Digital power mesurement


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  1. #1
    Prolific Poster shantanu@india's Avatar
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    Default Digital power mesurement

    Hi,
    I have an application where the power consumed by an inverter(VFD) driven 90kW motor has to be measured.
    As we know the PWM output of the VFD cannot be directly interfaced to standard analog or digital meters.....the modulating frequency(>1kHz)generates huge negative error.
    The only proven power meter with 100kHz bandwidth suitable to measure 3-phase power at the output of a VFD is Yokogawa 330 which is frightfully expensive....about 6000 USD with hall effect clamps !!!
    Any cheaper solutions??
    Thanks.
    Shantanu

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    Prolific Poster normnet's Avatar
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    Default Re: Digital power mesurement

    I have installed VFD's on a rooftop HVAC unit to control the condenser fan speed so this is of interest to me. Would measuring the power draw to the VFD in place of the output work as it is standard power I.E. amps and volts not being PWM'd at that point? There may be some inefficiency involved within the VFD but hopefully the input power is comparable to the output power.

    Norm
    Last edited by normnet; 25th March 2018 at 13:58.

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    Prolific Poster shantanu@india's Avatar
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    Default Re: Digital power mesurement

    Norm,
    The VFD efficiency is load dependent & you cannot exactly predict the true motor power by measuring the incoming power to the VFD by a separate meter.

    All VFD's have their own power measurement block which can be assigned either to the input or to the output. Generally a 4-20mA output is provided that can be attached to the required power reading.

    What fluke meter was that?......do you remember the type no. ?
    Shantanu

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    Prolific Poster normnet's Avatar
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    Default Re: Digital power mesurement

    After I thought about the Fluke meter I think it actually was the RMS option. I emailed Fluke support and asked if they offered a VFD power meter for 90kW as they show several on their site.

    Norm
    Last edited by normnet; 25th March 2018 at 14:50.

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    Prolific Poster shantanu@india's Avatar
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    Default Re: Digital power mesurement

    Norm,
    Fluke 1742 model shows voltage & current bandwidth of 42 Hz to 3.5 kHz.
    Does that mean that these meters can measure power accurately of 'dirty' waves as we would expect at the o/p of a VFD?
    I shall contact a Fluke agent in my locality.
    Thanks.
    Shantanu

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    Prolific Poster normnet's Avatar
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    Default Re: Digital power mesurement

    Quote Originally Posted by [email protected] View Post
    Norm,
    ...All VFD's have their own power measurement block which can be assigned either to the input or to the output. Generally a 4-20mA output is provided that can be attached to the required power reading...
    You are saying the VFD will output a 4-20mA signal in proportion to the power used of either input or output? If there were known values couldn't a PIC convert the signal to power readings? Did the VFD manual provide a conversion chart of the 4-20mA output readings?

    Norm

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  • #7
    Prolific Poster shantanu@india's Avatar
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    Default Re: Digital power mesurement

    Definitely Norm......many users depend on this 4-20mA output for motor power measurement by feeding it to a PLC analog input & displaying it in engineering units after scaling.
    I also have used this method many times in the past to avoid using an expensive power meter.
    But the catch lies in the fact that a VFD is not a measuring instrument & cannot be calibrated in a laboratory .......
    In my case the accuracy has to written & certified in black & white.
    Shantanu

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  • #8
    Senior Member Ken_K's Avatar
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    Default Re: Digital power mesurement

    Hi Shantanu

    I have no solution to your problem only some comments that may be of some use. I have been working on the design for a 60kW DC-DC converter and found current measurement a problem, no hall sensors tested had anything like the bandwidth required and all the tested devices had unacceptable hysteresis error. I found the Sensitec range of magneto resistive current sensors excelent with a 2MHz bandwidth almost zero drift and almost no measurable hysteresis error, very good indeed, a world away from the other sensors tested.

    https://au.rs-online.com/web/p/curre...ucers/7924180/
    https://docs-apac.rs-online.com/webd...6b81495e5d.pdf


    If you want to measure AC current only the bandwidth of a rogowski coil is outstanding.
    https://en.wikipedia.org/wiki/Rogowski_coil

    The RFI from a VF drive works against accurate measurements combined with the low output of a rogowski coil is a challenge, the magneto-resistive is better in this respect.


    As I said only suggestions here, I would be inclined to use a high speed adc to obtain voltage and current measurements at least at a few hundred thousand times a second.

    Studying some of the data sheets produced by various manufactures of power measurement IC's may give some ideas as to the best approach to take with the digital signal processing.

    Regards
    Ken K

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    Prolific Poster rcurl's Avatar
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    Default Re: Digital power mesurement

    Texas Instruments has some reference designs for 3-phase watt-hour meters that may be of interest: http://www.ti.com/solution/smart-e-meter-amr-ami

    Since these are used by the electric utilities the accuracy is pretty good even with nasty-looking waveforms.

    -Rick

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    Prolific Poster shantanu@india's Avatar
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    Default Re: Digital power mesurement

    H Ken,
    The Sensitec sensors seems to be very promising....thanks.
    What about voltage measurement......are there any line connected/galvanically isolated transducers around?

    The TI knowledge base is worth a read.....
    Last edited by [email protected]; 27th March 2018 at 11:47.
    Shantanu

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    Prolific Poster normnet's Avatar
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    Default Re: Digital power mesurement

    Quote Originally Posted by [email protected] View Post
    Norm,
    Fluke 1742 model shows voltage & current bandwidth of 42 Hz to 3.5 kHz.
    Does that mean that these meters can measure power accurately of 'dirty' waves as we would expect at the o/p of a VFD?
    I shall contact a Fluke agent in my locality.
    Thanks.
    The reply I received as follows:
    Thank you for contacting us, but Fluke doesnít have a model that logs the output of a VFD.

    Regards,
    Steve C
    Technical Support Engineer
    Fluke Industrial
    Norm

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    Prolific Poster shantanu@india's Avatar
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    Default Re: Digital power mesurement

    So Fluke is out of contention.....
    I fail to understand why meter manufacturers are not making their meters compatible to VFD outputs.......50% of the world motors with rating more than 5.5 kW are VFD driven nowadays.
    Shantanu

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    Prolific Poster normnet's Avatar
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    Default Re: Digital power mesurement

    Quote Originally Posted by [email protected] View Post
    So Fluke is out of contention.....
    I fail to understand why meter manufacturers are not making their meters compatible to VFD outputs.......50% of the world motors with rating more than 5.5 kW are VFD driven nowadays.
    As energy efficiency requirements increase for HVAC all the motors on a unit may someday be VFD as they may run almost continuously at varying lower speeds including the compressors in place of cycling on and off. One reason is the air moves more easily through the ducts at continuous lower speeds as opposed to cycling on at full capacity sometimes for just a few minutes. Of course on the hottest or coldest days of the year the unit would necessarily be running at full speed in order to meet the heating/cooling requirements.

    Norm

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    Senior Member Ken_K's Avatar
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    Default Re: Digital power mesurement

    Sadly I donít know of a simple solution for an isolated DC measurement.
    One could easily make non isolated measurements referenced to the test equipment ground by making a precision high voltage differential amplifier, if the input Z was kept low say at 1 meg good bandwidth could be obtained.
    As far as isolation of signals go it may be necessary to have a small floating power supply for a measurement circuit then using an RF coupler, opto coupler or optic fiber to couple back to the isolated equipment.
    I have used a dsPICís USART feeding one of these
    https://www.digikey.com/products/en/...ete/116?k=E96E
    via a length of cheap fiber (tested at 30 meters) into one of these
    https://www.digikey.com.au/products/...words=fb142-nd
    into my laptop.
    No more danger to myself from the high voltage high energy power supply.
    Much faster Fibre Optic Tx Rx are available for a higher price. The connector less fittings are interesting; if only using a couple meters of fibre one can pull the fibre out of the detector about 12mm before data dropout.
    The images are of a test setup only, a new PCB will be designed in the next few weeks, its only coms i/o will be fibre optic.

    High voltage end.

    Click image for larger version

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    Laptop end well away from RFI and high voltage, I have not seen one bad data packet!

    Click image for larger version

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    Shantanu the answer to your comment about not understanding why all the meters won't read the output from a VF drive may simply be it is not that easy.
    Processing power is cheap nowadays maybe even a dsPIC could cope.
    Ken

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    Prolific Poster shantanu@india's Avatar
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    Default Re: Digital power mesurement

    Sorry for seeing your post almost after a month Ken!!!
    Thanks for your views , but I am not treading the dsPIC path for the time being.....I think I have found a Fluke meter that serves my purpose.
    435-II is a 3-phase power quality analyzer with voltage & current input bandwidths of 10kHz.
    It is supplied with 3 voltage probes & 4 current clamps(clamps can be ordered separately)
    It was actually confirmed by a local Fluke representative.
    I am attaching the datasheet.
    Shantanu

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