Power Factor Correction

Power factor correction is the method to reduce the lagging power factor in inductive loads by fixing a high value capacitor across the phase and neutral close to the load. When the Voltage and Current are in phase with each other in an AC circuit, the energy from the source is fully converted into another form to drive the load and in this case power factor is in unity. When the power factor drops, the system becomes less efficient. As a rule a drop from unity to 0.9 in the power factor increases the current requirement to 15% or more.

A power factor of 0.7% increases the power requirement to around 40%. This is much severe in the case of inductive loads such as Motors, Refrigerators, Inverters etc. In these inductive loads, current “lags” the voltage leading to “lagging power factor”. But opposite condition occurs if current “leads” the voltage. This is called “leading power factor”. Power factor correction is the method to reduce the lagging power factor in inductive loads by fixing a high value capacitor across the phase and neutral close to the load. These capacitors have leading power factor so that it will neutralize the lagging power factor of the load. Power factor correction has the following advantages.

1. Load becomes more efficient
2. Prevents wastage of energy though heat
3. Maintains voltage stability

Over correction

It is important to note that, if the power factor capacitor (PFC) is connected at the entry point of power supply, say after the meter or in the distribution point, there is chance of “Over correction” when a fraction of the full load is switched on. This over correction causes heavy surge current flows to the load during switching that may destroy the load. So the PFC must be connected close to the inductive load, so that power correction occurs only in that load.

PF correction Capacitor

Power capacitors are huge non polarized metal film electrolytic type capacitors.

Power factor capacitor

These capacitors have self healing property. That is during momentary faults, small areas of the capacitor electrodes evaporate to restore its function again. There are two types of power factor correction capacitors.

1. Oil filled type in metal case and
2. Epoxy filled in plastic case

These PFCs can operate smoothly at a temperature as high as 70 degree. The PFCs have working voltage of 240 volt AC, 440 volt AC etc. Capacitance of PFC varies between 2 uF to 100 uF. The PFCs are also specified by its “Reacting power” KvAR.

PFC connection

Power factor capacitor is connected across the phase and neutral near the inductive load such as motor. Before connecting the PFC, make sure that the capacitor is sufficiently rated to the load capacity. It should be connected to the lines, only if the load is running and drawing current.
Note: Before connecting a Power factor capacitor, seek the opinion of an expert since the many parameters like voltage, current, load capacity etc are involved in power factor correction.


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  • Ahmad

    Hi, very interesting topic. I would like to ask that we have an electric motor (single phase) that is using about 3.8Kw at 180 volts. now a days our main transformer has become faulty and we nominally we are being supplied with 180 volts but when we switch on the motor the voltage drops to 50-80 volts, on which motor spins but unable to pick the load. we tried a 12000 watt stabilizer but the voltage but as voltage drop below hundred thus stabilizer was unable to help us. someone told me that installing PF capacitors would answer the problem. Is this true that if we install the PF capacitors then voltage drops to such extremes might be avoided? if yes that what values should we use. Regards

    • Jim Keith

      While power factor correction may be helpful, it is unlikely to solve the problem. You must determine why the source impedance is so high and correct it there. You did state that there was a transformer failure –it appears that the replacement has poor coupling (high leakage inductance). Locate a better transformer and perhaps oversize it to make the output voltage stiff.

  • mark

    Nice article. I was looking for something about lightly loaded fan motors, and power factor over correction. Nothing in the article is discussing variable condition on small motors, the formula were nice.

    • krokkenoster

      Thanks Keith for the info as I was only aware of it when I went to apprentice school 49 years ago and always wondered why not utilized more. Ever heard of a “Synchronous condenser”?

    • Jim Keith

      Mr. Mohankumar’s article is misleading –he makes the statement:
      “A power factor of 0.7% increases the power requirement to around 40%.”
      He should have stated that it increases the line current by around 40%. Since P = I²R, the losses in the connecting power line conductors increase by 2%.

      By reducing line current, only the losses in the power line resistance are reduced –not the load (motor). Since the line resistance is only about 1% of the load resistance, the consumer’s power savings is insignificant.

      On the other hand, it makes sense for the electric utility to correct the overall power factor because the total loss in their power generation and distribution system runs in the order of perhaps 5% –with the generator itself being the most lossy element.

      If you ever worked with small gasoline engine powered electric generators you will know how the voltage sags when the load is connected. For small generators e.g 2kW or 5kW, the generator source resistance is about 10 to 20% of the load resistance. In that case, power factor correction is beneficial and helps to increase the output voltage.


    I learned this and did experiments on this with tuned circuits but we were given the “correct” values just to demonstrate the effect. Up till now I could not get this gen! Thanks I appreciate this

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