Sunday, February 21, 2010

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.

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