Archive for January 22nd, 2012

Industrial Control Basics – Electric Relay Ladder Diagram

Sunday, January 22nd, 2012
     My daughter will be studying for her driver’s license exam soon, and I can already hear the questions starting.  “What does that sign mean?  Why does this sign mean construction is ahead?”  Symbols are an important part of our everyday lives, and in order to pass her test she’s going to become familiar with dozens of them that line our highways.

     Just as a triangle on the highway is a symbol for “caution,” industrial control systems employ a variety of symbols in their diagrams.  The pictures are shorthand for words.  They simplify the message, just as hieroglyphics did for our early ancestors who had not yet mastered the ability to write.  Ladder diagrams and the abstract symbols used in them are unique to industrial control systems, and they result in faster, clearer interpretations of how the system operates.  

     Last week we analyzed an electric circuit to see what happens when we put a relay to use within a basic industrial control system, as found in Figure 1.

Figure 1

     Now let’s see how it looks in an even simpler form, the three-rung ladder diagram shown in Figure 2.


Figure 2

     In industrial control terminology the electric relays shown in ladder diagrams are often called “control relays,” denoted as CR.  Since a ladder diagram can typically include many different control relays, they are numbered to avoid confusion.  The relay shown in Figure 2 has been named “CR1.” 

     Our ladder diagram contains a number of symbols.  The symbol on the top rung which looks like two parallel vertical lines with a diagonal line bridging the gap between them represents the N.C. contact.  This symbol’s vertical lines represent an air gap in the N.C. contact, the diagonal line is the relay armature which performs the function of bridging/closing the air gap.  This rung of the ladder diagram represents the contact when the relay is in its normal state.  

     In the middle ladder rung the N.O. contact symbol looks like two parallel vertical lines separated by a gap.  There is no diagonal line running through it since the relay armature doesn’t touch the N.O. contact when this particular relay is in its normal state.  The wire coil and steel core of this relay are represented by a circle on the bottom ladder rung.  The contact and coil symbols on all three rungs are labeled “CR1” to make it clear that they are part of the same control relay.

     Other symbols within Figure 2 represent the red and green bulbs we have become familiar with from our initial illustration.  They are depicted as circles, R for red and G for green, with symbolic light rays around them.  The pushbutton, PB1, is represented as we have discussed in previous articles on ladder diagrams.

     Just as road sign symbols are faster than sentences for drivers speeding down a highway to interpret, ladder diagrams are faster than customary illustrations for busy workers to interpret. 

     Next time we’ll expand on our electric relay by introducing latching components into the control system that will allow for a greater degree of automation.