Electrical job question: What is Power Factor ?

Types of Current:

There are two types of currents present in the AC circuits.

The current required by induction motors, transformers, fluorescent lights, induction heating furnaces, resistance welders, etc., may be considered to be made up of two separate kinds of current: magnetizing current and power-producing current.

Power-producing current:

This is also called working current  is that current which is converted by the equipment into useful work such as lighting a fluorescent light, making a weld, or pumping water. Hence the power required to perform a useful job or real work is termed as Active Power. The unit of measurement of the power produced is the kilowatt (kw).

Magnetizing current:

This type of current is also known as wattless, reactive, or non-working current. It is the current which is required to produce the flux necessary to the operation of induction devices. Without magnetizing current, energy (Active Power) could not flow through the core of a transformer or across the air gap of an induction motor. The unit of measurement of magnetizing volt-amperes is the kilovolt-ampere-reactive (kvar).

But the current that is read on an ammeter in the circuit is Total Current which include both magnetizing current and power producing current . Most a-c power systems require both kilowatts and kilovars. Power required to perform certain operation ( say running of induction motor) requires both active component of the power and magnetizing  or reactive component of the power. This total power is coined as "apparent power" having  the unit of measurement of  kilovolt-ampere (kva).

In simple,

Active power:

Power required to do the useful work (kW)

Reactive Power:

Power required to magnetizing effect (flux required)  the ac circuit and is important to deliver the active power in the circuit. (kvar)

Apparent Power:

Power combining both active power and reactive power.(kva)

Power factor:

• Power factor of a circuit may be expressed as the ratio of power-producing current in a circuit to the total current in that circuit.
• Another definition of power factor, which is generally more useful, is the ratio of kw or working power to the total kva or apparent power.

Thus,

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Methods to avoid low Power Factor

Some of the methods to improve power factor are:

1.     Induction motors have maximum power factor when they are fully loaded and power factor starts falling down when the motor loading is reduced and becomes worst during no load. Hence, it is important not to have motors too big for a specified job than  its required ratings. If however if the induction motor whose stator is delta connected is to run on less than half load for considerable time, it is adviced to connect the stator in star. This will not allow the drop in power factor to much extent. This is due to the fact that application of reduced voltage per phase gives rise to the reduced magnitude of the rotating magnetic field and hence less magnetising current and higher or better power factor.

2.     Reduction in the air gap of the induction machine is done as much as possible to enhance the power factor. This can be achieved by the use of ball or roller bearings in the place of sleeve bearings. A similar improvement in the transformer can also be achieved by employment of inter leaved instead of butt jointed cores. Grain oriented high permeability cores of the transformers will also reduce the magnitude of the magnetising current.

3.     Use of over excited synchronous motors in the place of induction motors.

4.     High speed Induction motors will always have better power factor compared to low speed induction motors. This is due to the fact that for a given power torque developed by the induction motor becomes less as the speed of the motor increases. Torque developed also depends on the square of the magnitude of the rotating flux. Therefore reduced torque relates to reduced magnitude of the rotating flux. Further, the number of poles of high speed induction motors are less. Therefore high speed motors requires less amount of magnetizing current. Also these motors being smaller in frame size are also more economical both from the point of view of initial cost and running cost.

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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.

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