Help!! How do you increase total harmonic distortion?

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escalator
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Help!! How do you increase total harmonic distortion?

Post by escalator »

Hi, I'm investigating the relationship between total harmonic distortion (THD) of an alternating current and power output in my physics project. I'm planning on using a multimeter to measure current frequency and then calculating THD using the equation THD=[√(I²-I₁²)]/I₁, where I₁ is the RMS of the fundamental wave. I have never done a practical using alternating current before (maybe there's a reason for that?) so I have a few questions on how to go about doing this:
  • 1. How do you increase THD? The sources I've read say that the most common way to do that is to add a nonlinear load to the circuit, but how do you make it so it's quantifiable and able to be adjusted?

    2. How do you obtain an alternating current? Is there a power source solely for a.c. that's commonly found in the lab? Or can the current from a wall outlet be reduced to a level that's appropriate for a lab experiment?

    3. Does the current have to be converted to d.c. in order to use an ammeter and voltmeter? Or should I simply use a multimeter on an a.c. to measure the final current/voltage?
Sorry if some of the questions don't make any sense :( If this experiment is not feasible, I'll probably scrap it and pick a more familiar topic. Thank you!
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Re: Help!! How do you increase total harmonic distortion?

Post by bfinio »

Hi escalator,

Sorry that you didn't get an initial reply to your post. This topic is more advanced that what we usually see on our forums. I write many of the electronics projects at Science Buddies but I'm a mechanical engineer by training and most of our projects are DC/battery-powered, so I'm not that familiar with the topic of THD. I will do my best to answer your questions and I will message one of our volunteers who's a retired electrical engineer to see if he can get you some better answers.

1. I am not sure and would need to read more about it.

2. Many electronics labs have function generators that can generate arbitrary waveforms (sine, square, triangle, etc), but these are not designed to drive a load or provide a lot of current. So whether you can use one sort of depends on the answer to #1 and what you're connecting to the circuit, and whether you'd need a separate amplification stage. Either way, don't mess with power directly from the wall outlet.

3. It depends on the multimeter. More expensive multimeters can measure both AC voltage and current. The two cheaper meters I have can measure AC voltage but not current.

Hope that helps,

Ben
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Re: Help!! How do you increase total harmonic distortion?

Post by rmarz »

escalator - I've read your outline of establishing a relationship between distortion in an AC waveform and power. I have some issues with that premise.

1) You can introduce significant harmonic distortion to a sinusoidal waveform and have no change in the amount of power in that waveform envelope. To me, that begs a question as to what you are trying to establish.

2) Your planned procedure would try to use voltmeters and ammeters to make AC measurements. As a suggestion, to understand or observe waveform harmonic distortion, I would think that access to an oscilloscope would be of greater starting benefit. Your suggested method has some real questions as to the "quantifiable" readings once distortion is induced.

3) You asked some additional questions regarding AC signal sources. Certainly an AC source could be a wall plug-in power supply that are used in so many devices, appliances and electronic products including doorbells, small battery chargers and battery operated tool chargers etc. The other source of a clean AC source would be an audio oscillator.

4) Your questions regarding use of a multimeter suggests you might want to review a Science Buddies resource on using multimeters.

You might want to think about your original hypothesis before continuing. I'd be happy to help.

Rick Marz
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