甚麼是power factor?

I like this question because i am an electrical engineering graduate.

(1)

Power factor is defined as a ratio of Real power to apparent power.

In a.c. circuits, there are capacitance and inductance. Unlike resistance, capacitance and inductance do not consume real power.

Hence, power factor measures how much power is really delivered as REAL power. So normally, the higher the power factor , the better it is.

Of course the maximum value of power factor is unity.
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(2)

In stepdown transformers, usually we use VA to describe the power level of the transformer. In fact, V means volt and A means ampere. They are units of voltage and current. So, VA value gives the power level. Large transformer has larger VA value. Note that VA is a kind of unit. VA is the unit of apparent power.

So given the power factor, I can calculate the real power the transformer delivers. The unit of real power is watt (W)

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(3)

after rectification, everything in ac will become dc. Since dc is unidirectional, we will not care about the VA anyway. The instantaneous voltage-current product gives the instantaneous power.


I hope I answer your question=)

Watt + VAr = VA
power factor = Watt / VA