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Last week we explored the topic of thermal expansion, and we learned how the bimetal contacts in a motor overload relay distort when heated. We also discussed how the overload relay comes into play to prevent overheating in electric motor circuits. Now let’s see what happens when an overload situation occurs.
Figure 1
Figure 1 shows a motor becoming overloaded, as it draws in abnormally high amounts of electric current. Since this current also flows through the electric heater in the overload relay, the heater starts producing more heat than it would if the motor were running normally. This abnormally high heat is directed towards the bimetal switch contacts, causing them to curl up tightly until they no longer touch each other and open up. They will only close again when the overload condition is cleared up and the heater cools back down to normal operating temperature. Let’s now take a look at Figure 2 to see how the motor overload relay fits into our example of a conveyor belt motor control circuit. Once again, the path of electric current flow is denoted by red lines.
Figure 2
The circuit in Figure 2 represents what happens after Button 1 is depressed. That is, the electric relay has become latched and current flows between hot and neutral sides through one of the N.O. contacts along the path of the green indicator bulb, the motor overload relay heater, and the conveyor belt motor. The current also flows through the other N.O. contact, the Emergency Stop button, Button 2, the electric relay’s wire coil, and the motor overload relay bimetal contacts. The motor becomes overloaded, causing the overload relay heater to produce abnormally high heat. This heat is directed towards the bimetal contacts, also causing them to heat up.
Figure 3
In Figure 3 the bimetal contacts have heated to the point that they have curled away from each other until they no longer touch. With the bimetal contacts open, electric current is unable to flow through to the electric relay’s wire coil. This in turn ends the magnetic attraction which formerly held the relay armatures against the N.O. contacts. The spring in the electric relay has pulled the armatures up, causing the N.O. contacts to open, simultaneously closing the N.C. contact. These actions have resulted in a loss of current to the green indicator bulb and electric motor. The red indicator bulb is now activated, and the conveyor motor is caused to automatically shut down to prevent damage and possible fire due to overheating. This means that even if the conveyor operator were to immediately press Button 1 in an attempt to restart the line, he would be prevented from doing so. Under these conditions the electric relay is prevented from latching, and the motor remains shut down because the bimetal contacts have been separated, preventing current from flowing through to the wire coil. The bimetal contacts will remain open until they once again cool to normal operating temperature. Once cooled, they will once again close, and the motor can be restarted. If the cause of the motor overload is not diagnosed and its ability to recur eliminated, the automatic shutdown process will repeat this cycle. Next time we’ll see how the overload relay is represented in a ladder diagram. We’ll also see how switches can be added to the circuit to allow maintenance staff to safely work. ____________________________________________
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Posts Tagged ‘N.O. contact’
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.
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.
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.
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.
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.
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.
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. ____________________________________________ |
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 1Now let’s see how it looks in an even simpler form, the three-rung ladder diagram shown in Figure 2. Figure 2In 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. ____________________________________________ |
Industrial Control Basics – Electric Relay Example
Saturday, January 14th, 2012| When a starving monkey is faced with two buttons, one representing access to a banana, the other cocaine, which will he push? The cocaine, every time. The presence of buttons usually indicates a choice must be made, and electric relays illustrate this dynamic.
Last week we looked at a basic electric relay and saw how it was used to facilitate a choice in electricity flow between two paths in a circuit. Now let’s see what happens when we put a relay to use within a basic industrial control system making use of lit bulbs.
Figure 1
Figure 1 shows an electric relay that’s connected to both hot and neutral wires. At the left side is our pushbutton and the hot wire, on the right two bulbs, one lit, one not, and the neutral wire. No one is depressing the pushbutton, so an air gap exists, preventing current from flowing through the wire coil between the hot and neutral sides. With these conditions in place the relay is said to be in its “normal state.” The relaxed spring positioned on the relay armature keeps it touching the N.C. contact. This allows current to flow between hot and neutral through the armature and the N.C. contact. When these conditions exist the red bulb is lit, and this is accomplished without the need for anyone to throw a switch or press a button. In this condition the other lamp will remain disengaged and unlit. Now let’s refer to Figure 2 to see what happens when someone presses the button.
Figure 2
When the button is depressed the air gap is eliminated and the coil and wire become magnetized. They will attract the steel armature closer to them, the spring to expand, and the armature to engage with the N.O. contact. Under these conditions current will no longer flow along a path to light the red bulb because an air gap has been created between the armature and N.C. contact. The current instead flows through the N.O. contact, lighting the green bulb. It will stay lit so long as someone holds the button down. If our monkey were faced with the scenarios presented in Figures l and 2 and a banana was placed in the position of the red bulb, the cocaine in the position of the green, he might find that the regular delivery of bananas that takes place when the relay is in the N.C. contact position is enough to keep him happy. In this state he might be so full of bananas he won’t want to expend the energy to engage the button into the N.O. contact position for the delivery of cocaine. Next time we’ll revisit the subject of ladder diagrams and see how they are used to denote the paths of electric relays. ____________________________________________ |
Industrial Control Basics – Electric Relay Operation
Monday, January 9th, 2012| It’s a dark and stormy night and you’ve come to the proverbial fork in the road. The plot is about to take a twist as you’re forced to make a decision in this either/or scenario. As we’ll see in this article, an electric relay operates in much the same manner, although choices will be made in a forced mechanical environment, not a cerebral one.
When we discussed basic electric relays last week we talked about their resting in a so-called “normal state,” so designated by industrial control parlance. It’s the state in which no electric current is flowing through its wire coil, the coil being one of the major devices within a relay assembly. Using Figure 3 of my previous article as a general reference, in this normal state a relaxed spring keeps the armature touching the N.C. switch contact. While in this state, a continuous conductive path is created for electricity through to the N.C. point. It originates from the wire on the left side, which leads to the armature pivot point, travels through the armature and N.C. contact points, and finally dispenses through the wire at the right leading from the N.C. contact. Now let’s look at an alternate scenario, when current is made to flow through the coil. See Figure l, below.
Figure 1Figure 1 shows the path of electric current as it flows through the wire coil, causing the coil and the steel core to which it’s attached to become magnetized. This magnetization is strong, attracting the steel armature and pulling it towards the steel core, thus overcoming the spring’s tension and its natural tendency to rest in a tension-free state. The magnetic attraction causes the armature to rotate about the pivot point until it comes to rest against the N.O. contact, thus creating an electrical path en route to the N.O. wire, on its way to whatever device it’s meant to energize. As long as current flows through the wire coil, its electromagnetic nature will attract the armature to it and contact will be maintained with the N.O. juncture. When current is made to flow through the wire coil, an air gap is created between the armature and the N.C. contact, and this prevents the flow of electric current through the N.C. contact area. Current is forced to follow the path to the N.O. contact only, effectively cutting off any other choice or fork in the road as to electrical path that may be followed. We can see that the main task of an electric relay is to switch current between two possible paths within a circuit, thereby directing its flow to one or the other. Next time we’ll examine a simple industrial control system and see how an electric relay can be engaged with the help of a pushbutton. ____________________________________________ |
