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engrjawwad05
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Join Date: Jan 2010
Posts: 23

Current Before and After VFD

02/18/2010 9:45 AM

Dear Friends,

hope u all are well n busy in your professional life.

Today, i installed a VFD (PUMA P5 series). after giving start command, when i checked the current through clamp meter before and after the VFD, i observed that both current(before and after VFD) are not equal. the current before VDF was almost one third of the current after VFD. how it possibles??
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JohnDG
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#1

Re: Current before and after VFD

02/18/2010 9:52 AM

Don't suppose you happened to check the voltage as well, did you?

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Johny451
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#2

Re: Current Before and After VFD

02/18/2010 10:46 PM

You have to be very careful measuring current on the output side of a VFD. The waveform is usually Pulse Width Modulation at between 2kHz and 16kHz. Most clamp meters can not handle this at all well.

It's OK to measure the input side, although this will have a high harmonic content due to the VFD input rectifiers, but on the output side it's better to believe the VFD display regarding output current.

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ffej
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#3
In reply to #2

Re: Current Before and After VFD

02/19/2010 12:15 AM

GA,

Misunderstanding the problems associated with measuring non sinusoidal waveforms probably account for half the world's "free energy", "over unity" and "zero point" schemes. (the other half are probably due to not understanding the differences between energy & power, temperature & heat etc).

The inventors ask themselves, what's more likely "I don't know how to measure unusual waveforms" or "I've discovered a wonderful effect that everyone else's missed".

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MalcolmK
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#6
In reply to #2

Re: Current Before and After VFD

02/19/2010 5:59 AM

The output motor current is close to sinusoidal, most current clamps give a good indication. Voltage is PWM and is very difficult without using moving coil or a higher perforamnce instrument.

Conversely, input voltage is easy to read (mains voltage) but, as you say, input current has a high harmonic conentent that most standard clampos will over read dramatically.

So, I'm sorry, but I do not quite agree with your analysis re currents and how easy it is to measure.

WRT original question, I use a Fluke 43 analyser for the input, very expensive but essential fot input current analysis, but a low cost CT to my multimeter (£20) will read most output currents OK for day to day needs +/- 5%.

As correctly described in another post, the output is motor current, no load could be 25-80% of FLC depending on the size of the motor, input current is Displacement Power Factor near unity and is affected by harmonics, you could add a choke and reduce input current too!!

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hetal_gohil
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#4

Re: Current Before and After VFD

02/19/2010 1:37 AM

please check the below:

input voltage; input current and input power factor.

output voltage; output current, output frequency and output power factor.

then check input power = output power + losses. (considering losses as negligible for vfd)

it becomes input power = output power.

which type of current meter was used is not specified? for accurate current reading at the input and output of vfd use good quality meter like fluke or use the analog meter.

hope your query would be solved?

do give feedback for the above.
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murugan.ramasamy
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#5

Re: Current Before and After VFD

02/19/2010 1:41 AM

Hi

Greetings of the day!

When you connect the VFD with motor and if motor is on no load or lesser load. The input current of VFD will be less than the output current, becuase the input power factor is near to unity.

where as the output power factor, you are aware it will be very and less and depends on load.

Hence the power is same. the input current will be less

Hope this clears your doubt.

Have a nice day!
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GW
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#7

Re: Current Before and After VFD

02/19/2010 11:07 AM

The input current will be near unity power factor at say 460vac.

The output at reduced speed (say 1/2 speed) and full load will have name plate output current, but approximately half voltage. So the motor power will be 1/2 rated.

Since the VFD is wasting very little power but its input is at line voltage the input current will be approximately 1/2 of rated.

Conservation of energy!
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Anonymous Poster
#8

Re: Current Before and After VFD

05/29/2010 4:28 AM

ABEY ATTOCK K KERAY TUM NE GHANT GHAR KA NAAM SUNA HAI??????? WAHAN JA KAR APNA SAWAL POST KARO NAZIM SAHB JAWAB DE DEINGE.......................
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Anonymous Poster
#9

Re: Current Before and After VFD

05/29/2010 5:32 AM

once again please check the below conditions:

input voltage, input current, input power factor.

output voltage, output current and if possible out put power factor.

then check the equation as below:

input power = output power + losses.

considering losses as negligible take input power = output power.

your query would be cleared.
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Anonymous Poster
#10

Re: Current Before and After VFD

06/05/2010 6:21 AM

I am also looking for VFD for my 2.2KW 3phase Motor... Ham pakistani kissi say kam nahein.. See my work below

www.my-woodcarving.blogspot.com
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