Magnetic Machines Help

Introduction

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.

A Relay

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.

Fig. 14-7 . ( a ) Pictorial drawing of a simple relay, ( b ) Schematic symbol for the same relay.

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.