very stupid question

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iwire

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Location
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I pulled these from Google

What is the definition of linear load?

An AC electrical load in which both current and voltage waves reach their peak simultaneously.


Nonlinear loads are any loads which draw current which is not sinusoidal and include such equipment as arc furnaces, gas discharge lighting, solid state motor drives, battery chargers, UPS systems, and the increasingly common electronic power supply.

Check out this EC&M article The How's and Why's of Harmonic Distortion
 

76nemo

Senior Member
Location
Ogdensburg, NY
Man I am dumb! Let's back up a minute, will you? I don't like the OP's title to his thread. There ARE NO STUPID questions, never. Sorry I missed that. Don't ever think any question is stupid. The only thing stupid is not asking the question.

Carry on,........
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
080614-0705 EST

A linear load is one, such as a resistor, where the waveform of the current is identical to the voltage waveform. However, there may be a time displacement of one waveform relative to the other.

Obviously a non-linear load will have some distortion of one of these waveforms relative to the other. Diodes, transistors, vacuum tubes, batteries, iron cored inductors, transformers, thermistors, motors, gas discharge lights, and arcs are some examples.

Consider a power transformer --- these are usually made with an iron core and are operated such that the excitation voltage forces the core partially into saturation. The core going into saturation causes more current proportionally during that portion of the cycle than if it was not saturating and thus a distortion of the current waveform relative to the voltage waveform.

A diode is bascially a one way device and thus current flows only during 1/2 of an AC cycle.

Some of the above mentioned non-linear devices are treated as linear devices when operated in limited portions of their operating ranges.

A linear device with pure sine wave excitation produces no harmonics in the current. Any non-linear device with the same excitation produces harmonics in the current.

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charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
iwire said:
I pulled these from Google
Please tell me that the Internet Search pointed you to Wikipedia! I love getting evidence that that "information" source is not to be trusted. :) (Sorry, Bob.)

As Gar has already pointed out, though perhaps in complex terms, the voltage and current need not reach their peaks at the same time, in order for the load to be called "linear." A resistor is a linear load, and it has that characteristic. But a coil and a capacitor also are linear, though they do not have their voltages and currents reach their peaks at the same times.

The simplest way I can say it will first require the reader to know what a "sine wave" looks like. I think most of us could draw one: smooth curve that goes above and below the zero line in an even pattern. If you are not sure, then Google "sine Wave," and look for a picture (somewhere other than Wikedpedia).

A load will be "non-linear" if you apply a sine wave as the voltage source, but you don't get a sine wave as the current flowing through the load. If between one positive peak and the next positive peak the curve is not smooth, if it has ups and downs within a single cycle, if it suddenly cuts off (current goes to zero) in the middle of a cycle (such as might happen with certain types of light dimming systems), then the load is "non-linear."
 

iwire

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Staff member
Location
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charlie b said:
Please tell me that the Internet Search pointed you to Wikipedia! I love getting evidence that that "information" source is not to be trusted. :) (Sorry, Bob.)

Of course it did, all searches now lead to Wikkipedia, but I chose to ignore those links for this type of info.

I am really not sure if your saying you trust Wikipedia or not. :-?

I don't, it can be edited at anytime by anyone. Heck we don't allow even allow that here in this forum.

http://www.msnbc.msn.com/id/17740041/[/QUOTE]
 

76nemo

Senior Member
Location
Ogdensburg, NY
I still haven't obtained the optical cable for my scope. Can't someone show some forms? I'd ask brian, but I know how busy he stays. Anyone have some forms saved to post???
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
080614-1257 EST

76nemo:

It took considerable searching to find the following plot:
http://www.ece.mtu.edu/faculty/bamork/FR_WG/Panel/MartinezMork_panel03.pdf
Then scroll down about to about 3/4 on the side scroll bar. Look for a plot titled "Steady-State Excitation - Case 2".
This is a plot of the sine wave excitation voltage and the resultant magnetizing current, This is a good illustration of a non-linear load.

I found this with Google using
plot of transformer magnetizing current as compared to the excitation voltage
and the result was about the 20th.

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charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
iwire said:
I am really not sure if your saying you trust Wikipedia or not. :-?
I do not, for the same reasons that you do not. Also for one other reason: My wife is a professional librarian. Being able to obtain two or more authoritative information sources to answer any reference question is a vital part of her job.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
There ARE NO STUPID questions, never.

Here is the scenario:

The telephone rings and I answer it. It's my mother and she asks to speak to my better half. Since she is standing next to me I just hand the phone over. My better half takes the phone and ends up talking for about 20 minutes. Upon hanging up the phone she looks at me and asks, "Who was that?"
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
K8MHZ said:
Upon hanging up the phone she looks at me and asks, "Who was that?"

0009362462632_500X500.jpg
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
For a linear load, the current is directly proportional to the voltage applied. What that means is that if you double the voltage, the current will double. If you raise the voltage by 10x, the current will go up by 10x. If you change the direction of the voltage without changing the amount of voltage, the current will also change direction, but keep the same value. And for zero volts applied, there is zero amps.

If you graph the voltage vs. current for a linear load, it would be a straight line going through 0,0.

Everything else in non-linear.
 
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