Posts Tagged ‘latched relay’

Industrial Control Basics – Emergency Stops

Sunday, February 26th, 2012
     Ever been in the basement when you heard a loud thud followed by a scream by a family member upstairs?  You run up the stairs to see what manner of calamity has happened, the climb seeming to take an eternity.  Imagine a similar scenario taking place in an industrial setting, where distances to be covered are potentially far greater and the dangerous scenarios numerous.

     Suppose an employee working near a conveyor system notices that a coworker’s gotten caught in the mechanism.  The conveyor has to be shut down fast, but the button to stop the line is located far away in the central control room.  This is when emergency stop buttons come to the rescue, like the colorful example shown in Figure 1.

emergency stop pushbutton

Figure 1


     Emergency stop buttons are mounted near potentially dangerous equipment in industrial settings, allowing workers in the area to quickly de-energize equipment should a dangerous situation arise.  These buttons are typically much larger than your standard operational button, and they tend to be very brightly colored, making them stick out like a sore thumb.  This type of notoriety is desirable when a high stress situation requiring immediate attention takes place.  They’re easy to spot, and their shape makes them easy to activate with the smack of a nearby hand, broom, or whatever else is convenient. 

     Figure 2 shows how an emergency stop button can be incorporated into a typical motor control circuit such as the one we’ve been working with in previous articles.

emergency stop button in motor control circuit

Figure 2


     An emergency stop button has been incorporated into the circuit in Figure 2.  It depicts what happens when someone depresses Button 1 on the conveyor control panel.  The N.C. contact opens, and the two N.O. contacts close.  The motor starts, and the lit green bulb indicates it is running.  The electric relay is latched because its wire coil remains energized through one N.O. contact.  It will only become unlatched when the flow of current is interrupted to the wire coil, as is outlined in the following paragraph.  The red lines denote areas with current flowing through them.

     Both Button 2 and the emergency stop button typically reside in normally closed positions.  As such electricity will flow through them on a continuous basis, so long as neither one of them is re-engaged.  If either of them becomes engaged, the same outcome will result, an interruption in current on the line.  The relay wire coil will then become de-energized and the N.O. contacts will stay open, preventing the wire coil from becoming energized again after Button 2 or the emergency stop are disengaged.  Under these conditions the conveyor motor stops, the green indicator bulb goes dark, the N.C. contact closes, and the red light comes on, indicating that the motor is not running.  This sequence, as it results from hitting the emergency stop button, is illustrated in Figure 3.

emergency stop button unlatches electric relay

Figure 3


     We now have the means to manually control the conveyor from a convenient, at-the-site-of-occurrence location, which allows for a quick shut down of operations should the need arise.

     So what if something else happens, like the conveyor motor overheats and catches on fire and no one is around to notice and hit the emergency stop?  Unfortunately, in our circuit as illustrated thus far the line will continue to operate and the motor will continue to run unless we incorporate an additional safeguard, the motor overload relay.  We’ll see how that’s done next time. 


Industrial Control Basics – Unlatching the Latching Circuit

Sunday, February 5th, 2012
     When I had the misfortune of getting stuck in my Uncle Jake’s outhouse as a kid, I would allow my hysteria to get the best of me and forget my uncle’s instructions on how to get out.  It was a series of raps and a single kick that would prove to be the magic formula, and once I had calmed myself down enough to employ them I would succeed in working the door’s rusty latch open.  Our relay circuit below has a much less challenging system to effectively unlatch the pattern of electric current.

      Figure 1 shows our latched circuit, where red lines denote the flow of current.

Latched Electric Relay Circuit

Figure 1


     If you recall, the relay in this circuit was latched by pressing Pushbutton 1.  When in the latched state, the magnetic attraction maintained by the wire coil and steel core won’t allow the relay armatures to release from their N.O. contacts.  The relay’s wire coil stays energized via Button 2, the red bulb goes dark while the green bulb remains lit, even though Button 1 is no longer actively depressed.

     Now let’s take a look at Figure 2 to see how to get the circuit back to its unlatched state.

Unlatching An Electric Relay

Figure 2


     With Button 2 depressed the flow of current is interrupted and the relay’s wire coil becomes de-energized.  In this state the coil and steel core are no longer magnetized, causing them to release their grip on the steel armatures.  The spring will now pull them back until one of them makes contact with the N.C. contact.  The red bulb lights again, although Button 2 is not being actively depressed.  At this point the electric relay has become unlatched.  It can be re-latched by depressing Button 1 again.

     Let’s see how we can simplify Figure 2’s representation with a ladder diagram, as shown in Figure 3.

Electric Relay Latching Circuit Ladder Diagram

Figure 3


        We’ve seen how this latching circuit activates and deactivates bulbs.  Next time we’ll see how it controls an electric motor and conveyor belt inside a factory.


Industrial Control Basics – Latching Circuit

Sunday, January 29th, 2012
     When I think of latches the first thing that comes to mind is my Uncle Jake’s outhouse and how I got stuck in it as a kid.  Its door was outfitted with a rusty old latch that had a nasty habit of locking up when someone entered, and it would take a tricky set of raps and bangs to loosen.  One day it was being particularly unresponsive to my repeated attempts to open it, and the scene became like something out of a horror movie.  There was a lot of screaming.

     When latches operate well, they’re indispensable.  Let’s take our example circuit from last time a bit further by adding more components and wires.  We’ll see how a latch can be applied to take the place of pressure exerted by an index finger.  See Figure 1.

Figure 1


     Our relay now contains an additional pivoting steel armature connected by a mechanical link to the original steel armature and spring.  The relay still has one N.C. contact, but it now has two N.O. contacts.  When the relay is in its normal state the spring holds both armatures away from the N.O. contacts so that no electric current will flow through them.  One armature touches the N.C. contact, and this is the point at which current will flow between hot and neutral sides, lighting the red bulb.  The parts of the circuit diagram with electric current flowing through them are denoted by red lines.

     Figure 1 reveals that there are now two pushbuttons instead of one.  Now let’s go to Figure 2 to see what happens when someone presses on Button 1.

Figure 2


     Again, the parts of the circuit diagram with current flowing through them are denoted by red lines.  From this diagram you can see that when Button 1 is depressed, current flows through the wire coil, making it and its steel core magnetic.  This electromagnet in turn attracts both steel armatures in our relay, causing them to pivot and touch their respective N.O. contacts.  Electric current now flows between hot and neutral sides, lighting up the green bulb.  Current no longer flows through the N.C. contact and the red bulb, making it go dark.

     If you look closely at Figure 2, you’ll notice that current can flow to the wire coil along two paths, either that of Button 1 or Button 2.  It will also flow through both N.O. contact points, as well as the additional armature.

     So how is this scenario different from last week’s blog discussion?  That becomes evident in Figure 3, when Button 1 is no longer depressed.

Figure 3


     In Figure 3 Button 1 is not depressed, and electric current does not flow through it.  The red bulb remains dark, and the green bulb lit.  How can this state exist without the human intervention of a finger depressing the button?  Because although one path for current flow was broken by releasing Button 1, the other path through Button 2 remains intact, allowing current to continue to flow through the wire coil.

     This situation exists because Button 2’s path  is “latched.”  Latching results in the relay’s wire coil keeping itself energized by maintaining armature contact at the N.O. contact points, even after Button 1 is released.  When in the latched state, the magnetic attraction maintained by the wire coil and steel core won’t allow the armature to release from the N.O. contacts.  This keeps current flowing through the wire coil and on to the green bulb.  Under these conditions the relay will remain latched.  But, just like my Uncle’s outhouse door, the relay can be unlatched if you know the trick to it. 

     Relays may be latched or unlatched, and next week we’ll see how Button 2 comes into play to create an unlatched condition in which the green bulb is dark and the red bulb lit.  We’ll also see how it is all represented in a ladder diagram.