In this article, we will learn about causes of low power factor and methods to avoid low Power Factor.
Causes of Low Power Factor
Some of the reasons for low power factor are
1. 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 lag behind the voltage. For the applied voltage, the magnitude of mutual flux in the case of a 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 the 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 of low power factor.
The power factor of the transformer and induction motor is further affected by the extent of it being loaded. Induction motors, for example, operate at a 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.
2. 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.
3. 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
4. Reactor
Reactors are also employed in central stations to minimize the fault current. This, therefore, is a cause of low power factor.
5. 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 power factor are
- 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 a considerable time, it is advised to connect the stator in a star. This will not allow the drop in power factor too 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.
- 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 interleaved instead of butt-jointed cores. Grain-oriented high permeability cores of the transformers will also reduce the magnitude of the magnetising current.
- Use of overexcited synchronous motors in the place of induction motors.
- 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. The torque developed also depends on the square of the magnitude of the rotating flux. Therefore reduced torque relates to the reduced magnitude of the rotating flux. Further, the number of poles of high-speed induction motors are less. Therefore high-speed motors require 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.
Excercise
Q. What is the necessity of power factor improvement?
Ans. Low power factor causes the ratings of generators and transformers, the cross-sectional area of the bus-bars and the contact surface of the switchgear, the size of the feeders and distributors, energy losses, voltage drops in generators, transformers, transmission lines and distributors to increase.
Q. What factors determine the economical limit of power factor correction?
Ans. Economic limit of power factor correction is governed by the relative costs of the supply and power factor correcting equipment.
