Causes of Low Power Factor ?

Some of the reasons for low power factor are:

Induction motors and Transformers:

Where ever the magnetising current is to be produced by the ac power such as in the cases of transformers, induction motors and reactors the power factor will be significantly low. The current drawn by these machines or equipment will always lags behind the voltage. For the applied voltage, the magnitude of mutual flux in the case of transformer or rotating flux in case of induction motor remains constant. The magnitude of magnetising current depends on the value of the reluctance of the magnetic path. Magnetic path in case of transformer does not have any air gap, but for induction motor air gap present between the stator and the rotor. Therefore the magnitude of the magnetising current is more case of induction motor than transformers. In industries 70% of the motors and drives used are induction motors due to low cost, robust operation but poor in power factor. Hence Induction motor is the potential source for low power factor.

Power factor of the transformer and induction motor is further affected by the extent of it being loaded. Induction motors for example operate at reasonable higher power factor of 0.85 at full load, 0.8 at 75% of full load, 0.7 at half full load, 0.5 at 25% of full load and as low as 0.1 on no load.

Arc Lamps:

Arc lamps and electric discharge lamps operate at low lagging power factor. An electric arc is essentially unstable and requires the use of ballast or choke to make it stable. It is due to the requirement of the magnetising current by the choke that all devices employing electric arc have low power factor.

Induction and Arc Furnace:

Induction heating furnace such as arc furnace  and induction furnace require high magnetising current and operate on very lagging power factor

Reactor:

Reactors are also employed in central stations to minimize the fault current. This therefore is a cause of low power factor.

Transmission lines:

Transmission lines also will have self inductance. Inductive reactance is small in multi-core cables but relatively large in case of overhead lines.

Some of the methods to improve the power factor are discussed earlier