Electric power is utilized to do some useful work like pumping of a fluid or conveying of some material or compression or expansion of a gas, etc. The power required to do this useful work is called the "Active Power" or "Real Power". The unit of active power is kilowatt (kW). But, to do this useful work, the electrical machine, as it needs to convert the electrical energy into motive energy, as in the case of motors, draws a certain other power called "Reactive Power". This is the inherent property of the induction machine and thus cannot be avoided. The unit of reactive power is kilovolt ampere (reactive) (kVAR). Strictly speaking, as this power is not doing any useful work this is unwanted power.
We also know that electric power is a product of voltage and current. So, for any useful work to be done, the power that is consumed from the mains by the machine can be calculated by multiplying the voltage (as read from a voltmeter) and the current (as read from an ammeter). This power is termed the "Apparent Power", as this is the power, which is "apparent" or "visible" as readings in the meters. The unit of apparent power is kilovolt ampere (kVA). Ideality requires that there shall be no difference between the apparent power and the active power, implying that the entire power that is drawn from the mains is used to do only useful work. But, in practice it is not so. Strangely, in the case of induction machines, it can be observed that there is always a difference between active power and apparent power. This means that not all the power drawn from the mains is used for doing actual work and some power is used in the induction machine itself. If this cannot be avoided altogether, one should at least know the ratio of actual power to apparent power (i.e.) to know what percentage of the power drawn from the mains is converted to useful work. This ratio is denoted by a factor called "Power Factor". This is the ratio of Actual Power to the Apparent Power. Obviously, the higher the power factor, the better is the utilisation of the power drawn. But, all induction machines have a very poor power factor.
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