Linear Load Power Factor Correction

- basics of linear load power factor correction used for correcting the power factor of motors, and inductive components.

Power factor correction tutorial includes:

    •  Power factor correction basics
    •  Linear load PFC
    •  Non-linear load PFC
    •  Passive non-linear load PFC
    •  Active non-linear-load PFC
    •  Power factor improvement summary

Many electrical systems present a linear load to the supply, and therefore need power correction to be applied in a linear fashion.

This linear load power factor correction enables the overall power factor to be brought within acceptable limits, using linear techniques.

The techniques required for linear load power factor correction have been established for many years and are well known and understood.

Linear load power factor correction basics

A linear load will present a constant load to the supply. This means that the effect of the load is constant over the period of a cycle.

The power factor correction for a linear load can therefore be supplied by presenting a reactive load of the equal and opposite sign. The linear load power factor correction is therefore applied by adding capacitors where there is an inductive load, and adding inductors where there is a capacitive load.

Motors typically present an inductive load to the supply, and therefore capacitors are added to neutralise the effect of the inductance and bring the power factor back to, or much closer to a figure of unity.

In terms of the industry jargon for linear power factor correction, inductors are normally said to consume reactive power and capacitors are said to supply it. This is despite the fact that the real energy is moving back and forth on each AC cycle.

Precautions

The reactive elements used to correct the power factor for linear loads can create voltage fluctuations and harmonic noise under some circumstances. This can occur when they switched in or out of circuit as the actual loads themselves take different levels of current or are turned on and off. Sometimes the linear load power factor correction elements will be applied remotely or to a complete site. When individual loads are turned on and off the level of power factor correction required changes.

Further problems can also arise when the reactive elements interact with the actual loads. As the load is reactive and the power factor correction applies the opposite form of reactance, this can create a resonant circuit and result in instability. As a result, careful analysis of the overall system of loads and linear power factor correction is required.

Automatic capacitor linear power factor correction

To ensure that the correct level of linear load power factor correction is applied, automatic systems are often employed.

Automatic linear power factor correction systems incorporate a number of capacitors that can be switched in according to the load and power factor correction required. This assumes an inductive load which is normally the case.

The linear power factor correction unit measures power factor of the network and then switches in the required number of capacitors to give the necessary power factor correction.

In this way the linear power factor correction unit is able to maintain the power factor above a certain level regardless of the load in the system.

Synchronous motor corrector

Linear load power factor correction can be applied by using an unloaded synchronous motor to supply the reactive power. The system works because the reactive power drawn by the synchronous motor is a function of its field excitation and this can be altered under these conditions to provide a capacitive load.

This form of linear load power factor correction is referred to as a synchronous condenser. The motor is started and connected to the electrical network where it operates at a leading power factor, thereby applying a reactive element to the system as required. The level can obviously be changed to meet the requirements of the time.

The great advantage of this form of linear power actor correction is that it can be varied more easily to give the required level of power factor correction.

Semiconductor power factor correction systems

Increasingly, semiconductor based systems are being used for linear power factor correction, particularly for high voltage and rapidly varying loads.

Semiconductor systems are able to respond more rapidly and also they require considerably less maintenance as the contactors used in capacitor systems will ultimately require replacement and synchronous motors have a limited life and require attention.

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