Constant Power Load Concept, a myth?

[itsEEE]

Hey everyone
I have some doubts over the 'concept' of Constant Power Load and its viability.

I have heard people often using this statement - " With drop in the voltage the current across the load increases as the power of the load remains same, which as a result increases the heating losses."

First, is this statement correct?

My opinion -

W=VxI ,

We as a source, the ones who has the control over the supply to the load, can control the voltage and thus depending upon the internal resistance/impedance of the load, the current should be controlled 'proportionally'.


Maybe it also depends upon the nature of the load as in Induction motor with decrease in the voltage, the current increases.
Then , I guess we should not generalize the statement I mentioned above.

Now please share your views on this and the concept of Const Pwr Ld.

Thank you

Your Current state is at high Potential
Hehe....

 

[3DDave]

Which "people" say this?

Second, conservation of energy indicates this would generally be true.

What matters is the nature of the control system. A constant load usually means a constant operating RPM or constant thermal dissipation.

Consider a filament light bulb controlled with a variac. As the voltage drops, the variac has to remain on longer, drawing more current, to keep the bulb brightness the same.

 

[itsEEE]

Dear 3DDave, thankyou for your valuable input.

Will you please explain that how is it possible that even on decreasing the applied voltage , the current to the load will remain same, as you said to maintain the brightness.

But is it possible? You mean a Variac is a constant current source irrespective of the voltage applied to the load?

Please help me out in understanding this concept that how the bulb is able to demand the same current.

Since I guess it is nothing to do with the Variac itself, as it is a different concept of increasing load current and where the is transfomer able to provide it.

Thank you

Your Current state is at high Potential
Hehe....

 

[3DDave]

Sorry - triac, not variac.

"the triac has to remain on longer, drawing more current"

 

[itsEEE]

No problem, but even in the case of TRIAC,I think we should not comment on that, as it is a semiconductor controlled device and there we are just controlling or we can say, creating such scenarios where the current is need to be constant.

But in normal conditions with linear loads or at very basic level, the resistive load, I guess they are not the constant power loads.

So the question is, wether the statement I mentioned at first is applicable everywhere with all types of load or not?

Thnx

Your Current state is at high Potential
Hehe....

 

[3DDave]

WHEN I WRITE THAT THE CURRENT ABSOLUTELY WILL NOT BE CONSTANT WHY DO YOU REPLY THAT IT WILL BE?

THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED. THE CURRENT IS CHANGED.

DO YOU UNDERSTAND THAT FOR CONSTANT POWER AT REDUCED VOLTAGE THE CURRENT IS CHANGED?

 

[rb1957]

well, that took a nasty turn ! ...

I'm not a sparky so forgive the ignorance but is the OP's question "How does the current increase if the voltage decreases ?" Is the OP, not to put words into their mouth, talking about a simple case where you have a single power source and a single power draw and is asking about the relationship between voltage and current coming from the source and how the load is affecting the source (and vis versa). As opposing the the complicated real world where there are multiple sources and loads, how does a single load work ??

The OP makes the point about he's in control of the voltage, so how/why does the load change the current ? Is this overlooking the power supply ... if the supply voltage drops then the supply current increases to match the power supplied to the source.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[IRstuff]

A constant power load means that the apparent load impedance MAY change to compensate for an input voltage change.

The only reason that the current will drop if the voltage drops is if the load is closer to a constant impedance than a constant power load.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[3DDave]

I wonder still:

How is it the OP "heard people often using this statement"? Who are the "people"
What is the exact" linear load" considered in this case. What is it linear in relation to?
What are "all types of load"
Why does the OP have the audacity to say "I think we should not comment on that"?
If a resistive load cannot be controlled to produce constant power then what load does the OP figure does require constant power regardless of voltage?
How lazy to type "Thnx" instead of "Thanks." Do that with close friends, not professionals.
"creating such scenarios where the current is need to be constant." is the opposite of what I wrote and gets to the heart of the matter:

What is it that ensures that the power required is constant? If the voltage drops then it has to be that the current increases.

Maybe the OP hasn't any idea of what constant power means and needs to go back to read that section of the textbook and stop talking to "people." Then they can return and post that textbook definition and explanation and if they have a question to ask there will be a basis for that question.

 

[3DDave]

Finally - is this load the load that is put on the power supply or the load that is put on the device.

Does a motor have to maintain a constant torque at a constant RPM or a constant thermal output like a constant power load tester does?

 

[waross]

If you have control of the voltage, it is what it is.
In industrial plants, the end user seldom has control of the voltage.
The voltage often varies as the load on the grid varies.
By far the greatest use of power in North America is driving integral horsepower induction motors.
Within limits, an induction motor approximates a constant power load.
Nothing is absolute because of the interaction of various effects, but consider a motor driving a pump.
A slight drop in voltage will cause a slight drop in torque.
The motor will drop slightly down the speed/torque curve to maintain the load.
The same torque at less Volts means more Amps.
But wait; As a pump slows down the torque demanded drops dramatically.
The drop in speed may be less than 1 RPM.
Like I said, there is no absolute due to interactions and this is one interaction.
The drop in RPM is much less than the drop in the torque demanded by the load at the slightly lower speed.
Throttling valves and dynamic head also interact slightly.
Within limits, an induction motor approximates a constant power load.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[itsEEE]

Thank You everyone

PS- 3DDAVE, Thank you

 

[rb1957]

c'mon OP ... how did we help ? what were you confused about ?/

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.