# Re: AC Power (back to basics)

#13009
Jacques Schonek
Participant

Back to the initial post on this topic, I have a few remarks:

– reactive power is a purely mathematical concept. It comes from the vector representation of voltage and current. The current component which is in phase with the voltage is the active current. The current component which is in quadrature is called “reactive” current. When multiplied by the voltage, it gives the reactive power. This is consistent with the definitions given by the International Electrotechnical Commission IEC. This is illustrated in one paper on this blog: “6/8 – Electrical equipment: improve power factor”

-reactive power does not oscillate between generator and load. In fact, when reactive power is present, the power transmitted from generator to load is positive during a part of the power line period, and negative during another part. This is illustrated in one paper on this blog: “4/8 – Electrical equipment: power harmonics, power factor”. This is why oscillation is often mentionned.

-in order to avoid confusion, let’s use the definitions used by IEC: apparent power (and not total power), active power (and not true power).

– the symbol for reactive power is “var” (with small letters). See the paper ” KVAR, kVAr or kvar? on this blog.

– transformers are rated in VA, kVA or MVA because the size of a transformer depends on the current AND voltage ratings, whatever the phase angle between voltage and current.

– motors are rated in kW because this is the mechanical power they can continuously deliver on the shaft. The number of kW of a motor is a mechanical unit, and not electrical unit. The electrical power absorbed by a motor can be calculated by taking account of efficiency.