# Power factor monitor in 3phase motor

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### #1 Guest__*

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Posted 27 January 2005 - 04:36 PM

Anybody knows a circuit to measure power factor on 3phase motor?
The circuit in the link http://www.designnot...ITS/pwrfact.htm
is just for one phase!
Instead of current transformer can I use hcpl-788j?

### #2 marke

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Posted 27 January 2005 - 06:31 PM

Use the same circuit for each phase, (three circuits) and combine the outputs.

You need a Current transformer to mearsure the current. An opto isolator will not carry the current.

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### #3 Guest__*

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Posted 28 January 2005 - 11:12 AM

Supposing that I implement this circuit, I will have a microcontroller (PIC) to know exactly what power factor I' ll have...
The output is a pulse at the AC frequency, in my case 50Hz, whose width is inversely proportional to the power factor. When the pulse width is zero, the power factor is one. But how do I know that power factor is for example 0.7? In this case what is the pulse width?
This ideia itīs correct...
25Hz-------PF=0.5
x------------PF=0.7
But itīs inversely proportional so 1-x=Final Power factor
And this ideia applies to all power factorīs.