Home Electrical Engineering Forum General Discussion how to find apparent power and reactive power?

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  • #10502

    admin
    Keymaster

    how to find apparent power and reactive power?

    is it different for static and rotating devices?

    sorry but i want to clear my doubt.

    #12238

    admin
    Keymaster

    S = P + jQ

    P = 1.732 VI Cos(phi)     Real power

    Q = 1.732 VI Sin(phi)      Reactive power

    S = 1.732 VI                  Apparent power

     

    Use root 3 for 3-phase i.e. 1,732

     

    Static or rotary same rules apply.

    #12240

    admin
    Keymaster

    shah said:

    how to find apparent power and reactive power?

    is it different for static and rotating devices?

    sorry but i want to clear my doubt.


    dear Shah, if you want to make the mathematical calculation then i dont know,

    if u want to measure it actually then use FLUKE POWER ANALYSERmodel

    43B or 435 your job will be done in minutes.

    #12244

    s
    Participant

    Hi

     

    Static or rotating device : this is a mechanical difference (even for AC inductive motors which have electrical rotating magnetic flux)

    But the general rule is : as long as there is inductance (or capacitor) inside the load, their is an apparent power and reactive power,

    Please note that reactive power is the power responsible of creating the magnetic flux (which is 90% of cases apply on rotating machines :)

     

    To know the apparent and reactive power on any machine, I'll use an inductive motor as an example,

    On every motor there is a name plate, which must indicates the apperant power (  Pn in W or KW or HP ) , Voltage ( in V ), and the phase shift ( cos phy ), and nominal current ( In in A)

    Those values are for the full load,

    for lower loads you should measure the current I and compare it to the nominal current In and devide with the calculated ration I/In=A

    P =Pn x A, Q=Qn x A

    Now for Q reactive power in VAR 

    Q=P x tg (Phy)

    Phy can be known by any scientefic calculator,

     

    Please note that the Harmonics effects are not considered here.

     

    Take care

    #12246

    apparent power is the balance power between real power (p)  and reactive power (Q) and has equation that P sqr and Q sqr  both under radical  and gives you aparent power (s) if it three phase times three when under the radical .

    #12247

    s
    Participant

    Hi

    One mistake at my post

    P is the real power in Watt , S is the apperant power in VA

    S=Square root (Psqr + Q sqr)

    Just replace the word “REAL” instead of “apperant” in my previous post

    #12252

    admin
    Keymaster

    nael.samman said:

    Hi

    One mistake at my post

    P is the real power in Watt , S is the apperant power in VA

    S=Square root (Psqr + Q sqr)

    Just replace the word “REAL” instead of “apperant” in my previous post


    apparant power= voltagex current (vA or KVA)

    REAL POWER= VOLTAGE X CURRENT X POWER FACTOR(COS Phi.) – WATTS /KW

     

    APPARANT POWER IS VECTOR SUM OF REAL POWER(KW) AND REACTIVE POWER KVAR(KVAR).

     kva=  square root    ( (kw)square+(kvar) square )

    power factor=kw/kva

    IF YOU KNOW ANY TWO OF THESE (KW,KVA .KVAR OR POWER FACTOR) YOU CAN CALCULATE OTHER PARAMETERS.

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