#196588  10/15/10 02:45 PM
Definition of a "Linear load"

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Registered: 07/05/04
Posts: 780
Loc: Winchester, NH, US


I always thought a linear load was one that exhibited resistor like behavior. Double the voltage, the current doubles. Then I thought about reactive loads. The current will change when you change the voltage, but the current doesn't immediately track the change in voltage. Are reactive loads also linear loads? thanks

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#196600  10/16/10 02:51 AM
Re: Definition of a "Linear load"
[Re: LarryC]

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Registered: 10/19/00
Posts: 2707
Loc: Anaheim, CA. USA


Larry,
In specific terms, a "Linear Load" would be described as something with nearly Zero Distortion, and a very High Power Factor. This would be a great definition of Linear Load in a Perfect World; a place where Right Triangles do not exist... But since perfection exists in concept only  and Right Triangles are a reality, we must revise the term per the large Monkey Wrench, tossed into the Machinery  and that Machine is known as "Linear Load".
Values involved:
1: "True Power" Specific Term = Wattage ("W" or "P"). Typically a Heat Component, but directly refers to Work (such as Horsepower, BTUs, Newtons). True Power is the usable portion of Electrical Transduction.
Nominal Unit is Kilo Watts (KW)  or 1,000 Watts.
Where used to describe TOTAL WORK DONE (Wattage over time), the relevant term is WattHour, or more appropriate  Kilo WattHour "KWH" (1,000 Watts drawn for 60 Minutes)
When Loads are connected to an Alternating Current Source, the True Power portion is represented as "Sine" on a Right Triangle  AKA "Baseline" or the base of the Triangle.
Per "X/Y/Z Coordinates", this would be represented on the "X" Axis.
.............
2: "Reactive Power" Specific Term = Volt Amps Reactive ("VAR").
This is the "Charging" Power of Loads and Circuitry.
With an Inductive Reactance Load, Reactive Power is the "Magnetizing Component".
Where Reactive Power is viewed, no "Actual Work" is performed directly. The Power "Bounces" between Source and Load.
Nominal Unit is Kilo Volt Amps Reactive (KVAR)  or 1,000 Volt Amps Reactive.
Where used to describe OVER TIME CONSUMPTION (VARs over time), the relevant term is Volt Amp ReactiveHour, or more appropriate  Kilo Volt Amp ReactiveHour (1,000 VARs drawn for 60 Minutes)
When Loads are connected to an Alternating Current Source, the Reactive Power portion is represented as "Cosine" on a Right Triangle  90 Degree Angle from the True Power baseline.
Per "X/Y/Z Coordinates", this would be represented on the "Y" Axis.
.............
3: "Apparent Power" Specific Term = Volt Amp ("VA").
This is the "Complete Power Package" for Loads, as it contains both the usable True Power, and the required Reactive Power.
All Loads and Circuits will contain both True and Reactive Power, and as such, the total drawn Apparent Power vs total drawn True Power determines the Load's Power Factor.
Nominal Unit is Kilo Volt Amps (KVA)  or 1,000 Volt Amps.
Where used to describe OVER TIME CONSUMPTION (VAs over time), the relevant term is Volt AmpHour, or more appropriate  Kilo Volt AmpHour (1,000 VAs drawn for 60 Minutes)
When Loads are connected to an Alternating Current Source, the Apparent Power portion is represented as the "Tangent" on a Right Triangle  connected between the "Far End" of the True Power Baseline, and the "Top" of the Cosine Reactive Power component.
Per "X/Y/Z Coordinates", this would be represented on the "Z" Axis.
..............
4: "Harmonic Distortion" Specific Term = Total Harmonic Distortion ("THD").
When AC is applied to Reactive Loads, the Reactive Load "Reflects" Energy back into the System, pressing this reflected Energy against the Source Device(s). Harmonic Reflections are viewed as Sine Waves being reflected back into the System, with frequencies being given Multiples of the Fundamental Frequency. Where Electrical Transduction is applied, the most apparent Harmonic Multiples are in the "Odd Order" of the Fundamental.
When applying the Harmonic Component (highest total reflected Energy, per relevant Harmonic Multiples) to the original drawn Load VA package's waveform, the results will be per a Total Harmonic Distortion value.
Per the THD reflected from a given Load, the total VA Load Package now has an additional Load segment added to the Fundamental Load  in multiples of the Synchronous Frequency.
.............. < end of terms description >

Basically, the Linear Load Characteristics refer to the True Power portion of an Apparent Power "Package" (total Load Volt Amps).
Case #1: Fixed, or "Pure Resistance" Load:
If the Load device was a "Pure Resistance Load"  such as an Electric Heating Element, the drawn Power would be mostly True Power. There will always be some level of Reactive Power, but in this case, the Load may be viewed as exhibiting a Power Factor of 1.0 (100% PF).
This is basically a Linear Load. Power is almost 100% True Power.
When viewing True Power, the level drawn from the Power Source is determined by the overall Resistance of the connected Circuitry vs the EMF (Voltage) measured between the two points of the Power Source.
If the connected Circuitry has a "Fixed" opposition level (Fixed Resistance or Impedance), any changes in the EMF will Directly Affect the amount of True Power drawn from the Power Source in a "Linear Fashion"...
* Increasing the EMF results in an increased Current Level, which is a Linear Result of the application against a Fixed opposition. With the increased level of both EMF (Pressure) and Amperes (Flow), a Linear increase of True Power is drawn from the Power Source.
This Linear Result of increased True Power draw is why Incandescent Lamps rapidly fail when the System Voltage exceeds 20% of the Lamp's Voltage rating.
* Decreasing the EMF results in a decreased Current Level, which is a Linear Result of the application against a Fixed opposition. With the decreased level of both EMF (Pressure) and Amperes (Flow), a Linear decrease of True Power is drawn from the Power Source.
This Linear Result of decreased True Power draw is why Incandescent Lamps become "Dim" when the System Voltage drops below the Lamp's Voltage rating.
The Light output is reduced, due to the reduction of drawn True Power; so the Lamp exhibits a Linear Output characteristic.
In the case of fixed opposition loads, the output is also Linear. ...............
Case #2: Variable Opposition Loads  Motors:
In the case of an Electric Motor  more specifically Series Connected Commutator Motors and Induction Motors, the Opposition component which affects the level of True Power drawn from the Source, is attributed to the actual Physical Load applied to the Shaft of the Motor.
Again we have a Linear Load characteristic: increasing the opposition of the Shaft's rotation, also increases the drawn True Power component  with an overall increase in drawn Horsepower plus Developed Horsepower.
Consequentially, decreasing the opposition for the Shaft's rotation also reduces the drawn Horsepower  which equates to a reduction of True Power drawn from the Power Source. Again, we have a Linear Load characteristic being applied to a Load.
This Load is by all means not something described as "Pure Resistance". The Load has a Power Factor below 1.0 (100%)  and in fact, the PF may drop as low as 40%!
However, the "True Power Component" will have a Linear Reaction to changes in the Circuit's values (or levels). Changing the opposition connected to the Motor's output Shaft directly affects the drawn True Power, which also affects the Output Energy (Transduction from Electrical Energy to Mechanical Energy).
With True Power, any change in Pressure (EMF / Voltage) AKA "E", also changes the Intensity of Charges flowing (Current / Amperes) AKA "I". Increased "E" results in increased "I"  which also increases "P"; decreased "E" results in decreased "I"  which also decreases "P".
The Reactive Power component will change with variations in "I", as result of variations in "E".
When the Load exhibits these variations, the input Apparent Power Package is directly affected, yet may appear as if the drawn VA remains steady.

Case #3: Reactive Loads with NonLinear Components:
In situations where the Power Factor of a given Load is very low  like a Zero Power Factor, the connected Circuitry does not draw a significant level of true Power, and therefore is a NonLinear situation.
Whereas the idea of 100% Power Factor and 100% Pure Resistance applies only in Fairyland (conceptual only), so is the extreme opposite  0% Power Factor and 0% Pure Resistance. The connected Circuitry plus the Reactive Load will have some level of Pure resistance. Where ever Heat is "produced", you will find Pure resistance.
Heat is a transduction of True Power, so there will always be a Linear Component involved. In addition, any Sound, Light or other types ElectroMagnetic Radiation (Radio Frequencies, Microwaves, etc.) generated by the load is also a transduction of true Power.
Where a connected Load reflects Harmonic Energy back into the System, the THD component is a NonLinear Load.

I will end this post here, so remarks can be made per the information given.
Let me know if I addressed the original question adequately.
Scott
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#196602  10/16/10 10:33 AM
Re: Definition of a "Linear load"
[Re: Scott35]

Cat Servant
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Registered: 01/22/05
Posts: 5316
Loc: Blue Collar Country


I suppose it's more about whether something is 'sort of' a linear load or not  and just what question we're trying to answer.
For example, your air conditioner is a 'linear load' when it's running  but, over time, the circuit itself sees periods of low draw and high draw.
The same applies to our reasoning when we size services; we recognize that not every circuit will be operating at full draw, all the time.
I don't think the issue of simple current draw is what all the concern over 'linear loads' is about. Instead, I think folks are really asking about harmonics, those pesky things created by electronics that play havoc with the neutral line.
If that's the case, then a simple 'current lead' or 'current lag' isn't the issue. Rather, we're looking at 'displaced' current, where current used at 'time A' is held back, then added to current at 'time B.' Now we have a nonlinear load.

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#196616  10/16/10 08:13 PM
Re: Definition of a "Linear load"
[Re: renosteinke]

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Registered: 07/05/04
Posts: 780
Loc: Winchester, NH, US


Scott and Reno, In summary, pure resistive loads are linear loads, most reactive loads are linear loads, and most modern (SMPS) electronic loads are NOT linear loads. True statement? Larry C

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#196641  10/18/10 07:28 AM
Re: Definition of a "Linear load"
[Re: LarryC]

Member

Registered: 06/16/04
Posts: 1273
Loc: Sacramento, CA


Yes, Larry. That is it.
Chopping circuits trigger harmonics like crazy.
In practical terms  that is what constitutes a 'nonlinear load.'
The original term, which came from electronics/ signal quality has morphed into an expression to describe nonideal electrical loads.
Nonideal means HARMONICS and so, higher frequency resonant waveforms.
Alternating Conductors are really Wave Paths for Energy.
The wave nature of electricity tends to get down played during apprenticeships. Typically, circuits are portrayed as being DC logic. Hot in Drain out. But, in real AC circuits the wave nature of power is paramount.
A tremendous amount of posting here and every other forum revolves around the troops getting up to speed on the nature of wave physics WRT alternating power.
And harmonics/ nonlinear loads is topic number one!
Cheers.
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