non linear loads

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raider1 said:
. . . motors are non-linear.
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Motors are not non-linear. If the motor is supplied via a variable frequency drive, then the VFD itself is non-linear. Perhaps you are confusing the notion of power factor (i.e., current and voltage not being in phase with each other) with the notion of a non-sinusoidal wave form.
 
charlie b said:

Motors are not non-linear. If the motor is supplied via a variable frequency drive, then the VFD itself is non-linear. Perhaps you are confusing the notion of power factor (i.e., current and voltage not being in phase with each other) with the notion of a non-sinusoidal wave form.
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You are correct, I need to learn to not post before my coffee has kicked in.:)

Chris
 
Anything with a "switching" power supply will be a non-linear load. Computers are the best example, but switching power supplies are everywhere now, even some wall-warts are switching supplies.

Most light dimmers will also be non-linear...just about anything that reduces voltage by chopping up the waveform.
 
raider1 said:
You are correct, I need to learn to not post before my coffee has kicked in.:)

Chris
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Chris you posted at 1:10 PM! When does the coffee kick in? (and you have used this excuse before)
 
Mr. Bill said:
Any type of electronics is a non-linear load. Computers, TVs, fluorescent lighting if it has an electronic ballast.
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Is that true? I thought that many modern electronic devices were designed to limit or eliminate harmonics?
 
chris kennedy said:
Chris you posted at 1:10 PM! When does the coffee kick in? (and you have used this excuse before)
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I posted at 8:51 AM Mountain Standard Time.:)

Yes I have used this excuse before, due to the fact that my brain no worky without lots of caffeine.:D

Chris
 
infinity said:
Is that true? I thought that many modern electronic devices were designed to limit or eliminate harmonics?
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What if all nonlinear devices would limit THD to around 1%?
We would still need to count the neutral as a current carrying conductor.
I always thought 310.15-B-4-c should specify a THD (guess I should have authored a proposal...)
 
Any electrical device can be described by its 'current versus voltage characteristic'. Basically this a graph of the current flowing through the load versus the voltage applied to the load, although when you get into detail you need to consider time characteristics (current flowing through the load _now_ as a function of the voltage applied to the load both now and in the past), and there are always more factors that you can consider (eg, how the current versus voltage characteristic changes with temperature or the phase of the moon :) )

If the current versus voltage graph is a straight line, then you have a simple resistive load, and you can describe the load in terms of V/I, or _resistance_. If you apply a nice sinusoidal voltage to a resistive load, then a nice sinusoidal current will flow through that load.

But for many loads, the current versus voltage graph is _not_ a straight line. That is why they are called 'non-linear'.

For example, the rectifier input stage to a switching power supply draws essentially no current until the supply voltage gets high enough to forward bias the diodes into the filter capacitor. A graph of current versus voltage will have _zero_ current until some threshold, then the current will suddenly rise as the threshold is crossed. Apply a nice sinusoidal voltage to that load, and the current flow graph is a set of spikes, located where the peak of the sine curve should be.

Modern switching power supplies can use various techniques to draw reasonably sinusoidal load current in phase with the applied voltage. IMHO they should be considered linear, although a detailed analysis would show them to be non-linear.

-Jon
 
Let us also be aware that the notion of "harmonics" (i.e., non-linear loads) is not the same as the notion of "power factor" (i.e., loads for which there is a phase shift between current and voltage).

In inductive loads, such as motors, and in capacitive loads, such as, well, capacitors, the peak value of current and the peak value of voltage do not take place at the same moment in time. One of the peaks takes place earlier than the other. The value of "power factor" is derived from the amount of time that separates the two (to be precise, it is the cosine of the phase angle between the two). But both curves, the voltage curve and the current curve, remain purely sinusoidal. Also, the amount of time between two successive voltage peaks is exactly the same as the amount of time between two successive current peaks. That is, whatever phase angle separates current from voltage, that phase angle remains the same from cycle to cycle (unless, of course, you do something silly like changing the amount of mechanical load that the motor is being called upon to support).

A motor is a linear load. So too is a capacitor bank. There are no harmonics associated with either. I am of course speaking in the purely hypothetical world of perfect equipment. If you supply power to the motor from a variable frequency drive, then the VFD will be a source of harmonics. But the motor will not.
 
I would like to add arcing equipment as a non-linear load such as welders and electric production furnaces, ect... now would it be safe to say that a non-linear load is one where the impedance changes with applied voltage... Now another question is how would you describe the non-linear loads effect on the aging utility supply system?
 
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