In a transformer let us assume a load is connected consuming certain active power; say 5000 W. Now if we put a different load of low power factor consuming same amount of active power, now here the current has to increase right? Here the active power can be written as VIcos(theta) and reactive power as VIsin(theta). Now as power factor decreases current increases, it means reactive power (VIsin(theta)) also increases (as current increases and theta increases), but why it is said that reactive power is constant? In an induction motor,reactive power is taken constant. And on loading of the motor,power factor is taken as increasing.

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    \$\begingroup\$ Where is it said that reactive power is constant? \$\endgroup\$ – MrGerber Mar 20 '18 at 15:24
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    \$\begingroup\$ ''it is said that reactive power is constant'' Says who? \$\endgroup\$ – Joren Vaes Mar 20 '18 at 15:25
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    \$\begingroup\$ Also, please put spaces after your punctuation! Right now it's really hard to read. \$\endgroup\$ – MrGerber Mar 20 '18 at 15:26
  • \$\begingroup\$ You are going to have to clarify your question. You start with real power 5000W, so pf=1. Then you add reactive power with low pf, say 5000VAR, pf = 0.707. So reactive power went up, which is definitely not constant. So you have to clarify or add more detail. You also tag power factor correction. Why? \$\endgroup\$ – StainlessSteelRat Mar 20 '18 at 16:36
  • \$\begingroup\$ Better, but now you have introduced an induction motor, so now we have three states. \$\endgroup\$ – StainlessSteelRat Mar 20 '18 at 17:07

Here is some data for a 22 kW, 3-ph, 400 V induction motor at no-load, and 0.25, 0.5, 0.75, 1.0 and 1.25 X rated load. The reactive power (kVAR) more than doubles between no-load and 1.25 X rated load. The power factor increases even though the reactive VA increases because the real power increases more than the reactive VA increases.

Someone might expect the reactive power to remain fairly constant because the magnetizing current is assumed to be constant if the voltage and frequency remain constant. However the effect of the increased current in the stator and rotor leakage reactances apparently represents a significant contribution to the reactive VA.

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Transformers are also assumed to have a constant magnetizing current as long as the applied voltage and frequency remain constant. The reactive VA of a transformer may also have a significant contribution to to leakage reactance, but the reactive VA of a transformer is relatively insignificant compared to the real and reactive power of the load.

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