-rotation by the commutator . This keeps the force going in the same angular direction. The shaft is carried along by its own angular momentum so that it doesn’t come to a stop during those instants when the current is being switched in polarity.
The Electric Generator
An electric generator is constructed somewhat like a conventional motor, although it functions in the opposite sense. Some generators also can operate as motors; they are called motor/generators . Generators, like motors, are energy transducers of a special sort.
A typical generator produces ac when a coil is rotated rapidly in a strong magnetic field. The magnetic field can be provided by a pair of permanent magnets (Fig. 14-9). The rotating shaft is driven by a gasoline-powered motor, a turbine, or some other source of mechanical energy. A commutator can be used with a generator to produce pulsating dc output, which can be filtered to obtain pure dc for use with precision equipment.
Fig. 14-9 . A simple type of ac generator.
Practice problems of these concepts can be found at: Magnetism Practice Test
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