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Many electrical systems include electromagnetic contact relays in their control and actuation circuits. Essentially, such relays comprise an electromagnet and a moving armature carrying metallic blades (Fig. 4.1). Each blade is attached to the armature at one end. The other Open image in new window Fig. 4.1 Contact relay. end is free and can, according to the position of the armature, either be free or in contact with a contact point mounted on a blade affixed to the relay. The assembly of the blade and the contact point associated with it constitutes what is known as a “contact” which may either be open or closed. Dependant upon whether or not the coil of the electromagnet be excited by the passage of a current, the moving armature takes up one of 2 positions: so long as no excitation current is applied the armature takes up the “rest” position; when an excitation current is applied the armature is attracted into a second position (“attracted” position). Thus it is possible, in general, to recognise two, principal types of contacts (Figs. 4.2 & 4.3): Open image in new window Fig. 4.2 Normally open contact (“make contact”). Open image in new window Fig. 4.3 Normally closed contact (“break contact”). |
