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Magic Number 2652 Explained

Testing a capacitor under load means testing it while it is in an operating circuit. To do this you measure the operating capacitor amp draw and voltage and then apply them to the formula

Why does this work and where does the 2652 come from? To answer these questions, we need to understand what a capacitor does in an AC motor circuit.

What Does a Capacitor Do?

A run capacitor’s job is to add enough capacitive reactance to offset the inductive reactance of the winding it is in series with. Current in an inductive (magnetic) load lags the voltage. This means that the current peaks AFTER the voltage. Since the current and voltage are out of phase with each other, they don’t work together, causing inefficiency. Adding a capacitor in series with an inductive (magnetic) load helps correct this because capacitors cause the current to peak BEFORE the voltage. The amount of capacitive reactance needed depends upon the inductive reactance of the motor.

Like resistance, capacitive reactance is measured in ohms. The capacitive reactance produced by a particular capacitor varies with both the frequency of the AC current and the microfarad capacity of the capacitor. Higher frequencies and higher microfarad capacity both decrease capacitive reactance. The formula is

 This means that capacitive reactance is equal to the inverse of the product of 2 x pi x frequency x Farad rating. Through the magic of algebra we know that we can swap the XC (capacitive reactance) and C (capacitance) terms to get our formula for capacitance. That gives us the formula

 This can be rewritten as  

In this formula, 2π is a mathematical expression for a cycle. Recalling that the circumference of a circle is twice the radius times π, the expression 2π represents a complete turn of a circle if we are not concerned with the circle’s radius. Frequency is represented by f, which is always 60 cycles in North America. Together, 1/2πf calculates the effect of frequency on capacitive reactance.

works out to 0.00265258 for 60 cycle power. This would produce an answer in Farads, but we normally work with microfarads. Multiplying this by 1000 to get 2652.58 produces an answer in microfarads. This is usually rounded to 2652.

What about the capacitive reactance, XC? Remember that capacitive reactance is measured in ohms and that ohms can be found by dividing volts by amps. So we can substitute the capacitor voltage divided by the capacitor amps for the capacitive reactance. However, since 1/XC is the inverse of capacitive reactance, the fraction is flipped to perform the multiplication, placing amps on top. Together the two terms become

Teach the Process , part 2

In an earlier post entitled “Teach the Process”, I made the observation that many students across all levels of education fail to gain a thorough understanding of the subject matter being taught. I named standardized, multiple-choice tests as the primary culprit. It is not that these assessments don’t have value. It is that they should just be a part of the learning tapestry, not the overarching goal. Nobody’s educational goal should be to pass a multiple choice test. The goal should be to understand the subject matter. Really, this gets back to how we learn. I found a quote by Ruth and Art Winter describing what learning is. According to the Winters, “Learning is the ability to make sense out of something you observe based on your past experience and being able to take that observation and associate it with meaning.” Not just storing away facts, but organizing and associating these facts in our minds so that we can use previous experiences to understand new ones. In our brains, we make new connections between neurons when we learn new things. The more connections you have to any given “fact”, the better the likelihood you will be able to recall it and use it for meaningful association. It is a teacher’s job to help students make these connections. We need to show how the data are related and engage the student’s interest. To achieve good results, teachers need use a variety of techniques beside the standard, and somewhat boring lecture.

Visual aids help catch student attention and can illustrate things that are awkward to describe in words. For example, try writing out the detail for connecting a standard gauge manifold. Each valve on the system needs a name, each valve on the manifold needs a name, you have to describe which way valves are being turned or positioned. It gets complicated and is very confusing to read. Now replace that with a video showing the connections. It is much easier to understand and a whole lot less confusing.

Manipulatives are items that students hold in their hands that help them learn. This is actually a term used for kindergarten classes. I have found that what works with kindergarteners also works with adults. So if you are talking about electric meters, each student should have a meter in their hand during the lecture. Then design the lesson around the fact that the students can actually handle and operate the meters during the lesson.

Analogies are great for helping students understand concepts. Dave Boyd of Appion used to compare pulling a vacuum through a 1/4” charging manifold  to the traffic jams created by merging two lanes of traffic into one. Everyone has experienced that and can relate that experience to the gas slowing down as it travels through the manifold.

Scaffolding is a way to help students reach a higher level of understanding by building information one piece at a time. It can be more successful to describe a complicated procedure in small steps. One way we do this at Athens Tech is to wire a project resembling a packaged air conditioner one circuit at a time. Students often are glassy eyed when they see the whole thing, already done. But each circuit is pretty simple. So doing one at a time gets the job done.

All of these techniques involve exploring relationships. The more ways you can describe something and how it relates to other things, the more brain connections you build. The more connections you build, the better your chance of using that information. I will be speaking at the upcoming National HVACR Education Conference by HVAC Excellence in Las Vegas on Tuesday, March 22. Come see me and we can brainstorm about more ways to make connections.

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