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I understand that to correct the power factor you have to choose a capacitance such that the reactive power from the circuit is cancelled by the reactive power of the circuit using this method.

Circuit with power factor corrected:

capacitor placed in parallel

Why does one place the capacitor in parallel (as opposed to series)?

Thanks in advance

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    \$\begingroup\$ One practical reason is that the capacitor would cause a voltage drop at the load. Another is that the capacitor would have to carry all the load current rather than just the reactive part. \$\endgroup\$ – Transistor Apr 2 '16 at 13:43
  • \$\begingroup\$ Thanks. So would power factor correction still be achieved if the 80uF capacitor was placed in series but for those reasons you gave it is better to place it in parallel? edit: oh I think it's because in the calculations the voltage drop across the capacitor has to be 240Vrms \$\endgroup\$ – Jamila Apr 2 '16 at 13:49
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Without the capacitor, the source has to provide all the energy (St): the actual energy consumed by the load (Pt) and the energy stored in the inductive part of the load (Qt). The inductive part makes the source supply a lot more current than necessary, since a lot of that current goes into setting up a magnetic field that stores some of the energy generated by the source.

With the capacitor in parallel, there is now an additional source of energy, which can take up some/all of the burden of supplying current to the inductive load (when it resists changes in current till it sets up its field), after which the source takes over again and recharges the capacitor. So the apparent power S (and thus energy) drawn from the source is reduced and is much closer to the true power P actually being used by the load.

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Current can only flow in a closed loop, so a series capacitor cannot keep reactive current from flowing through the distribution grid, which is the very thing that power factor correction seeks to avoid in order to avoid the resistive losses of that current travelling long distances through practical conductors. Basically, the only way a series compensating capacitor could be effective for power factor would be to tune out the ability of the machine to draw power at line frequency at all, which would make it non-operational.

In contrast, parallel connection of an appropriately sized capacitor keeps the reactive current local, constrained to short low-loss wiring runs.

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