CHAPTER 3: RELAY

RELAY

3.1 INTRODUCTION OF RELAYS

The relay is an electromagnetically device.The relay offers a simple ON/OFF switching action and response to a control signal.

3.2 RELAY PRINCIPLE

     The electrical relay offers a simple ON/OFF switching action in response to a control signal.Figure 3.2 illustrates the principle.When a current flows through the coil of wire a magnetic field is produced.This pulls a movable arm that forces the contact to open or close.This might then be used to supply a current to a motor or perhaps an electric heater in a temperature control system.

     Time-delay relays are control relays that have a delayed switching action.The time delay is usually adjustable and can be initiated when a current flows through the relay coil or when it ceases to f;ow through the coil.

3.3 APPLICATION OF RELAYS IN INDUSTRIES

     Relays are used in the control of fluid power valves and in many machine sequence controls such as boring,drilling,milling and grinding operations.

3.4 SYMBOL OF RELAYS

Below is the common electrical symbols of relay based on the function of relay.

3.5 SPECIFICATION OF RELAY

 

      The standard voltage for relay used in machine control is 120 volt.The coils on electro-mechanical devices such as relay,contactors and motor starter are designed so as not to drop out(de-energize) until the voltage drops to minimum of 85% of the rated voltage.The relay coil also will not pick up (energize)until the voltage rises to 85% of the rated voltage.This voltage level is set by the National Electrical Manufacturer Association (NEMA).

3.6 RELAYS AND SWITCH LOGIC CIRCUIT

       Relays are widely applied in electromagnetic devices.Figure 3.6(a) and 3.6(b) shows a typical relays appearance.When the relay is not energized,the spring keeps the armature away from the coil.This produces an air gap and the main contact presses against the normally closed contact.’When the relay is energized,the armature is attracted and moves toward the coil.This eliminates the air gap and the main contact touches the normally open contact and completes that circuit.The circuit with normally closed contact is opened.The relay acts as a single -pole double-throw switch.Many different contact arrangement are possible.

 

 

     

     Relays require a given current for pull-in.Once they pull-in,less current is require to hold them in the closed position.This is because the air gap is eliminated when the armature pulls in.The air has quite a bit more reluctance than the iron circuit and eliminating it means that less emf is required to overcome the spring tension.

   The switch logic circuit application in relay (Figure 3.6(c) shows a relay with (NO contacts).One contact is used as an interlock around the START push button.Thus, a interlock circuit is a path provided for electrical energy to the load after the initial path has been opened.The second relay contact is used to energize a light.Remember that when a relay coil is energized,the NO contacts close.The circuit can be DE-energized by operating the STOP push-button switch.