| 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. ____________________________________________ |
Archive for February, 2012
Industrial Control Basics – Electric Motor Control
Sunday, February 19th, 2012| Electric motors are everywhere, from driving the conveyor belts, tools, and machines found in factories, to putting our household appliances in motion. The first electric motors appeared in the 1820s. They were little more than lab experiments and curiosities then, as their useful potential had not yet been discovered. The first commercially successful electric motors didn’t appear until the early 1870s, and they could be found driving industrial devices such as pumps, blowers, and conveyor belts.
In our last blog we learned how a latched electric relay was unlatched at the push of a button, using red and green light bulbs to illustrate the control circuit. Now let’s see in Figure 1 how that circuit can be modified to include the control of an electric motor that drives, say, a conveyor belt inside a factory.
Figure 1
Again, red lines in the diagram indicate parts of the circuit where electrical current is flowing. The relay is in its normal state, as discussed in a previous article, so the N.O. contacts are open and the N.C. contact is closed. No electric current can flow through the conveyor motor in this state, so it isn’t operating. Our green indicator bulb also does not operate because it is part of this circuit. However current does flow through the red indicator bulb via the closed N.C. contact, causing the red bulb to light. The red and green bulbs are particularly useful as indicators of the action taking place in the electric relay circuit. They’re located in the conveyor control panel along with Buttons 1 and 2, and together they keep the conveyor belt operator informed as to what’s taking place on the line, such as, is the belt running or stopped? When the red bulb is lit the operator can tell at a glance that the conveyor is stopped. When the green bulb is lit the conveyor is running. So why not just take a look at the belt itself to see what’s happening? Sometimes that just isn’t possible. Control panels are often located in central control rooms within large factories, which makes it more efficient for operators to monitor and control all operating equipment from one place. When this is the case, the bulbs act as beacons of the activity taking place on the line. Now, let’s go to Figure 2 to see what happens when Button 1 is pushed.
Figure 2
The relay’s wire coil becomes energized, causing the relay armatures to move. The N.C. contact opens and the N.O. contacts close, making the red indicator bulb go dark, the green indicator bulb to light, and the conveyor belt motor to start. With these conditions in place the conveyor belt starts up. Now, let’s look at Figure 3 to see what happens when we release Button 1.
Figure 3
With Button 1 released the relay is said to be “latched” because current will continue to flow through the wire coil via one of the closed N.O. contacts. In this condition the red bulb remains unlit, the green bulb lit, and the conveyor motor continues to run without further human interaction. Now, let’s go to Figure 4 to see how we can stop the motor.
Figure 4
When Button 2 is depressed current flow through the relay coil interrupted. The relay is said to be unlatched and it returns to its normal state where both N.O. contacts are open. With these conditions in place the conveyor motor stops, and the green indicator bulb goes dark, while the N.C. contact closes and the red indicator bulb lights. Since the relay is unlatched and current no longer flows through its wire coil, the motor remains stopped even after releasing Button 2. At this point we have a return to the conditions first presented in Figure 1. The ladder diagram shown in Figure 5 represents this circuit.
Figure 5
Next time we’ll introduce safety elements to our circuit by introducing emergency buttons and motor overload switches. ____________________________________________ |
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. ____________________________________________ |
