The dc motor

Chapter The dc motor

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • When there is no current flowing in the coil, the steel rod is held down by the forceof gravity. But when a pulse of current passes through the coil, the rod is pulled forciblyupward so that it strikes the ringer plate. This plate is like one of the plates in a xylo-phone. The current pulse is short, so that the steel rod falls back down again to its rest-ing position, allowing the plate to reverberate: Gonggg! Some office telephones areequipped with ringers that produce this noise, rather than the conventional ringing orelectronic bleeping ernitted by most phone sets.A relayIn some electronic devices, it is inconvenient to place a switch exactly where it shouldbe. For example, you might want to switch a communications line from one branch toanother from a long distance away. In many radio transmitters, the wiring carrieshigh-frequency alternating currents that must be kept within certain parts of the cir-cuit, and not routed out to the front panel for switching. A relay makes use of a sole-noid to allow remote-control switching.A diagram of a relay is shown in Fig. 8-8. The movable lever, called the armature,is held to one side by a spring when there is no current flowing through the electro-magnet. Under these conditions, terminal X is connected to Y, but not to Z. When a suf-ficient current is applied, the armature is pulled over to the other side. This disconnectsterminal X from terminal Y, and connects X to Z.There are numerous types of relays used for different purposes. Some are meant foruse with dc, and others are for ac; a few will work with either type of current. A normallyclosed relay completes the 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 in the illustration (Fig.8-8) can be used either as a normally open or normally closed relay, depending on whichcontacts are selected. It can also be used to switch a line between two different circuits.Some relays have several sets of contacts. Some relays are meant to remain in onestate (either with current or without) for a long time, while others are meant to switchseveral times per second. The fastest relays work dozens of times per second. These areused for such purposes as keying radio transmitters in Morse code or radioteletype.The dc motorMagnetic fields can produce considerable mechanical forces. These forces can be har-nessed to do work. The device that converts direct-current energy into rotating me-chanical energy is a dc motor.Motors can be microscopic in size, or as big as a house. Some tiny motors are beingconsidered for use in medical devices that can actually circulate in the bloodstream orbe 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, producingmagnetic fields. The attraction of opposite poles, and the repulsion of like poles, isswitched in such a way that a constant torque, or rotational force, results. The greaterthe current that flows in the coils, the stronger the torque, and the more electrical en-ergy is needed.The dc motor145