Be a Problem solver

Have you ever known someone that excels in finding the cloud behind the silver lining? Some people have a knack for finding the negative to any set of circumstances. Sometimes students can get the impression that a service technician’s job is to find problems. But in truth, the technician’s job is to solve problems. Yes, you have to identify a problem to solve it, but solving the problem is the goal. Customers and employers are more impressed by people that find solutions than people that find problems. Most people already have more than enough problems, so the last person they want around is someone who generates more problems than solutions. On the other hand, people that excel in finding solutions are a joy to have around. Encourage your students to offer solutions to the problems they find. They need to become practiced in providing solutions, not just identifying problems. This can sometimes be a matter of attitude. Rather than finding reasons why something cannot be accomplished, look for ways to overcome the barriers to get the job done. This attitude should start with the way you interact with your students. Make sure to recognize what they do correctly, and not simply point out their errors. Of course you need to correct their errors, but it should be done constructively, not destructively. Always offer a better way when correcting students who are struggling. The student who is trying to glob on solder that is balling up and falling off really does not have to be told that what he is doing is not working; he can see it for himself. What he needs is someone to offer a better way. Students who gain skills after struggling at first appreciate the accomplishment more. You can teach more than any particular skill when you help a student overcome a learning barrier, you teach them how to be a problem solver.

Understanding Relays and Contactors

I promised last week to reveal my secret for teaching relays. I know I sound like a broken record, but the first step is making sure that the students have the necessary electrical foundation knowledge. Before introducing relays and contactors, students must clearly understand circuits, loads, and switches. A relay is essentially an electrically operated switch. A contactor is really just a bigger relay. The key to understanding relay operation is realizing that a relay is actually two electrical devices: a set of contacts and a coil. The contacts are a switch. They are wired in circuits exactly like switches and they behave like switches because they are switches. Relay coils are loads; specifically, solenoids. They wire in circuits like a solenoid and they behave like solenoids because they are solenoids. The purpose of the relay coil is to operate the relay contacts. Energizing or de-energizing the relay coil makes the contacts open or close, but the coil and contacts are almost always in separate circuits. This is difficult for many students to understand. What they see is a box with a lot of electrical connections. To demonstrate how a relay works I ask the student to wire a toggle switch to control a light. Then I ask them to wire another circuit to a relay with a 120 volt coil the same way, with a toggle switch controlling the relay coil. Make sure and reinforce the idea that the coil is a load. The student should operate the circuit and hear the relay click when the switch is closed. Have the student check the continuity across the contacts with the coil de-energized, then have them operate the relay coil and check the continuity again. If you are using a relay with both normally open and normally closed contacts they should check both. Now have the student combine the circuit to the light they wired earlier with the relay by replacing the toggle switch controlling the light with the relay contacts. This illustrates that the contacts are just a switch. Now have the student operate the relay to see it control the light. Next, mention that the coil and contacts do not have to be the same voltage. Have the student wire a toggle switch and a 24 volt transformer to control a 24 volt relay coil. Then have them use the relay contacts to control a 120 volt light. If you want to extend the HVAC/R component analogy you can replace the toggle switch with a thermostat, explaining that a thermostat is simply a switch. Before leaving the exercise, find a schematic diagram and show the circuit with the relay coil and the circuit with the relay contacts to illustrate that they are in separate circuits. You can point out that the labeling identifies which relay contacts are controlled by which relay coils. Figure 32-23 in Fundamentals of HVAC/R shows a simple diagram with a 24 volt contactor controlling a compressor. The figure shows that the coil in the 24 volt circuit is controlling the contacts in the 230 volt circuit. Building the circuits to the relay in steps helps students understand how relays operate by relating the relay coil and the relay contacts to components the student already understands.

There are many resources for teaching relays and contactors in Fundamentals of HVAC/R. Unit 29 contains good discussion and illustrations of how relays operate in 29.7 Relays and Contactors. Contactors are discussed and illustrated in 32.16 Contactors in Unit 32. Diagrams using relay and contactor circuits are discussed extensively throughout Unit 33.

Teaching Ladder Diagrams

Reading electrical diagrams is a crucial skill that HVAC/R students must master. That is why all HVAC/R curriculums spend a lot of time discussing how to read electrical diagrams. Many students struggle with this even though every HVAC/R instructor I have ever met emphasizes diagrams. I believe this is because students must learn several pieces of information and then weave them all together in order to truly understand electrical diagrams. You really can’t understand diagrams if you don’t understand the basic function and operation of standard electrical components. Learning symbols for components that you don’t understand is like memorizing words in a foreign language without knowing what they mean. You still can’t read. Understanding basic circuits is also a must. Students should be able to construct basic circuits consisting of loads and switches. Finally, they must have a good concept of how the system they are working on heats or cools. If all this is in place they usually will have no problem learning how to read electrical diagrams. I find that students who are the weakest with diagrams are usually also weak in one or more of these foundation areas. After all, a diagram is simply a representation of a collection of circuits that are made up of electrical components. You really can’t understand an abstract representation if you don’t understand the actual component that you can see and touch. So we start by having students build basic circuits with switches and lights in order to learn how loads and switches work in a basic circuit. Loads should have a voltage drop when current passes through them, switches should not. Then we introduce components and relate them to either loads or switches. So thermostats are switches while motors are loads. Introduce symbols and have students draw symbols for selected components. Then have them identify components on a unit using the unit diagram. The most difficult components for students to grasp are usually contactors and relays. Understanding relays is a big stumbling point for many students. I have a method for teaching relays, but I will save that for another article. Just know that there is no use trying to explain a ladder diagram that contains 24 volt control relays to someone that does not understand how relays work. After explaining basic ladder diagrams and showing several examples ask the students to draw a simple diagram of a packages air conditioning unit without looking at an example. You don’t want a copy of the book; you want to see how much they understand. There will be lots of weeping and gnashing of teeth. Students will complain that they are not artists and that drawing diagrams is not necessary to service systems. However, you will find out quickly how much they understand about circuits, components, and diagrams. People that understand diagrams can draw them. Be patient, suggest corrections, and have them revise their diagrams until they have something that will work. This typically takes several revisions for many students. Finally, have them wire the circuit that they drew using the actual components as much as possible. For safety we have a simple rule: the instructor has to check the circuit before it is energized. The pride the students get from seeing their circuit operate will offset the anguish they experienced getting there. Gradually add more complex circuits – two stage cooling, cooling and heating, heat pumps, or whatever makes sense for your area. After the students have a good command of ladder diagrams you can start discussing the many variations they will see in different manufacturer’s diagrams. There are as many variations as there are manufacturers, adding to the difficulty for aspiring technicians.

Diagrams are discussed extensively in Fundamentals of HVAC/R in units 31 Electrical Components and Wiring Diagrams, 32 Controls, and 33 Control Circuits and Diagrams. There are some great exercises in MyHVACLab to help students learn electrical diagrams including an interactive electrical symbol recognition activity and a virtual wiring exercise that aligns a ladder diagram to the actual components as they are wired.

Problem Solving

HVAC/R Service is about practical problem solving. All the tools, technology, training, and literature are just there to help us identify and solve problems. The primary skill that any service tech needs is problem solving. The most important tool at your disposal is your mind. I am frequently asked why we make students do BTU calculations of ice turning to steam, series parallel ohms law problems, gas law calculations, or any host of other primarily mental exercises that nearly all HVAC/R students must suffer through. Usually, the students asking are doing the most suffering. Although I can justify all of the above as an endeavor to garner a deeper understanding of the principles which make HVAC/R systems work, I usually tell them that you can’t become a champion weight lifter by lifting marshmallows. Service techs are not paid to connect gauges or take voltage readings, they are paid to solve problems. Obviously techs need to be familiar with all the tools at their disposal, but we should never overlook the fact that their primary tool sits on their shoulders. I am afraid the current focus on standardized testing throughout our educational system has not prepared our students for practical problem solving. Rather, they are used to selecting the best solution from a very limited set of answers. My prescription? Lots of work that requires students to recognize and define the problem, systematically find the cause, and offer a solution. Assign work that requires students to provide written answers. Ask questions whose answers have not been explicitly stated, but require students to put two pieces of information together. If you get an answer that is way off base, try and ask leading questions to help the student work their way through a solution to the problem. The exact answer is not as important as the process. Encouraging students to use their minds to solve problems is crucial. The shop is a great place to work on problem solving because real life problems are never as simple as a, b, c or d. But don’t accept “the part is bad” as an answer. The student should be able to explain what the unit is doing wrong, what is the cause, how they determined the cause of the problem, and what their proposed solution is. Truth be told, I believe this approach could work in a lot of fields besides HVAC/R. The leaders in any field are always people who have recognized and defined problems and then devised solutions.

Teaching Capacitive Reactance

A capacitor is basically an electrical spring. It stores an electrical charge when the voltage increases, and it releases that electrical charge when the voltage starts to drop. Capacitors are used smooth out the output of DC power supplies. When a capacitor is connected in an AC circuit it alternately charges and discharges with the AC current. This has the effect of causing voltage to peak after the current peaks. Note that this is exactly the opposite of inductive reactance which causes current to peak after the voltage peaks. Adding a capacitor in series with an inductive load offsets the inductive reactance so that the voltage and current peak together. Capacitors are used with PSC motors to improve their efficiency by offsetting the inductive reactance of the motor windings. This is why PSC motors became the standard motor for compressors and fans. Several years ago I had a furnace with a shaded pole indoor blower motor. When it died, I decided to replace it with a similar size PSC motor. The replacement PSC blower motor was slightly larger in horsepower than the original shaded pole motor, but drew only half as many amps while operating. Of course today PSC motors are being replaced with brushless DC, or ECM motors for efficiency savings that are equally dramatic.

You can do a demonstration of the effects of inductive and capacitive reactance by wiring a 60 watt light, a 120 volt 10 watt unit bearing shaded pole motor, and a 10 MFD run capacitor in series to a 120 volt power source. Wire a switch in parallel to the shaded pole motor and another switch in parallel with the capacitor. The switches will allow you to take the motor or capacitor out of the circuit by simply bypassing them. Ideally, you want to use an oscilloscope to show the source voltage and the voltage across the light. The light will remain in phase with the circuit current since the light is a resistive load. The phase relationship between the source voltage and the voltage across the light will show the relationship between voltage and current. Operating the switches will show the effects of inductive reactance, capacitive reactance, and the combination of the two. Even without an oscilloscope you can show that the voltage across the three devices adds up to more than the applied voltage. This shows why run capacitors must be rated for a higher voltage than the voltage to the unit.

I did a powerpoint presentation for the National HVACR Educators & Trainers Workshop this past March that includes an interactive virtual lab to show inductive and capacitive reactance. You can download the presentation at the HVAC Excellence web site. It is listed in the free downloads as "Practical Labs." Students can click on the switches to control them and see the results. For more ideas on teaching alternating current fundamentals check out Unit 29 Electrical Power and Circuits in Fundamentals of HVAC/R.

Dry Out!

Its spring, the weather is warming up, flowers are blooming, and our thoughts turn to dehumidification. In humid climates like the southeast, removing humidity from the air in warm weather is just is just as important to comfort as reducing its temperature. Being warm blooded, our body normally produces more heat than it needs and then regulates our temperature using different cooling mechanisms. The primary cooling mechanism is evaporation of perspiration from our skin. Dry air makes us feel cooler because it accelerates the evaporation from our skin. Humid air makes us feel warmer because the evaporation process is slowed down. Dehumidification can be the difference between being comfortable and being uncomfortable at 78°F. Dehumidification can save energy by reducing the amount of sensible cooling required for comfort. Many people with oversized air conditioning systems essentially over cool their house to be comfortable because their systems do not run long enough in mild weather to reduce humidity, so they do not feel comfortable until they reach temperatures of 70°F in their house. It takes several minutes for most air conditioning coils to get cold enough to sweat. An over sized system will often satisfy the thermostat and shut off shortly after the coil reaches dew pont. A properly sized air conditioning system will run longer, allowing longer operation with an evaporator operating below dew point and removing more water from the air. Systems with ECM blowers and thermidistat controllers have a special dehumidification mode that reduces system airflow for dehumidification. This increases the latent system capacity and decreases its sensible capacity. Two stage cooling systems can help by allowing longer system operation at moderate loads. All of these are a big improvement over the typical oversized single capacity system with a PSC blower. However, an air conditioner is still not a dehumidifier.

If you are really serious about dehumidification you need a dehumidifier. In a nutshell, a dehumidifier is an air conditioner with a single blower that moves air first over the evaporator, and then over the condenser. The air first passes over the evaporator where it is cooled to dew point to remove water, and then the same air passes over the condenser where it is reheated to a temperature slightly above its original temperature. If you have a basement in the southeast you NEED a dehumidifier. My basement stays below 80°F all the time without air conditioning, but it feels warm without my dehumidifier operating. In the past most dehumidifiers have been small console types that are noisy, inconvenient, and typically undersized. Several companies now offer whole house dehumidifiers that can be integrated into a complete comfort system for your house. They have enough capacity for a house, are far quieter, and do not require you to empty a bucket twice a day. Therma-Stor has a good short animation on why basements have high a relative humidity and how a dehumidifier addresses this problem. You will find other articles on their website which help address specific dehumidifier applications. They also have a psychrometric chart in a round format that looks like a ductulator that are great for teaching psychrometrics. To read more about the effect of humidity on comfort, check out Unit 61 Fundamentals of Psychrometrics in Fundamentals of HVAC/R. To download an interactive pschrometric chart for free, go to HandsDown Software.

Free RSES Jounal for Students

I truly believe that one of the best career decisions anyone can make is to choose something they love. After all, most of us are going to spend the majority of our conscious lifetimes at work. So if you hate your work, your life will be miserable. By contrast, if you enjoy your work, your life will be more fulfilling and enjoyable. However, young people often have not experienced enough of the world to really know what they will like. Unless they grow up in a family that works in the HVAC/R field, most teenagers really know very little about it. We should be cultivating a love of the HVAC/R field. One way is to expose your students to the wide array of careers in the HVAC/R field through professional journals. I bring in my RSES Journals to school and strategically place them around the class and lab for the students to pick up and read. OK, so maybe I leave them laying around – the effect is that students do read them. They will ask me about articles they read and are interested in. Many students use them as resources in other classes. Students have used RSES Journals for research articles and even for speech class. One student discovered that he could stand up and give an extemporaneous speech on air conditioning anytime based on RSES articles he read. After a few speeches on the refrigeration cycle, scroll compressors, and heat pumps the teacher told him the next speech could not be on air conditioning. She later told me that she never met anyone before who loved his field of study so much. That should be our goal – to instill respect for the craft and pride in learning and practicing it. RSES would like to help us out. They are offering FREE one year subscriptions to HVAC/R students. To get their free subscription they need to go to the RSES website http://www.rses.org/studentjournal.aspx The good folks at RSES did stress that this offer is for students only.

I think getting the RSES Journal for free is a great start at getting a whole new generation of HVAC/R technicians interested in a great organization and a rewarding career. Right after getting all your students signed up for a free RSES Journal subscription, consider thanking RSES by joining JOIN RSES. If you are already a member, thanks!