This chapter will describe some of the applications of electromagnetism which are commonly in use. It should be noted, however, that no complete list of such appli- cations is possible, because this list is almost endless. The principles put forward in Chapter 6 apply to all these devices.
Two of the most important types of electromagnetic equipment are not men- tioned in this chapter. These are the generator and the motor, which are introduced in Chapters 9 and 11, respectively.
7.2 Bells and buzzers
The operation of bells and buzzers is identical, and units of these types are used widely to give audible warnings. To a large extent, electromagnetic bells and buzzers have now been replaced by electronic sounders, but they still hold a very important place. There are a number of types of electromagnetic bell.
The signal-stroke bell has applications where simple signalling is necessary. It consists of an electromagnet which, when energised, attracts a soft-iron strip called an armature against the pull of a flat spring strip. A striker supported by the armature sounds the gong (Figure 7.1). When the current is switched off by releasing the signalling push, the spring strip returns the armature to its original position.
The trembler bell is the most widely used type, and is similar in construction to the single-stroke bell, but has the addition of a set of contacts P which are opened by movement of the armature towards the electromagnet (Figure 7.2). This de-energises the magnet and allows the armature to return to its original position, closing the contacts and repeating the cycle. While the supply is maintained, the armature is in a continual state of vibration, its striker hitting the gong repeatedly. The rate of striking depends on the flexibility of the spring strip, and the weight and size of the moving system.
The buzzer has no striker and normally oscillates more rapidly; some adjustment of movement, and hence of buzzer note, being possible with the contact screw.
The continuous-ringing bell is particularly useful for alarm systems, since it will continue to ring after the external operating circuit has been broken. The construction
spring strip
armature coils
gong striker
Figure 7.1 Single-stroke bel
spring strip
P
striker gong
Applications of electromagnetism 125 pivot relay arm pull to reset striker gong coils
Figure 7.3 Continuous-ringing bell
Figure 7.4 Polarised bell
(Figure 7.3) includes a relay arm which drops when the bell first rings, closing an internal operating circuit, which continues to ring the bell until reset by lifting the relay arm to its original position. This can be done by means of a pull cord or by a solenoid operated remotely.
The polarised bell is mainly used for telephone circuits where a low-frequency AC supply is used. A permanent-magnet system (Figure 7.4) is fitted with two oper- ating coils connected in series but wound in opposite directions. Alternating current
will alternately weaken and strengthen the magnetic fields in the side limbs, one being strengthened while the other is weakened. The moving system will thus be attracted to each side in turn, the striker sounding each bell in turn.
7.3 Bell indicators and circuits
There are many applications of systems which require that a large number of widely spaced pushes should be used to indicate, at one central position, that service is required. Hospitals, hotels, restaurants and many other semi-public buildings need to be so equipped. Fire and burglar alarm systems are other examples of installations where the point of origin of the alarm may need to be indicated at some central or remote point.
The range of bells and buzzers with differing tones is strictly limited so the indicator board was developed to cater for this situation. Usually, only one bell or buzzer sounds, the board giving a clear indication of the position of the push that was operated. Electronic indicator boards, with filament lamps or light emitting diodes (LEDs) which are kept on by electronic latching circuits until reset, are very com- mon. These are not electromagnetic devices and thus have no place in this chapter. Electromagnetic types are still common, however, so their operation is described.
There are three basic types of electromagnetic indicator unit:
1 Pendulum-type indicator: The indicator ‘flag’ is attracted to a solenoid while the press is pushed, falling away and swinging to give indication when it is released. This is a simple and inexpensive system, but the indication ceases after a time when the flag stops swinging. The pendulum type has largely been replaced by the two other types.
2 Mechanical-reset-type indicator: The coloured disc of this unit is not normally seen through the window on the front of the indicator, but is pulled into view by a solenoid coil when the circuit concerned is energised. It then remains in the indicating position until reset by the mechanical operation of a rod or lever. 3 Electrical-reset-type indicator: This is similar in operation to the mechanical-
reset type, but is returned to the normal position by a reset solenoid operated from a control push normally situated adjacent to the board (Figure 7.5).
operating coil reset coil
Applications of electromagnetism 127 transformer bell mains supply bell push 8V 4V indicator unit
Figure 7.6 Four-way indicator unit and circuit
Units of these types are made up into boards containing any required number of ways, each way suitably labelled. Figure 7.6 shows how a four-way indicator board can be connected. Additional boards may be connected in series with the first if indication at other points is necessary.
7.4 Relays and contactors
Relays
A relay is similar in construction to the single-stroke bell, but a set of contacts are opened or closed by operation, instead of a gong being struck. This enables one circuit to operate another (Figure 7.7), a very low current often being sufficient to close the contacts of a relay, and thus operate a high-power circuit. Some relays have a large number of contacts, and can be used in complicated circuits for a wide variety of switching purposes. As in other applications, electronics is becoming common in electromagnetics, and many relays are now solid state, using thyristors or triacs.
A relay usually has an operating coil wound on a magnetic circuit with a mov- ing section, or armature, held open by a spring. When the operating current is switched on and sets up a magnetic field, the armature is closed and operates the contacts.
Contactors
Contactors are simply very large relays, enabling a heavy load to be switched on and off with a very small operating current. A few applications are as follows.
thermostat
main contacts supply heater
Figure 7.7 Principle of relays and contactors
1 Motor starting: the contactor coil, and hence the motor, is controlled by ‘stop’ and ‘start’ pushbuttons; the contactor also ensures that the supply leads to the motor are broken in the event of a supply failure, so that the motor cannot restart automatically when the supply is restored.
2 Timeswitch contactor: a heavy heating load can be switched by a timeswitch with low-rated contacts; the timeswitch controls the coil of the contactor, which controls the load.
3 Remote switching: the principle is shown in Figure 7.7.
7.5 Telephones
Operation of a telephone is based on the fact that noise consists of a series of pressure waves in the air. These pressure changes are caused by the vocal chords in speech, and alternate bands of high and low pressure moved outwards in ever-widening circles from the noise source, like the ripples on a pond into which a stone has been thrown. These waves vibrate the eardrums, making it possible to hear the sound.
The telephone changes these variations in pressure into corresponding changes in an electric current, which can be made to flow for long distances before giving up energy to convert the current variations back into pressure changes.