cos(phi) and induction motors, reactive power ?

[StoppTidigare]

Hi all, I have a question about something that I've never completely understood.
It is about reactive power:

When it is said that cos(phi) is, say, 0.7 (I just pick it from the air).
Does it mean that the motor requires this phasedifference and doesn't work well if it doesn't get it, or does it mean that it pulls the phase between S and P to "phi", and that this phase difference will be availble for all other consumers ?

As I have been told one never pays for the apparent power. How do the power suppliers measure the P, becuase this is what the consumer pays for. The meter that everyone has with a rotating disk, does it measure real power P ?

Kindest regards,
StoppTidigare

 

[jacekd]

or does it mean that it pulls the phase between S and P to "phi"

Almost true, it pulls the phase between U and I.

The meter that everyone has with a rotating disk, does it measure real power P ?

Yes, usually. Some of them measure reactive power.

 

[mandrei]

In a pure resistor:

U and I are in Phase
cos(phi)=1 (power factor=1)
People at the energy plant are happy: you consume exactly what they deliver.
You are happy: all the energy is disipated by your R (in your home)


In a pure L:
I is delayed in respect with U by 90 degree
cos(phi)=0
L acumulate energy (but not get hot because is ideal), then this L will deliver exactly the same energy back to the electrical plant.
The problem is that returned I have now an 90 degree lag and is merged (added) with original I from electrical plant.
Because wires from electrical plant to your home have some R, all this returned energy will get disipated on that R (of the wires)

Aparently you do not consume any energy (all is returned)
But peoples at electrical plant observe a loss of energy in theyr system and have no posibility to catch you!!
They are not happy...

Therefore, for large L consumers (like motors) there are strict regulations from energy plants to have at least a cos(pfi)>0.9 (Not to heat theyr wires with unregistered energy ...)

:wink: :wink: :wink:

 

[nicleo]

Good explanation from mandrei ...
Something to add ... conventional meter (based on rotating disk) is designed to measure the active power, assuming that the V and I are pure sinewave. However, due to the increasing use of power electronics or semiconductor devices, the current might be distorted and the meter might not measure correct. However, I do not know whether the error (due to high harmonics) is beneficial to customer or power supplier. Anyone may clarify about this ...

 

[marie65]

The powermeters (even the electronic types) have a limited bandwith, so the armonics produce by high power comutation devices like triacs ,mosfet, igbt are measure incorectly by powermeters. The energy is lost on power wires not only by heat but also by EM radiation due to high frequency of the armonics. I think that the power plants are not very happy about armonics in their power lines from both technical and economical point of view.

 

[mostafa0020]

The rated power factor descreibes the equivalent circuit of the IM on rated speed, torque and voltage.
Leakage inductance makes it not to equal to unity and allways to be a lagging power factor,
The reactive power isn't a real power that can be measured as it is not an average power like the real one,it just describe the usefulness of the drained current for a certain load,I enclose a good papper regarding such concepts!!worth to see it indeed!!

 

[nicleo]

The powermeters (even the electronic types) have a limited bandwith, so the armonics produce by high power comutation devices like triacs ,mosfet, igbt are measure incorectly by powermeters. The energy is lost on power wires not only by heat but also by EM radiation due to high frequency of the armonics. I think that the power plants are not very happy about armonics in their power lines from both technical and economical point of view.

It's true that powermeters have limited bandwidth. If I'm not mistaken, according to a IEEE standard (can not remember the title of the document), we just need to measure up to 50th (or 51st or 52nd) harmonics. I believe this is for POWER application. For EMC testing, higher bandwidth technique should be used.