Operation of control Relay

An electrical relay is a magnetic switch. It uses electromagnetism to switch contacts. A relay will usually have only one coil but may have any number of different contacts. Figure 6-2 illustrates the operation of a typical control relay. With no current fl ow through the coil (deenergized), the armature is held away from the core of
the coil by spring tension. When the coil is energized, it produces an electromagnetic fi eld. Action of this field, in turn, causes the physical movement of the armature. Movement of the armature causes the contact points of the relay to open or close. The coil and contacts are insulated from each other; therefore, under normal conditions, no electric circuit will exist between them. The symbol used to represent a control relay is shown in Figure 6-3 . The contacts are represented by a pair of short parallel lines and are identifi ed with the coil by means of the letters. The letter M frequently indicates a motor starter, while CR is used for control relays. Normally open (NO) contacts are defi ned as those contacts that are open when no current fl ows through the coil but that close as soon as the coil conducts a current or is energized. Normally closed (NC) contacts are closed when the coil is de-energized and open when the coil is energized.Each contact is usually drawn as it would appear with the coil de-energized. Control relay coils and contacts have separate ratings.
Coils are rated for the type of operating current (DC or AC) and normal operating voltage. Contacts are rated in terms of the maximum amount of current the contacts are capable of handling at a specifi ed voltage level and type(AC or DC). Control relay contacts generally are not normally designed to carry heavy currents or high voltages. The contacts are usually rated between 5 and 10 amperes, with the most common rating for the coil voltage being 120 VAC.