Figure 7.1 Electromechanical Relay (EMR)
Electromechanical Relay ( EMR) is a magnetic switch. It turns a load circuit ON or OFF by energizing an electromagnet, which opens or closes contacts in both electric and electronic circuits. EMRs may be used in the control of fluid power valves and in many machine sequence controls such as drilling, boring, milling, and grinding operations. A relay will usually have only one coil, but it may have any number of different contacts. Electromechanical relays contain both stationary and moving contacts.The moving contacts are attached to the plunger. Contacts are referred to as normally open (NO) and normally closed (NC).
Figure 7.2 Parts of Electromechanical relay
Basic parts and functions of electromechanical relays include:
- Frame: Heavy-duty frame that contains and supports the parts of the relay.
- Coil: Wire is wound around a metal core. The coil of wire causes an electromagnetic field.
- Armature: A relays moving part. The armature opens and closes the contacts. An attached spring returns the armature to its original position.
- Contacts: The conducting part of the switch that makes (closes) or breaks (opens) a circuit
Figure 7.3 Mechanical operation of Electromechanical relay
When the coil is energized, it produces an electromagnetic field. Action of this field, in turn, causes the plunger to move through the coil, closing the NO contacts and opening the NC contacts. Normally open contacts are open when no current flows through the coil but closed as soon as the coil conducts a current or is energized. Normally closed contacts are closed when the coil is deenergized and open when the coil is energized. Each contact is usually drawn as it would appear with the coil deenergized. Most machine control relays have some provision for changing contacts normally open to a normally closed, or vice versa. It ranges from a simple flip-over contact to removing the contacts and relocating with spring location changes.
In general, control relays are used as auxiliary devices to switch control circuits and loads such as small motors, solenoids, and pilot light. The EMR can be used to control a high-voltage load circuit with a low-voltage control circuit. This is possible because the coil and contacts of the relay are electrically insulated from each other. From a safety point of view, this circuit provides extra protection for the operator. Another basic application for a relay is to control a high-current load circuit with a low-current control circuit.This is possible because the current that can be handled by the contacts can be much greater that what is required to operate the coil.
Sources :
http://www.galco.com/comp/prod/relay.htm
Petruzella, Frank. (1995). Industrial Electronics. Mc Graw Hill.202-204
