Power Factor Improvement Methods Summary

- summary of the different power factor improvement methods, discussing their advantages, disadvantages and applications.

Power factor improvement is often governed by the electrical energy supply company applying penalties to large users if the power factor of their energy use falls outside certain limits.

However there are also benefits in correcting the power factor in terms of gaining more efficient use of energy.

To achieve the optimum power factor it is necessary to use the correct method for the particular application in question.

A number of different methods of power factor improvement are available. The correct method needs to be chose for any given application.

Capacitor power factor improvement

This is a simple method of providing power factor improvement. It is often applied to areas where machines using electric motors are used. These motors are inductive. Applying a capacitor neutralises the power factor error.

Often these systems monitor the power factor and switch in further reactance (capacitive) to provide the required power factor improvement

Points to note:

  • Load type:   Capacitors are used to provide linear load power factor improvement. The capacitors are only able to provide a change in phase and are not able to cater for the issues surrounding non-linear loads.
  • Capacitive & inductive reactances cancel:   For the system to work correctly the capacitive reactance must cancel out the inductive reactance. This may require banks of capacitors to be switched to enable the power factor improvement to be in place as the load varies and the power factor changes.
  • Beware instability:   The situation of capacitive and inductive reactances cancelling out equates to resonance of a tuned circuit. Care smut be taken when deigning these systems to ensure instability does not arise.
  • Applicability:   This form of power factor improvement is normally applied to large workshops using electrically driven machines and other similar situations.

Synchronous motor power factor improvement

The use of synchronous motors is another method of providing power factor improvement. The motors are run without a load and are able to provide the capacitive load required to ensure the power factor is improved.

This form of power factor improvement operates because the reactive power drawn by the synchronous motor is a governed by its field winding excitation. This can be altered to provide a variable capacitive load.

This type of load factor correction has now generally been superseded by other solid state methods.

  • Load type :   This type of power factor correction is only applicable to linear loads such as motors and other inductive components. It does not accommodate non-linear loads such as electronic power supplies.
  • Large motor required:   For this system to work, the motor must be running all the time. It is run in a non-loaded fashion to give the capacitive reactance required.
  • Motor expense:   Synchronous motors are not cheap and the capital cost needs to be remembered when considering this option.
  • Limited motor life:   As the synchronous motor needs to be run continuously it requires maintenance and also has a limited useful life. Both items add costs to this solution.

Filter power factor improvement

Filtering the input signal to remove harmonics is a method used to provide power factor improvement. Removing harmonics generated at the input can aid the input signal to return to a better power factor. As harmonics will be at multiples of the line input frequency, a filter can be devised with a cut-off just above the line frequency to give sufficient attenuation of the harmonics to return the waveform to an acceptable form.

  • Load type:   This form of power factor correction is used with non-linear loads which might be electronic power supplies. The scheme only removes the harmonics of the signal to return it to a sine wave format.
  • Performance:   This form of power factor improvement is easier to achieve than other forms of non-linear load power factor correction, but it is not as effective as active power factor improvement.
  • Size:   As frequencies are low, and line input voltages often high, component sizes are large.
  • Cost:   The inductors and capacitors needed for a low frequency filter are large and hence costly
  • Worldwide operation:   A filter to provide power factor improvement for worldwide operation si difficult to configure because line frequency varies between 50 and 60 Hz and also voltages may change.

Active boost power factor improvement

This form of power factor correction or improvement uses active circuitry in the form of a switch mode power supply boost circuit. By controlling the times when charge can be applied to a reservoir capacitor, the input current can be maintained in synchronisation with the voltage.

  • Load type:   This form of power factor correction is used with non-linear loads.
  • Application:   This scheme can be accommodated relatively easily within the power supplies of small computers. As circuitry is already present for a switch mode power supply, the power factor improvement circuitry can be incorporated relatively easily and without an unacceptable cost increase.
  • Performance:   This form of power factor improvement is accepted as being the most effective for non-linear loads.

Each type of power factor improvement or correction has its own advantages and disadvantages. These factors must be taken into account when choosing the optimum form of load factor improvement.

By Ian Poole

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