Power Usage-Voltage and Current Interaction

[Humble2000]

We are doing a power quality monitoring at the main service. Atsome particular time of the day a voltage sag happened which the main 347V drpped down to 120V and caused all the equipment to shutdwon for around 16Cycle.
I understaned the basic rule of power P=V*I .Is this true for all loads?
any type of load?
Heating lopads,motors...?
Is there an instance whcih if the voltage dropps the current droops too?Well obviously the same power would not be delivered, just need some expert feedbacks.

Thank you,

 

[davidbeach]

Loads can generally be split into constant impedance and constant power (other variations possible and some limitations on voltage range for which constant power applies). As voltage goes down at a constant power load, current goes up, but as voltage goes down at a constant impedance load, current also goes down.

 

[Humble2000]

So a heating elements are constant impedance.
How aout HVAC systems or Pumps or air conditioners,motors...
How to caractorize them?

 

[davidbeach]

Motors try to be constant power, over some range of voltage.

 

[ScottyUK]

Switching power supplies are remarkably close to 'ideal' constant power loads. Fluorescent lighting can do funny things depending on the ballast. Closed loop systems such as rectifiers can behave in a more-or-less constant power mode.


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Sometimes I only open my mouth to swap feet...

 

[Humble2000]

Davidbeach,

what range of voltage?where can I get the detailed info?
How about HVAC systems or Pumps or air conditioners?
Well pumps or compressors are motors..
when the voltage dropes down and current goes up,is this the indication that most of the plant loading is constant power loads such as motors?but if the voltage goes lower than certain vaue, then it might behave as constant impedance right?

 

[itsmoked]

Humble2000; Motors of any type normally found in the areas you mention are constant power sources to the extent that as the voltage drops their current rises. It rises until the IR losses and supplying transformer cores and their IR losses start limiting the available current. Usually around that point things start breaking, letting out their smoke, and tripping off, a generally unpredictable and chaotic situation. You likely won't find ANY documents clearly describing to a level high enough, to allow you any "planning", for your specific facility/situations.

Resistive loads will shed to the equation Power = V²/R and so are definitely not constant power. But of course the current will drop linearly.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[CJCPE]

Humble2000:
Regarding P=V*I, for AC power, P=V*I*pf where the power factor (pf) is 1 for resistive loads and less than 1 for motors. For balanced 3 phase, P=V*I*pf*1.732 where V is line to line volts and I is line current.

For additional information about what happens to motors when voltage droops, look at:

http://www.reliance.com/mtr/pwrspymn.htm.

 

[rbulsara]

Just curious, what does your power/voltage/current question has to do with the voltage sag, rather severe one, that shutdown the loads?

 

[Humble2000]

Thanks everyone for their valuable responces.

wELL, I NEED TO KNOW what type of load with this charactristics can cause a voltage sag of 35% for 285ms .
There is not a huge motor in the plant?
and the problem suspected to be initiated inside the plant.

 

[jghrist]

The voltage sag at the main service was probably caused by a fault somewhere else on the utility's distribution system. It could have been caused by the starting of a large motor in the plant, but if that were the case, the plant has an inadequate distribution system and you would have problems a lot more often.

 

[rbulsara]

As jghrist said, I suspect the sag is due to utiltiy side faults than internal loads. Check with your utiltiy company if something occured at the time of the sag.

 

[Humble2000]

I like to send you a picture of this fault...
Can I do this?

 

[itsmoked]

faq238-1161

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com