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Earth Leackage Protection In Vfd


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#1 ndeepak

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Posted 13 February 2008 - 02:33 AM

Hellow !
being a new user pl overlook the violation of etikates if went wrong.

I built vfd 220volt, 750W. i ve provided earth leackage protection using differential CT.
When i turn on the motor at any speed it shows more than 50 volts leackage voltage with respect to ground.
the motor is tested for 2Kv insulation test. What can be the possible reasons?

#2 jraef

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Posted 14 February 2008 - 07:19 AM

It is difficult to get consistent accuracy from CTs on the load side of VFDs. Manufacturers usually use Hall Effect transducers for that function.
"He's not dead, he's just pinin' for the fjords!"

#3 marke

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Posted 14 February 2008 - 10:25 AM

Hello ndeepak

Most VFDs have a high frequency PWM modulated "carrier" output voltage with very fast switching.
There is significant capacitance between the motor windings and the motor frame, so the high frequency component of the output waveform is capacitively coupled to the motor frame and this is a leakage current.
If you filter the output of the differential CT so that it is insensitive to this high frequency component, the problem should go away.

Best regards,
Mark.

QUOTE
Hellow !
being a new user pl overlook the violation of etikates if went wrong.

I built vfd 220volt, 750W. i ve provided earth leackage protection using differential CT.
When i turn on the motor at any speed it shows more than 50 volts leackage voltage with respect to ground.
the motor is tested for 2Kv insulation test. What can be the possible reasons?





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