Power Factor

When getting the total power of a motor it is practically expressed as true or real in Kilowatt (KW) or apparent power in kilovolt ampere(KVA). Get the Kw rating/s at the motor/s nameplate/s then add those KW that would your total of motors.

hi

" when we measure the power conumed we need to know the total power factor or not"

.... sure, you need that, but as you have both of a Wattmeter(for measuring the active power in W) & a VARmeter (for measuring the reactive power in VAR),so this measurements can be done by both the WATTmeter & VARAMETER..in other words, by these two devices you can measure the total consumed power in a network...

IF you don't have such electric power measuring devices, you need to check the full-load current, the voltage and the power factor of each motor,as a result, you can measure the total consumed power by adding up the consumed power of all motors.....

This process for the calculation is correct only if the motors are physically connected together at the same nodes (for instance in a paralell connection). Then, it is important to really consider the electrical circuit where the motors are connected first and after that you have to find out the Thevenin equivalent which gives you directly the pf by adding all VARs (∑VAR+ and ∑VAR−) also ∑Ws. Be sure to consider also if you have monophase or three phases motors. In conclusion, you have to consider the electrical circuit topology. Hoping this for some help!

Your answer is very good and practical. It does not need any meters to measure and will give a good approximation of the P.F.

for the other answers: Where they recommend to use Wattmeters and Varmeters: Well since you have these devices lined up to measure these values, why do it on every motor then do the additions etc? Do the measurements at the source of the supply to these motors and get the answer directly.

Thanks msamad.

your info really helped. but i got some ambiguities, as described by one of the guest.

How should be the circuit connection to make the suggested calculation work.

At the plant site, i experienced almost all the motors in parallel, connected to the common switchgear.

At the generator, the PF meter display the power factor, how does it calculate the power factor?

If the generator is equipped with a digital metering device that gives the kW and kVAr or at least measures them, the the Power factor is calculated as explained by msamad.

There are different types of meters in the market. The analogue types will measure a differential value and the scale will reflect a P.F. (details too complicated for you I presume: Just thrust thr results if the quality of the meter is good!).

That is all made to confuse, with regards, unnecessarliy.

Motors are connected in parallel, unless there is a very specific reason to put them in series, and that reason I have not come across in any industrial situation, though they may exist.

As far as you are concerned, neglect that condition.

But if it is a real life scenario, the pfs on motor name-plates are extremely optimistic, as I mentioned. These are the full load power-factors (ie when motor is running at rated load) - again in actual life it is a bit rare situation.

You will be working under much less pf , may be as low as 0.2 to 0.3 or even worse if a motor is running idle (on no load).

Better is to monitor the system KVAR/ pf at the supply point (may be incoming transformer output point, as we do - the panels have the meters showing pf, KVAR, KW,... for all the major nodes). The incoming 11KV is brought down to 415V then is controlled through multiple ACBs of 400-800A each. Each of these ACB panels have the meters.

For us the pf compensation is done by each of these major load points. You can of course bunch them up and do it at a single point too.

Just to note: the motor pf on the name plate are full load pfs (As is the KW)

On actual running condition, the KW is likely to be less and more it is less from the rated, poorer will be the actual pf.

So in actual life, you are going to run with much worse condition (a la pf) than what you calculate vis a vis name plate values.

Just to confuse you a bit more- motors are not the only devices that affect the site's total power factor. ANY electrical device that is not a true resistive load will have or cause some power factor.

The primary "bad thing" about power factor is its impact on power wiring temperature rise (internal to the plant) and its impact on the utility grid and the amount of energy that the utility supplier must feed into the grid to satisfy your power needs- the lower the power factor, the higher the required energy input.

The main thing to understand is that power factor is the algebraic statement of the relationship between APPARENT POWER (kW- what you measure with an ammeter or some other kW metering device) and ACTUAL POWER (kVA that it takes to drive the device). Looked at in a force vector diagram, kW (apparent power) is the longer leg of a right triangle, reactive power is the shorter leg, and actual power is the hypotenuse of the triangle. Power factor is the CoSine of the angle formed by the apparent power and the actual power. As the reactive power gets larger, the actual power gets longer and the CoSine gets smaller (because the hypotenuse is getting larger). If the power factor for any particular sum-main feeder gets too low, the power flow will begin to overheat the wiring- National Electric Code basic design for sizing wiring assumes a 0.8 power factor so iv it is lower than that, the wiring is not sized to carry the true power without overheating. In addition to "hot" wires that may cause a breakdown of insulation, the ability of the wiring to carry power is compromised and that feeder will see a voltage drop beyond normal expectations.

From the utility side- If your overall plant power factor falls below some magic number that the utility has posted in its rate structure, you will be financially penalized for the impact that your bad power factor is having on the utility's grid.

The easiest solution for all of these problems is to install capacitors at all major motor feeds (in the motor control center) or install capacitor load banks at all major power feeders (sized to offset the measured power factor per feeder) or, at least, install a central capacitor load bank at the utility inlet connection to at least avoid any financial penalties. If you do not know how to size these capacitor banks, any electrical consulting engineer who works on industrial service can do this very quickly.

Hope this explanation helps you determine how you want to proceed.

There's no point in calculating it because a given motor's power factor depends on how loaded it is. For example, a motor working under 40% load has a power factor that's different from when it's operating at 80%.

If you're interested in the intellectual exercise or if you have five wattmeters and VAR (volt-ampere reactive) power meters that give you how much each motor is consuming at a given time, the watts are 'real' power and the VARs are 'reactive' power. VARs are 90 degrees out of phase with watts, so you add them as you would the perpendicular sides of right triangles: total apparent power in VA (volt-amperes) = sqrt((total watts)-squared + (total VARs)-squared).

Add watts to watts and VARs to VARs. Then calculate your power factor, by dividing watts by total apparent power. Your power factor is the cosine of the angle between 'real' power (watts) and 'apparent' power. The higher the power factor, the more 'real' power there is in the apparent power.

Cheers! DZ

POWER FACTOR---

WHICH FACTORIES THE POWER AND THE LEFT

WHICH REMAINS AFTER FACTORISATION GET

CONVERT IN REAL OUT PUT i e HEAT OR

MECANICAL.

P.F.= 0.5

APPARENT POWER IS =100 VA

ACIVE POWER = 100*O.5= 50 watt

THIS 5O WATTS GETS CONVERT IN TO REAL OUT PUT.

Theoritically as long as you know the power triangle method formula at the trigonometric relations of KW(real power), KVAR(reactive power), KVA(apparent power) with respect to power factor(PF) angle, plus application of mathematical ability.

In your statement you mentioned POWER FACTOR, LOAD FACTOR.

THEY ARE NOT THE SAME AND THEY ARE DIFFERENT.

THE TOTAL pOWER CONSUMED IS THE VECTORIAL SUM OF THE LINE CURRENT WHICH IN TURN DEPENDS ON THE INDIVIDUAL POWER FACTOR OF THE MOTOR.

DHAYANANDHAN.S

Fix a PF meter in the switchboard feeding these motors