Power Factor

For calculating the Power Factor, you need the Active Power and the Total Power.

As you know Power Factor equals to Total Power divided by Active Power.

Total Power is the multiplication from the RMS current and the RMS voltage

Active power is the mean value from the instantaneous multiplication of current and voltage.

• S = Irms x Urms
• P = 1/T ( I x U)
• PF = P/S

Both calculations can be done totally analog or totally digital.

For the analog way, you need

• two RMS converting circuits for calculating Urms and Irms
• one quadrant analog multiplier for calculating the Total Power
• an analog four quadrant multiplier for calculating UxI instantaneous
• output filter to calculate the mean value of UxI instantaneous, this gives you the Active Power
• an analog dividing circuit to calculate the Power Factor from Active and Total Power

For the digital way, you need

• two fast A/D converter IC's with sample and hold circuit
• two arrays to put away the instantaneous voltage and current values
• a fast computer to calculate
• the RMS value from current (Irms)
• the RMS value from voltage (Urms)
• the product form Irms and Urms this gives Total Power
• the mean value from the product of each instantaneous voltage and current value this gives Active power
• the ratio Active Power over Total Power, this gives the Power Factor

Additional to this you will need an auto ranging circuit both for voltage and current

If you want to measure three phase power factor, to do calculate

• six times the RMS values (voltage and current from each phase).
• three multiplications from Irms and Urms which gives you Total Power
• the addition from the 3 Total Powers
• three multipiers for each phase to get Active Power
• the addition from the 3 Active Powers
• the ratio Active and Total Power

Keep in mind that you can have also a running current into the Neutral line wire. This current also must be measured.

Or you can buy a complete power meter which does it al for you.

The rules and calculations are the same as the one used at lower frequency but the components are different. The capacitors will be film capacitors rated for the 10 KHZ. The switching of banks if needed is likely to be done by MOSFET or IGBT switches.

If this is for your induction heater or a motor drive, you should reconsider why you have to control the power factor on the output as it might not be a good idea to add anything there. Make sure that you know what you are doing as it can damage the inverter.

Dear marcot,

You r right.. I'm talking abt the M.F Induction heater. There is a Motor Generator set based Induction Hardening machine of 130kw.I need to match the power capacitor tappings(connected across the Output wires(2 wires). usually it is 60kvar) for every particular Job at output coil (around 60mm dia). the manual method of capacitor matching is taking much time & not satisfactory..

I wish to measure the PF on any meter, whether it's LAG or LEAD?. so I can increase & decrease the capacitance offline to get good heat output. I've tried analog PF meter by using PT & CTs. But, failed...

Pls Suggest Any Idea....

Regards,

There are few power factor meters that will operate at these frequencies. The highest I have found was for 1 KHz and was very expensive.

I suggest that you use the zero crossing of the voltage through a comparator to trigger a A/D converter on a current sensor. The polarity and amplitude of the current sampled would be almost a function of the power factor. A few CMOS chips and a A/D converter with display driver powered by a battery supply should make a simple device. You could also program a microcontroller if you are up to it.

This is a nice term project for a student in electronics.