| When my daughter was seven she found out about Ohm’s Law the hard way, although she didn’t know it. She had accidentally bumped into her electric toy train, causing its metal wheels to derail and fall askew of the metal track. This created a short circuit, causing current to flow in an undesirable direction, that is, through the derailed wheels rather than along the track to the electric motor in the locomotive as it should.
What happened during the short circuit is that the bulk of the current began to follow through the path of least resistance, that of the derailed wheels, rather than the higher resistance of the electric motor. Electric current, always opportunistic, will flow along its easiest course, in this case the short, thick metal of the wheels, rather than work its way along the many feet of thin metal wire of the motor’s electromagnetic coils. With its wheels sparking at the site of derailment the train had become an electric toaster within seconds, and the carpet beneath the track began to burn. Needless to say, mom wasn’t very happy. In this instance Ohm’s Law was at work, with a decidedly negative outcome. The Law’s basic formula concerning the toy train would be written as: I = V ÷ Rwhere, I is the current flowing through the metal track, V is the track voltage, and R is the internal resistance of the metal track and locomotive motor, or in the case of a derailment, the metal track and the derailed wheel. So, according to the formula, for a given voltage V, when the R got really small due to the derailment, I got really big. But enough about toy trains. Let’s see how Ohm’s Law applies to an unregulated power supply circuit. We’ll start with a schematic of the power supply in isolation.
Figure 1The unregulated power supply shown in Figure 1 has two basic aspects to its operation, contained within a blue dashed line. The dashed line is for the sake of clarity when we connect the power supply up to an external circuit which powers peripheral devices later on. The first aspect of the power supply is a direct current (DC) voltage source, which we’ll call VDC. It’s represented by a series of parallel lines of alternating lengths, a common representation within electrical engineering. And like all electrical components, the power supply has an internal resistance, such as discussed previously. This resistance, notated RInternal, is the second aspect of the power supply, represented by another standard symbol, that of a zigzag line. Finally, the unregulated power supply has two output terminals. These will connect to an external supply circuit through which power will be provided to peripheral devices. Next time we’ll connect to this external circuit, which for our purposes will consist of an electric relay, buzzer, and light to see how it all works in accordance with Ohm’s Law. ____________________________________________ |
Posts Tagged ‘electric motor’
Industrial Control Basics – Disconnect Switches
Sunday, March 25th, 2012| Last week our kitchen ceiling fan and light combo decided to stop working. We don’t like eating in the dark, so I was compelled to do some immediate troubleshooting. As an engineer with training in the workings of electricity I have a great respect for it. I’m well aware of potential hazards, and I took a necessary precaution before taking things apart and disconnecting wires. I made the long haul down the stairs to the basement, opened the circuit breaker in the electrical panel, and disabled the flow of electricity to the kitchen. My fears of potential electrocution having been eliminated, my only remaining fear was of tumbling off the ladder while servicing the fan.
Just as I took the precaution to disconnect the power supply before performing electrical maintenance in my home, workers in industrial settings must do the same, and a chief player in those scenarios is the motor overload relay discussed last week. It automatically shuts down electric motors when they become overheated. Let’s revisit that example now.
Figure 1
Our diagram in Figure 1 shows electric current flowing through the circuit by way of the red path. Even if this line were shut down, current would continue to flow along the path, because there is no means to disconnect the entire control system from the hot and neutral lines supplying power to it, that is, it is missing disconnect switches. Electric current will continue to pose a threat to workers were they to attempt a repair to the system. Now let’s see how we can eliminate potential hazards on the line.
Figure 2
In Figure 2 there is an obvious absence of the color red, indicating the lack of current within the system. We accomplished this with the addition of disconnect switches capable of isolating the motor control circuitry, thereby cutting off the hot and neutral lines of the electrical power supply and along with it the unencumbered flow of electricity. These switches are basically the same as those seen in earlier diagrams in our series on industrial controls, the difference here is that the two switches are tied together by an insulated mechanical link. This link causes them to open and close at the same time. The switches are opened and closed manually via a handle. When the disconnect switches are both open electricity can’t flow and nothing can operate. Under these conditions there is no risk of a worker coming along and accidentally starting the conveyor motor. To add yet another level of safety, disconnect switches are often tagged and locked once de-energized. This prevents workers from mistakenly closing them and starting the conveyor while maintenance is being performed. Brightly colored tags alert everyone that maintenance is taking place and the switches must not be closed. The lock that performs this safety function is actually a padlock. It’s inserted through a hole in the switch handle, making it impossible for anyone to flip the switch. Tags and locks are usually placed on switches by maintenance personnel before repairs begin and are removed when work is completed. Now let’s see how our example control system looks in ladder diagram format.
Figure 3
Figure 3 shows a ladder diagram that includes disconnect switches, an emergency stop button, and the motor overload relay contacts. The insulated mechanical link between the two switches is represented by a dashed line. Oddly enough, engineering convention has it that the motor overload relay heater is typically not shown in a ladder diagram, therefore it is not represented here. This wraps up our series on industrial control. Next time we’ll begin a discussion on mechanical clutches and how they’re used to transmit power from gasoline engines to tools like chainsaws and grass trimmers. ____________________________________________
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Industrial Control Basics – Motor Overload
Sunday, March 4th, 2012|
Last summer my wife and I did a lot of work in the garden. Many holes were dug, bags of garden soil lifted, and plants planted. It’s a new garden, and my wife has very big plans for it, so needless to say there was a lot of work to be done. On more than one occasion we would end the day moaning about our body aches and how we had overdone it. The next day we would hurt even worse, and we’d end up taking time off to recuperate. Pain is your body’s way of telling you that it needs attention, and you’d better listen to it or you may have an even heavier price to pay down the road. Electric motors can get overworked, just like our bodies. Motors are often placed into situations where they are expected to perform tasks beyond their capability. Sometimes this happens through poor planning, sometimes due to wishful thinking on the user’s part. Motors can sustain damage when stressed in this way, but they don’t have a pain system to tell them to stop. Instead, motors benefit by a specific type of electric relay known as an overload relay. But before we get into how an overload relay works, let’s get a better understanding of how overloads happen. Suppose we’re back in the telephone factory discussed in previous blogs, watching a conveyor belt move phones through the manufacturing process. An electric motor drives the conveyor belt by converting electrical energy into mechanical energy. Everything is moving along normally when all of a sudden a machine malfunctions. Telephones start piling up on a belt, and the pile up gets so bad the belt eventually gets jammed and its motor overloaded. If the electricity flow to the motor isn’t shut down promptly by means of a nearby emergency stop button or an astute operator sitting in central control, then an even bigger problem is in the making, that of a potential fire. When electricity is applied to motors they begin to operate, and their natural tendency is to want to keep operating. They do so by continuously drawing energy from the electric current being supplied to them. The greater the workload demand on the motor, the more current it requires to operate. When motors become overloaded as in the scenario presented above, they continue to draw energy unless forced to a stop. The result is an overabundance of current flowing through the motor and no outlet for its task of converting electrical energy into mechanical energy. And where is all that pent up energy to go? It becomes heat energy trapped inside the motor itself, and this heat can build up to the point where the motor becomes damaged or even bursts into flames. Next time we’ll look at how overload relays work to keep electric motors from overheating, just as our body’s pain sensors protect us from overdoing it.
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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 – Pushbuttons
Monday, December 26th, 2011| I always enjoy watching impatient people waiting for an elevator. They press the button, and if it doesn’t come within a few seconds they press it over and over again, as if this will hurry things up. In the end they must resign themselves to the fact that the elevator will operate in its own good time.
Pushbuttons, although simple in appearance like the big, red “Easy” button that’s featured in a certain business supply chain’s commercials, are actually complex behind the scenes. They perform important functions within the industrial control systems of a huge diversity of mechanized equipment. Last week we introduced ladder diagrams, used to design and document industrial control systems, and we’ll now see how they depict the action of pushbuttons within two commonly used industrial settings, the “normally open” and the “normally closed.”
Figure 1
Figure 1(a) shows a pushbutton hooked up to an electric motor. When no one is pressing it a spring in the pushbutton forces the button to rest in the up position, allowing an air gap to exist in the electrical circuit between hot and neutral and preventing current from flowing. This type of switch is characterized as a “normally open” switch in industrial control terminology. In Figure 1(b) someone depresses the button, compressing its spring and closing the air gap, which allows current to flow and the motor to operate Figure 1(c) shows the ladder diagram version of 1(a). Now let’s take a look at Figure 2 to see a different type of pushbutton, one that’s characterized as “normally closed.”
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Industrial Control Basics – Ladder Diagrams
Sunday, December 18th, 2011| The other day I pressed the button to activate my electric garage door opener and nothing happened. I pushed again and again, still nothing. Finally, I convinced myself to get out of the car and take a closer look. A wooden board I had propped up against the side of the garage wall had come loose, wedging itself in front of the electric eye, you know, the one that acts as a safety. The board was an obstruction to the clear vision of the eye. It couldn’t see the light emitter on the other side of the door opening and wouldn’t permit the door opener to function.
The basic manual control system we looked at last week operates similarly to the eye on a garage door opener. If you can’t “close the loop,” you won’t get the power. Last week’s example was as basic as things get. Now let’s look at something a bit more complex. Words aren’t always the best vehicle to facilitate understanding, which is why I often use visual aids in my work. In the field of industrial control systems diagrams are often used to illustrate things. Whether it’s by putting pencil to paper or the flow diagram of software logic, illustrations make things easier to interpret. Diagrams such as the one in Figure l are often referred to as “ladder diagrams,” and in a minute we’ll see why.
Figure 1 Figure 1(a) shows a basic manual control system. It consists of wires that connect a power switch and electric motor to a 120 volt alternating current power source. One wire is “hot,” the other “neutral.” The hot side is ungrounded, meaning that it isn’t electrically connected to the Earth. The neutral side is grounded, that’s right, it’s driven into the ground and its energy is dissipated right into the earth, then returned back to the power grid. In Figure 1(a) we see that the power switch is open and an air gap exists. When gaps exist, we don’t have a closed electrical loop, and electricity will not flow. Figure 1(b), our ladder diagram, aka line diagram, shows an easier, more simplified representation of the manual control shown in Figure 1(a). It’s easier to decipher because there’s less going on visually for the brain to interpret. Everything has been reduced to simple lines and symbols. For example, the electric motor is represented by a symbol consisting of a circle with an “M” in it. Now, let’s turn our attention to Figure 2 below to see what happens when the power switch is closed.
Figure 2 The power switch in Figure 2(a) is closed, allowing electric current to flow between hot and neutral wires, then power switch, and finally to the motor. The current flow makes the motor come to life and the motor shaft begins to turn. The line diagram for this circuit is shown in Figure 2(b). You might have noticed that the line diagrams show in Figures 1(b) and 2(b) have a rather peculiar shape. The vertically running lines at either side depict the hot and neutral legs of the system. If you stretch your imagination a bit, they look like the legs of a ladder. Between them run the wires, power switch, and motor, and this horizontal running line represents the rung of the ladder. More complicated line diagrams can have hundreds, or even thousands of rungs, making up one humongous ladder, hence they are commonly referred to as ladder diagrams. Next week we’ll take a look at two key elements in automatic control systems, the push button and electric relay, elements which allow us to do away with the need for human intervention. ____________________________________________ |
Food Manufacturing Challenges – HACCP Design Principle No. 7
Sunday, November 27th, 2011| Ever overdraw on your checking account or max out a credit card? It’s not hard to do if you’re not keeping track of things. How can we manage household expenses without some sort of record keeping?
Away from home, in the business sector, record keeping becomes even more important. In fact, it’s the very thing covered by HACCP Design Principle No. 7. Principle 7: Establish record keeping procedures. – This HACCP principle requires that all food manufacturing plants maintain records to show they implemented a HACCP plan, are following all principles, and the plan is working effectively. Let’s look at an example. In keeping with the directive of HACCP Design Principle 7, the engineering department of a food manufacturing plant must keep records for each design project. The design record for a new cookie forming machine would contain things like engineering calculations to determine strength requirements of machine parts and supports, as well as power requirements for the electric motor that drives the machine. This design record would also contain documentation concerning materials selected to construct the machine, as well as dimensioned mechanical drawings of the machine and its parts. These dimensioned drawings will show all physical dimensions of the machine and its constituent parts. The record would also contain test results and analysis of the results. Lastly, the design record must include a risk analysis of potential hazards that could result. Other activities include identification of CCPs, establishment of critical limits, and other factors in accordance with HACCP Design Principles 1 through 5. In other words, the record must be complete, bearing witness to an effective adherence to HACCP Design Principles 1 through 5. Principle 7 also encompasses guidelines set in place through Design Principle 6, which calls for the establishment of procedures to govern Principles 1 through 5. A complete record would contain the procedures themselves, along with any revisions. It would also contain documentation that the procedures were reviewed and approved by management along the way. Finally, of what use would records be if they were incomplete, disorganized, and outdated? A document control system not only establishes procedures, but assigns responsibilities to personnel within the department for filing design records to make sure that everything is up to snuff. This system would encompass everything, from the creation of engineering documents, to their timely entry into the record keeping system. We have now exhausted our discussion on HACCP Design Principles. We’ll switch to a new topic next time, examining some basic concepts behind the control of industrial equipment and machinery. ____________________________________________
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Industrial Ventilation – Local Exhaust Ventilation Fans
Sunday, May 22nd, 2011| When something is said to be the “heart of the operation,” one usually imagines that it is integral to whatever is being discussed, and it is probably centrally located. The human heart fits this description well. This amazing organ, centrally located within your chest cavity, moves blood, nutrients, oxygen, and carbon dioxide through your body with amazing efficiency. During a twenty four hour period it can pump as much as 2,000 gallons of blood through 6,000 miles of arteries, veins, and capillaries.
At the heart of a local exhaust ventilation system is its fan. Like the human heart, it is a model of efficiency. It first creates a vacuum in the intake hood, which is strategically located at a pollution source, pulling in contaminated air and leading it through ductwork. Sometimes the fan leads the air to a filter or other air cleaning equipment, but eventually the dirty air is exhausted through a stack leading outdoors. There are two main types of fan, axial and centrifugal. You’re probably most familiar with the axial type, because they’re the type commonly used in tabletop, box, and oscillating fans in your home. These have blades that look like a propeller on an airplane, and they work by drawing air straight through the fan. As helpful as they are within a personal setting, axial fans are not typically used in local exhaust ventilation systems because the electric motor that drives the blades is in the path of airflow. This setup can create a problem if the air flowing over the motor contains dust and flammable vapor. Dust can cause the motor to get dirty and overheat. Flammable vapor can ignite if the motor wiring fails and creates an electrical arc. Because of the technical difficulties presented by an axial type fan, centrifugal fans are what are most often used in industrial settings. One such fan is shown in Figure 1.
Figure 1 – Centrifugal Fan The blades of a centrifugal fan are fully enclosed in air tight housing. This housing keeps any dust or fumes from leaking out into the building. The electric motor that drives the fan can be safely located outside of this housing, where it is dust-free and there are no flammable vapors. If you look inside the housing you will see that the moving part, known as the impeller, resembles a squirrel cage. See Figure 2.
Figure 2 – Centrifugal Fan Impeller This impeller is made up of many blades, set up within a wheel configuration. When an electric motor causes the wheel to rotate, air is made to move off the blades and out of the impeller due to centrifugal force. This air is sent crashing into the fan housing, shown in Figure 1, which is curved like a spiral to direct the air into an outlet duct which is connected to ductwork that leads to the exhaust stack. As air leaves the impeller, more air rushes into its center from the inlet duct to occupy the empty space that’s been created. Hence, as long as the motor keeps spinning the impeller, air will flow through the fan. In order for all this to work effectively, the centrifugal fan must be the right size, one that is capable of providing enough suction to capture contaminated air at the hood source, then overcoming the resistance to air flow that is presented by ductwork, filters, and other air cleaning devices. Because air resistance factors such as these impede the fan’s ability to move air through the system, the fan must be of sufficient strength make up for these factors. To size up the right centrifugal fan for the job, engineers must calculate the resistance to airflow that is expected to be encountered, and to do this they use data supplied by manufacturers of component parts, as well as tabulated data that is readily available in engineering handbooks. Just as a lawn mower engine won’t provide sufficient energy to power a car, an undersized fan won’t be able to move air through a system which is beyond its capacity limit. Next time, we’ll finish our series on local exhaust ventilations systems by looking at the last component in the system: the exhaust stack. _____________________________________________
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Dynamic Brakes
Monday, May 31st, 2010|
Last week we looked at how a mechanical brake stopped a rotating wheel by converting its mechanical energy, namely kinetic energy, into heat energy. This week, we’ll see how a dynamic brake works. Chances are you have directly benefited by a dynamic braking system the last time you rode in an elevator. But, to understand the basic principle behind an elevator’s dynamic brake system, let’s first take a look at the electric braking system in Figure 1 below.
Figure 1 – A Simple Electric Braking System Here the brake consists of an electric generator wired via an open switch to an electrical component called a resistor. The weight is attached to a cable that is wound around a pulley on the generator’s shaft. As the weight freefalls, the cable unwinds on the pulley, causing the pulley to turn the generator’s shaft. Unlike last week’s mechanical brake which required a good deal of effort to employ, a dynamic braking system requires very little. All that needs to be done is to close a switch as shown in Figure 2 below. When the switch is closed, an electrical circuit is created where the resistor gets connected to the generator. The resistor does as its name implies: it resists (but doesn’t stop) the electrical current flowing through it from the generator. As the electrical current fights its way through the resistor to get back to the generator, the resistor gets hot like an electric heater. This heat is dissipated to the cooler surrounding air. At the same time, the weight begins to slow down in its descent. But how is this happening? The electric braking system can be thought of as an energy conversion process. We start out with the kinetic, or motion energy, of the freefalling weight. This kinetic energy is transmitted to the electrical generator by the cable, which spins the generator’s shaft as the cable unwinds. Electrical generators are machines that convert kinetic energy into electrical energy. This energy travels from the electric generator through wires and a closed switch to the resistor. In the process the resistor converts the electrical energy into heat energy. So, kinetic energy is drawn from the falling weight through the conversion process and leaves the process in the form of heat. As the falling weight is drained of kinetic energy, it slows down.
Figure 2 – Applying the Electric Brake Okay, now let’s get back to dynamic brakes on elevators. An elevator is attached by a cable to a hoist that is powered by an electric motor. When it’s time to stop at the desired floor, the automatic control system disconnects the elevator’s electric motor from its power source and turns the motor into a generator. The generator is then automatically connected to a resistor like the one shown in the electric brake above. The kinetic energy of the moving elevator is converted by the generator into electrical energy. The resistor converts the electrical energy into heat energy which is then dissipated into the surrounding environment. The elevator slows down in the process because it’s being robbed of kinetic energy. When the dynamic brake slows the elevator down enough, a mechanical brake is introduced, taking over to bring the elevator to a complete stop. This two-fold process serves to reduce wear and tear on the mechanical brake’s parts, lengthening the operational lifespan of the system as a whole. Next time, we’ll tie everything together and show how mechanical and dynamic brakes work together in a diesel locomotive. _____________________________________________
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