Power Factor Correction for High Power Inverters

This is chapter 5: PFC of the DRSSTC design guide

What Is Power Factor and PFC Meaning

PFC is short for Power Factor Correction. So before we talk about doing any correction, let us first understand what power factor is. Power factor is a measure of energy efficiency. Presented in a percentage, where a lower value is a worse result, indicating a poor efficiency. Power factor is the ratio of working power (measured in kilowatts kW), to apparent power (measured in kilovolt amperes kVA). The difference between kVA down to kW is the reactive power kVAR. This is calculated using Cosinus to the phase shift angle.

The demand power or Apparent power, is the measured amount of power used to run a load. It is calculated by multiplying Volts with Ampere (V x A). It is often presented in its 1000s SI unit as kVA.

Power Factor Formula and Definition

Power Factor = Working Power / Apparent Power, therefore the formula can also be written as PF = kW / kVA.

A poor power factor can result in:

  • Excessive heat damage to wires, cables and other circuit components.
  • Reduction in the amount of available useful working power
  • A required increase in conductor, cable and circuit component sizes

Tesla Coil Power Factor Correction

It would be optimal to feed all DRSSTCs from a boost converter with PFC frontend, but it can be a complex task to undertake for the power ratings we need. There is three ways to deal with power factor problems in regard to the average Tesla coil experimenter.

1. Do nothing about it. This just means that AC side components like wires and rectifiers will have to have higher ratings and that you do not get as much real power on the DC link side as you are drawing in apparent power on the AC side, due to the reactive power drawn from the bad power factor caused by the capacitive load. This is the easy and cheap solution. Expect power factor to be 0.5 to 0.6.

2. Passive power factor correction with a AC side choke / inductor can help a little. It is however heavy, bulky and might not be easy to find cheap. If such a choke can be found at a scrap yard, it is a easy and cheap solution, if it has to be bought its more like easy and expensive. If you use a variable autotransformer / variac to adjust the input voltage, it will also act as a choke and give you better power factor. Expect power factor to be 0.6 to 0.8.

These two simulations show the difference in current waveform (yellow) from no passive inductor and a 5 mH inductor.

3. Active power factor correction with a boost converter. Complexity and cost is high and designing a multi kW PFC unit is not an easy task. 3-phase active PFC is even more complex. Very complex task, expensive and time consuming. Expect power factor to be 0.95 and above.