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Power Factor Circuit Help


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#1 chisam14

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Posted 22 October 2007 - 02:44 AM

I am designing a circuit to demonstrate the effects of capacitors on the power factor. I have a couple different single-phase motors (120V) that will manually be switched on. I have a relay to engage the capacitors when the power factor drops below a certain level. I have everything so far that I need for the demonstration but I can not figure out a way to have a device measure the power factor and energize/de-energize my relay to turn on my cap bank. If anyone could help with this it would be greatly appreciated

#2 marke

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Posted 22 October 2007 - 03:44 AM

Hello chisam14

Welcome to the forum.

There are number of special controllers available for doing this, but they are primarily three phase and they also cost money.
I presume that you want something simple and low cost??


Best regards,

#3 chisam14

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Posted 23 October 2007 - 12:53 AM

Yeah I am lookin for something simple and low cost since this is just for demonstarion and yes it is single phase.

#4 marke

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Posted 23 October 2007 - 05:10 AM

I do not know of a simple controller that will do what you want, there will be something somewhere, however if you are electronics literate, it is easy to build something to do the job for you.
Would you consider building a controller from scratch?
For some background, see Power Factor Measurement

Best regards,

#5 chisam14

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Posted 23 October 2007 - 03:46 PM

Thanks for the diagram. I would be interested in building one from scratch. I have basic electronic background, nothing too in-depth. I could build the circuit in the diagram if I knew the resistor/transformer values, but how could I use that to energize my caps without some sort of programming? Thanks

#6 marke

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Posted 24 October 2007 - 08:07 AM

The diagrams are only intended as a means of showing the basic principles, but someone with a reasonable grasp of electronics could make something work form the information in the wiki article.
The current transformer would need to be sized for the magnitude of the current and the shunt resistor on the secondary needs to match the CT.
The PT needs to have a primary rating equal to the supply voltage and the secondary voltage would be less than a third of the supply rails applied to the op-amps.
The bias resistors apply half of the supply rails to one input of the op-amps.
The output waveforms are applied to the input of an XOR gate and the output of the XOR gate is then applied to a filter with a time constant much lower than line frequency. This filter could be a simple R C filter, or an active filter with a sharper cut off.

The output of the filter will essentially be a DC voltage representative of the displacement angle. You can apply this DC voltage to an input of a comparator. The second input of the comparator can be connected to a potentiometer and the output of the comparator can drive a relay to switch the capacitors. This is a very crude solution. There would normally be ON delays and OFF delays associated with the switching relay. A further improvement is to add some hysteresis to the comparator.

I do not have a complete circuit diagram that I can provide for you to build from, but it is not a difficult project for someone with a little competence.

Best regards,




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