Teaching Alternating Current Concepts

My students sometimes must think that I am a sadist. Many of them make it clear that working math formulas is not a favorite activity – so I give them ohm’s law, temperature conversions, and gas law formulas to get them started out right! I always get a rise out of my students when after a steady diet of Ohm’s Law formulas I reveal that Ohm’s Law does not really work for most HVAC/R circuits. They calm down a bit when I tell them to substitute impedance for ohms, it sounds like an easy fix. The trick is explaining properties like impedance, inductive reactance, and capacitive reactance in a way that makes sense without resorting to LCR formulas (inductive capacitive resistive). People who find series-parallel Ohm’s Law calculations challenging will not find LCR formulas any help in understanding alternating current principles. Instead, I rely on explaining the general concepts, analogies and demonstrations. Alternating current characteristics are discussed in Unit 29 Electrical Power and Circuits in Fundamentals of HVAC/R.

Most students can understand the concept of alternating current generating a voltage in the conductor. I start with explaining that inductive reactance is an opposition to changes in current flow produced by magnetic devices. I explain that instead of the wires moving inside a magnetic field the magnetic field is moving around the wires. The fact that this induced voltage is counter the original can be explained using an analogy to jumping out of a boat: you go one way and the boat goes the other. Finally, discuss the current characteristics of all AC inductive devices – the current surge when they are first energized. Since there is no magnetic field when they are first energized, there is also no inductive reactance and a very high current rushes through the device. A magnetic field is established, which creates inductive reactance, and the current drops. This is a very important concept for the students to grasp.

The effects of inductive reactance are easy to demonstrate. Have the students measure the resistance of an inductive alternating current motor and calculate the expected amp draw using Ohm’s Law. Then run the motor and measure the amp draw, noting that it is much lower than Ohm’s Law predicted. If you have a clamp on meter with peak hold you can also show how much higher the inrush current is than the operating current. Run a multispeed motor and measure the voltage generated across the unused speed taps to demonstrate the counter electromotive force being generated in that part of the winding. Wire a 115 volt solenoid coil and a 60 watt 115 volt light in series. Leave the core out of the solenoid coil. You may have to experiment to find a paring of light and coil that allows the light to burn dimly and keeps the solenoid from overheating with the core out. After the circuit is energized insert the core into the solenoid coil. The light will get dimmer or even appear to go out. You can discuss how the iron core increases the magnetic field and therefore increases the inductive reactance. Finally, wire a small 115 volt shaded pole motor in series with a 115 volt light. Operate the circuit and measure the voltage across both devices. The students will see that the sum of the effective voltage measured across the motor and the light is higher than 115 volts. Then stop the motor and observe the light – it gets brighter. This is because the motor’s inductive reactance is lower when it is not spinning. I find demonstrations like these are more helpful to the students than anything else I have tried in trying to explain inductive reactance. With a little imagination and time you can come up with your own demonstrations using materials already available in your shop. Next week I will discuss capacitive reactance demonstrations.

Are Your Students Thirsty?

We are all familiar with the saying “You can lead a horse to water but you can’t make him drink!” Well maybe not, but you can make him thirsty and he will search out the water. The same can be said for students and knowledge. Some just have more thirst for learning than others. It can be frustrating to spend time constructing lessons and then have people not pay attention. Those same folks will be lost souls in the lab because they did not prepare. They don’t exhibit a thirst for knowledge. It would be easy to just write these students off and deal with the people who want to learn. The trouble is, they are not always the same students. Because students are people, they come to class with distractions that can get in the way of learning. You cannot cure their sick child, balance their bank statement, or get them a date with the cute girl across the hall, but you can create a thirst for air conditioning knowledge that can replace these worries for an hour or so.

There are many methods employed to get students focused on the material. They are as individual as instructors. One overlooked technique for teaching is testing. There is nothing like an impending test to create a thirst for knowledge. They want to know what is on the test. So right before a test, give a quick review of things you really want them to know. Keep the number of items covered to a minimum so they can remember them. Of course the time is limited as well because you still have a test to give. What you cover should be helpful to the students when they take the test, otherwise they will not listen the next time. Try not to just give away questions, but cover the material in a more general fashion. For example you can briefly discuss the relationship of voltage, resistance, and current by offering a few shout outs like “when resistance increases the current” … class responds “decreases.” Then the test can have some word problems that call for understanding the relationship of voltage, resistance, and current.

For most students, the test just makes them thirstier. They want to know how they did right after they complete the test. If time permits, grade the test immediately upon completion. My favorite technique for this is to have the students grade their own tests. I have them put away their pens and pencils and I hand out markers. Then I go through the test, answer each question, and discuss any issues. Students will ask questions at this point that they leave unasked in class. The discussion becomes a powerful lesson. Of course you still have to grade them. Students occasionally neglect to mark a few wrong answers when grading their own papers. The point is really not to have them do the grading, but to see how they did and learn from the experience. A word of caution: do not have them grade each other’s test. I know teachers may have used that technique on you in the past, but it is a violation of FERPA. FERPA is a relatively new federal law that guarantees students the right to privacy regarding their educational records. Since the point is for the students to learn how they did, it would not make sense for them to be grading someone else’s test.

What about cheating? Certainly some students will view this as an opportunity to “answer” the questions while they are being discussed. That is why they must put away their pens and pencils – to avoid temptation. But in the end, who are they cheating – themselves! Cheating in a class you are not compelled to take where you are preparing for your future just really does not make sense. When a student turns in a test with a low grade they gave themselves I have learned two valuable things: I did a poor job teaching that particular student and that student is honest.

I have a one question test for all of you: Are you making your students thirsty?

Free Online HVAC/R Training!

Many of our students are accustomed to 24/7 access to electronic information. I know that most instructors simply don’t have the time to create online content. If you are like most of us, you are already working at home to prepare for and manage your classes, especially now that enrollments have swelled. But I love for my students to do research on their own time after class; it shows real interest in HVAC/R and commitment to our trade. One way to provide access to online training material is to point students towards information offered by component and equipment manufacturers. There are some well done power point presentations, videos, and articles that manufacturers post for free on their web sites. Forward thinking manufacturers have figured out that showing technicians how their products work, how they should be installed, and how they should be services saves them time and money. Time spent educating technicians in advance saves time trying to do it over the phone in an emergency. Money spent on training saves money on warranty returns. Companies who take the time and resources to make training materials freely available to the HVAC/R community should be supported, applauded, and encouraged because educational resources freely offered are a gift to the HVAC/R community.

Our students can benefit from these offerings. It takes some time to dig around the internet for them, but much less time than it would take to produce comparable materials yourself. I will share a few of my favorites. I am sure there are resources that I don’t know about. If you know of one, please share it with the rest of us by posting it on a comment to this article.

I believe my favorite site contains three very well done online training courses for electric motors by US Motors, a division of Emerson. The modules are Motor Cross Referencing, Motor Mechanical Terminology, and Motor Efficiency. After the students complete the training, they can receive a certificate of completion in the mail. These are not 10 minute time wasters with glossy promo photos and little information. It will take the students a while to work through each course, but they will learn a great deal about motors in the process. Click on US Motors to go there.

Another favorite is the Yellow Jacket site. They have videos on Refrigerant Recovery, Evacuation, and Tubing Tools & Techniques. They are very well done and cover the basics of these important subjects. They are available in a small format that will stream on the web, or in a larger file size that can be downloaded for larger screens and better resolution. They do use their products in the videos and the videos do promote their products a little. I am fine with that. Click on Yellow Jacket to go there.The address is

Fluke has a boatload of articles, demos, videos, and simulations. The materials are very well done. I especially like the electrical safety materials. Fluke will send you training videos on DVDs for free. All these materials are obviously designed to show how to use Fluke products, but most of the information is applicable to testing in general. And again, I don’t mind the materials talking about Fluke products since they are providing them for free. Some of the more interesting features they have on the web site are virtual meter demonstrations. You can “use” a meter online by going to the page with information on a meter and choosing “Virtual Demo.” The Fluke site has so much that it is easy to get lost navigating the site. But the search is worthwhile, they have so many HVAC/R products and do a great job explaining their use that just looking at what is available and what it does is helpful for the students. Their HVAC/R specific materials can be found at Fluke

Several compressor manufacturers have great technical specs available online including

Tecumseh, Copeland, Bristol and Carlyle

The Carlyle site has a program that you can download called Compressor Selection software It is a complete refrigeration system simulation! You select the compressor, the refrigerant, choose the condensing and evaporating temperatures, and the program will show the capacity, energy use, temperatures, and pressures throughout the system. Some time spent playing with this can really explain how changes in system variables affect the whole system.

If you are using Fundamentals of HVAC/R, consider using the on line MyHVACLab online course management and content that is designed to work with the book. There is no cost to the school to use MyHVACLab; students must purchase an access code. If purchased packaged with the book, the cost can be as little as $7 more than the book alone. MyHVACLab provides a complete HVAC/R course that ties in with the book. Of course you can edit, delete, or add to the default course that is provided, but you start out with a complete course.

Lab Magic!

Most of us think of labs as a chance for students to practice the practical application of HVAC/R skills: brazing, charging, wiring, or troubleshooting to name a few. Certainly, students must practice their practical skills in the lab. I believe that the lab can also be instrumental in teaching more abstract concepts like gas laws or ohm’s law.

I have found that most HVAC/R students are visual and tactile learners. They learn more by seeing, touching, and doing than by reading, writing, and listening. Frequently the desire “to do” is part of what motivated them to study air conditioning in the first place, as opposed to a more academic pursuit. They arrive at the HVAC/R class excited and ready “to do”, and then we provide them with an opportunity to learn gas law formulas and ohms law. Normally we teach these subjects with an extra helping of mathematical equations. The numbers in the equations are typically all presented as part of a hypothetical example or problem. For those of us with a good understanding of the theory and the concept, the meaning is clear. For students who are still trying to understand the difference between a volt, an amp, and an ohm the problem simply becomes an exercise in math. They are not really interested in math formulas, and telling them the formulas are good for them is a bit like forcing them to take their castor oil. To overcome this, get them in the lab early and use numbers from real objects.

For ohms law, have the students measure the resistance of a strip heater, measure the voltage at the source, and then have them use ohm’s law to predict the amp draw. Next, have them operate the heater and measure its amp draw. The readings won’t be perfect, but they will be close enough to get the point across. Then do the same thing with two heaters in series and then two in parallel. They can read the individual resistance of each heater and then the combined circuit resistance to show how series and parallel resistances work. The key is to have the numbers associated with something real.

Gas behavior also works well. You can talk about the boiling point being tied to the pressure all day long and not really get the point across that boiling does not have to occur at a high temperature. This is because the information contradicts the student’s life experience – the only thing they have seen boil was hot. Put water in a flask, attach the flask to a vacuum pump, and start the vacuum pump. The water will boil at room temperature. Invite the students to touch the flask and ask questions. I have yet to see anyone who does not like this experiment. It is simple, does not take a lot of equipment, and really makes an impact.

There are many other similar labs we run whose purpose is to demonstrate an important concept. Students can quickly learn fundamental electrical circuits in the lab as well. Just make sure students know not to energize any circuits before having an instructor checks them. After the students start to connect the concepts to something that is real, then they will start to ask questions. Now you can start to use hypothetical examples and the students have a mental image of a real device to give the discussion meaning.

For some more ideas about these types of labs, take a look at the Lab Manual by David Skaves. It contains an entire section of labs labeled as Properties labs. These labs teach students about physics properties that make HVAC/R work. Although this Lab Manual is written to accompany Fundamentals of HVAC/R, it works well with any text because it is organized by subject matter with sections on Fundamentals, Properties, Refrigeration, Accessories, Controls, Electricity, Maintenance, Gas Heat, Oil Heat, and Electric Heat. Each section has several labs.

Are You Lecturing in Russion?

We have our fair share of acronyms in the HVAC/R industry. Used properly; acronyms speed up communication by reducing long polysyllabic phrases to a few letters. For example, speakers would get a bit winded if they used the phrase “heating, ventilating, air conditioning, and refrigeration” repeatedly in a conversation. The abundance of industry specific acronyms and the use of more than one acronym for the same item can be truly bewildering to students. For example, one major valve manufacturer uses the acronym TEV to represent “Thermostatic Expansion Valve,” while another uses the acronym TXV for the same thing. They do not represent two different types of components, just two different ways to abbreviate the word "expansion."

When introducing a new topic, using acronyms can lead to confusion and misunderstanding. Worse, students can start to pick up the jargon, but use it incorrectly. It can be difficult for teachers who have had years of experience with HVAC/R techno-speak to remember that the acronyms we casually throw around often have no meaning for our students. Imagine listening to a lecture in which every important concept was in Russian! For example: “The evaporator delta T is controlled by the TEV adjustment, the return air wb, and the CFM.” Now I believe that most any air conditioning instructor understands this sentence, but it is essentially unintelligible to many air conditioning students. It might as well be in Russian! Replacing the HVAC/R specific jargon with the wingdings font it looks like “The evaporator delta T is controlled by the TEV adjustment, the return air wb, and the CFM.” THAT is what this sentence looks like to a new student!

In general it is a good idea to resist using an acronym for something until that item or process has been discussed. Otherwise, the acronym will appear to students to be a mysterious grouping of letters used by air conditioning shaman to communicate with each other. The acronyms and jargon are like a secret language which they are not familiar with. I believe that language which is largely acronym free promotes better understanding, but some acronyms are so common that students really do need to eventually learn them. It is much easier to remember an acronym if you understand what the letters stand for. When using an acronym for the first time make sure and explicitly spell out what the letters represent, this will increase student’s understanding and retention of the term.

Of course, students must still learn the HVAC/R language. Having a solid grasp of the terminology is necessary to make use of essential technical literature produced by equipment manufacturers. In Fundamentals of HVAC/R, we always use the complete word or phrase before introducing an acronym. It helps to explain concepts plainly, and then introduce the technical terminology that is used to refer to the concept. The students are more likely to remember the terminology if it is logically connected to something they understand.

Fundamentals of HVAC/R has a unique abbreviation and acronym dictionary to help students learn the language of HVAC/R. The acronym dictionary is very useful when students are reading industry literature and need help with a particular abbreviation or acronym. They can find out exactly what is meant by a particular acronym or abbreviation in Appendix C which lists the definition of common acronyms and abbreviations used in HVAC/R.

Show Me the Money!

There is no secret that all of us teaching air conditioning are experiencing a boom in enrollment. A frustration that many of these new students share is limited financial resources. Truth be told, that is precisely why they are air conditioning students – they are working to improve their financial fortunes. Sometimes they end up trying to decide whether to buy the required texts, or the required tools. Both can represent a sizeable investment. An unemployed student can find it difficult to afford both. Although the schools are trying to help, most schools have limited resources right now as well. Some major corporations and industry organizations are offering significant scholarships to people studying Air Conditioning and Refrigeration. The ones that I know about are the AHRI Rees Scholarship, the Grainger Tools for Tomorrow Scholarship, and the Home Depot Trade Scholarship. I am sure there are more. The AHRI Rees Scholarship Foundation awards approximately 15 scholarships of up to $2,000 each to qualified students who are enrolled in an HVACR program at an accredited institution. The Grainger Tools for Tomorrow Scholarship provides a $2,000 scholarship plus a custom set of tools from Grainger to approximately 100 trade students nationwide. Home Depot is offering $1,000 scholarships to 500 trade students. Home Depot is also going to award the school each scholarship student attends an additional $1,000. One of these scholarships could be the difference between a promising student finishing or having to drop out for financial reasons. Together that is 615 more future trades professionals who do REAL work right here in the USA to build a brighter tomorrow. God bless them.

However, to win a scholarship you must apply. You would be surprised how difficult it can be to get wrench minded students to fill out scholarship applications, write essays, and apply their literary skills for a chance to win a scholarship. I am proud to say that the first Tools For Tomorrow Scholarship recipient at Athens Tech was one of my students, Ricky Byrd (Article). Ricky was a very deserving recipient and he represents my program and our profession well. There were other capable students who chose not to apply because they were required to write a short essay. The folks from Grainger came to my department to award Ricky the scholarship and that changed everyone’s perspective. Think of the scholarship application as practice for seeking employment. To sell yourself to future employers you need to be able to explain in writing and verbally what you can offer that they cannot do without. If you just sit around waiting for the world to discover how brilliant you are, you most likely will be waiting for a long time.

For information on how to apply check out the following links.

AHRI Rees

Grainger

Home Depot

If you don’t snag one of these scholarships, don’t despair. Often local organizations have scholarships that are not as well publicized. Check with local contractor associations or local RSES chapters to see if they have any scholarships.

The BIG Showis Almost Here

The AHR Expo is coming up January 25 in Orlando Florida. The AHR Expo is the granddaddy of all air conditioning industry shows. It is co-sponsored by two of the HAVC/R industry heavy hitters: ASHRAE and AHRI. This is truly a world class show with people and companies from all over the globe. My experience has been that it is really impossible to look at all the exhibits in a day because there is so much to see. If you are located anywhere close to Orlando and can find a day to visit the expo, it is well worth your time. Instructors and students can get so much free literature that it will take you hours just to read it all. There are many free seminars every day for discussion of new technologies and products. The AHR Expo is usually where HVAC/R companies roll out their latest and greatest, so it is a perfect place to see where the industry is heading. Perhaps most useful to instructors are the contacts you can make with OEM suppliers, manufacturers, wholesalers, and industry technical gurus of all stripes. Of course the expo will be located in the land of fun and sun, making the overall trip more pleasant. However, you will probably have to wait until after the show to see Mickey because there is just so much to do at the expo. Click on AHR Expo to learn more about the show.

Naturally, when you get home you will be thinking about how to incorporate what you saw in the show into your program. One way is through links to the online literature of HVAC/R companies. A good way to do this is to add links in MyHVACLab at appropriate places in the program. This gives the information some structure and makes it more meaningful than a long list of unrelated hyperlinks. Although on advantage of MyHVACLab is that it is a complete program out of the box, another big advantage is that it is easy to modify to fit your needs. Click on MyHVACLab to learn more about MyHVACLab or Fundamentals of HVAC/R, the AHRI endorsed text that MyHVACLab supports. So pack your sunscreen and head down to the land of fun and sun to see the world’s largest air conditioning show!