and function of relay ladder diagrams.
RELAY
OBJECTIVES
General Objective : To apply the concept of relay.
Specific Objectives : At the end of the unit you should be able to:
· Identify the main uses for a relay
· Identify the application of relay
· Show how the control relay is constructed mechanically
· Identify the specification of relay according to NEMA standard
· Draw the relay and switch logic circuit
· Identify the function of ladder-type diagram of relay
· Draw the schematic diagram and the ladder-type diagram of relay
3.1 INTRODUCTION OF RELAYS
The relay is an electromechanical 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 flow 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 RELAY
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 electromechanical devices such as relays, contactors and motor starters are designed so as not to drop out (de-energize) until the voltage drops to minimum of 85% of the rated voltage. The relay coils 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 RELAY AND SWITCH LOGIC CIRCUIT
Relays are widely applied in electromagnetic devices. Figure 3.6(a) and 3.6(b) shows a typical relay 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 arrangements are possible.
Relays require a given current for pull-in. Once they pull in, less current is required 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 mmf is required to overcome the spring tension.
The switch logic circuit application in relay ( Figure 3.6( c ) shows a relay with two NO contacts). One contact is used as an interlock around the START push button. Thus, an interlock ircuit 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.
Figure 3.6 (d) shows the addition of a selector switch, fuse, pilot light and a second relay. When the selector switch is operated to the ON position, electrical energy is available at the two vertical sides of the circuit. The green light is energized, showing that the operation has been completed.
One additional relay contact is added in the circuit from relay 1 CR. This contact closes when the relay1 CR is energized and it, in turn, energizes a second relay coil 2 CR. The operating circuit can be de-energized by operating the STOP push-button switch.
