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I've beenstudying Real(True), Active and Apparent power in AC circuit.

I have understood all the concepts including phasor diagram, lagged sinusoidal waveform,the relationship between those three power.

However, I'm stuck on a this one concept at the moment and I feel really stupid. The definition for real power is "Real/True power is power dissipated by a load (resistor) and it is the actual power transferred to the consumer".

But doesn't dissipated power due to load mean that it has lost that amount of power because of the resistance and the power is wasted as heat?

TM1
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    If the load is a resistor, then it's wasted as heat. If it's a light bulb, some of it becomes light rather than heat so we don't consider it wasted. If it's an oven, we don't consider the heat wasted. If it's a PC, then some computations are done along with producing heat. Etc. – The Photon Nov 29 '17 at 02:07
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    The resistor is just a placeholder for "something useful here" like the consumer's house. But note that the consumer's house will generally provide both a resistive and a reactive load - only the resistive part is useful to her. –  Nov 29 '17 at 11:08

1 Answers1

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Real power P in W is consumed by the circuit.

  • A resistor converts electrical energy into heat.
  • A lamp converts electrical energy into light and heat.
  • A motor converts electrical energy into mechanical energy.

Real power does something useful, so the name Real or True power.

We also need reactive (what you are calling Active) power to make some circuits work. Motors and Fluorescent lights have magnetic circuits, which require lagging reactive power Q in VAR. Reactive power is not consumed but goes back and forth between the source and the load. Reactive power is stored in magnetic fields of inductors (and electric fields of capacitors) and returned to the source as field is collapsed.

Apparent power S in VA is the full impact of the circuit on the source. All three powers are related by the power triangle.

Power Triangle

Real is useful power but the source has to put in apparent power, so the power factor is a kind of efficiency in a circuit. $$ pf = \frac P S $$ We do power factor correction (add capacitors [leading reactive power] in parallel with the load) to improve the power factor of large loads with poor power factors.

StainlessSteelRat
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