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Hello Lewis



Could anyone tell me more about "resonance" ?

When you add power factor correction, you are adding capacitance to the system. You add power factor correction, because the system is inductive, and consequently you have both capacitance and inductance in the system. If you have a circuit that has both capacitance and inductance, it will have a resonant frequency. If the impedance in paralles with the resonant circuit is very low, the Q of the circuit will be very low and there will be little problem. If, however, the circuit impedance is high, the Q will also be high and in that case the circuit can resonate and generate high voltages. This is the principle that all radio recievers use!


how to switch the capacitors to correct the power factor?

Using a power factor relay, you control contactors that switch the capacitors in and out of circuit. The power factor relay measures the power factor and if the measured power factor is higher than the highest power factor, a bank is opened, if the measured power factor is less than the set power factor, a bank is switched on. If it is still lower, another bank is switched on.

Best regards,

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Hello Lewis


How can I solve the problem of resonance?

Resonance is not easy to fix. The best thing to do is avoid it altogether if possible. (Don't fit capacitors!) Where there is the potential for resonance, the first step to take is to try to make sure that the resonant frequency is not on a harmonic frequency of the supply.

Avoid adding capacitance to a lightly loaded supply.


what capacitance of the capacitor should I use to insert to the system? What does it depend on?

The capacitance values used are dependant on the system that you are correcting. If you are using automatic correction, you need to study the system and determine :


  • Maximum correction required

  • Minimum correction required

  • stepsize required

  • target power factor

You determine the maximum correction by esatblishing the worst case load conditions, and similarly the lowest correction from the minimum load conditions. The step size is determined by the load increments and the capacitor increments required to keep the power factor within you required bounds.

Best regards,

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Hello Marke


Thanks for your help first.

Assuming I need to correct the power factor for a residential building.

1:What should I pay more attention ?

2:How can I monitor the resonance?

3:What is the "stepsize"?

4:Why sould I set the switching delay time?

5:How the switching sequence should be?




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Hello Lewis


The first question I would ask, is why correct a residential building? Generally, the power factor is not too bad and residential buildings are generally charged on KWHr only in which case there is no advantage in power factor correcting them!

I don't believe that there are any hard and fast or correct answers to your questions, you just do what seems best for your application.


Best regards,

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