measuring 3ph voltage using a reducing probe?

Discussion in 'General Electronics Chat' started by kokkie_d, Jul 21, 2015.

  1. kokkie_d

    Thread Starter Active Member

    Jan 12, 2009
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    0
    Hi,

    I want to measure the line-line voltage (415V) or Line-neutral voltage (240V) from a 3phase power supply. I am also measuring the current using a hall sensor. yes, I am after the power factor and power of this 3ph system when loaded.

    I have a NI module which can measure up to 250V so I can measure line-neutral. But it got me wondering, is there a voltage probe that reduces the voltage by a factor of 10 (say) much like a probe for an oscilloscope. I measured the 10x reduction of an oscilloscope probe using a multi meter on a single phase source (240V) and found the voltage to have been reduced to 11V. which isnt a factor of 10 reduction and made me wary of using this to measure voltage on mu NI module (they are expensive).

    The question: is there an of the shelf product that reduces voltage by a factor that is reliable to use by a standard multi meter and not a scope? I need to log the data for the system I am building (and yes I can do this via oscilloscope but would like a more integrated solution; keeping all data together).


    regards
     
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Are your input channels fully isolated (from each other and from ground)
     
  3. kokkie_d

    Thread Starter Active Member

    Jan 12, 2009
    72
    0
    Hi,

    The two modules I have are: NI 9219 which has
    - 250Vrms channel to channel isolation
    - Available channel-to-earth ground double-isolation barrier for safety, noise immunity, and high common-mode voltage range
    - 10ms response rate (borderline nyquist - UK is 50Hz)
    -
    and the NI 9207
    - which has channel to ground isolation
    - Each channel must remain within ±10.2 V of common
    - 2ms conversion in high speed mode

    yes, I reported earlier I had a module that can measure 250Vrms I was wrong. I dont have that module. So I kinda need the other solution.

    regards
     
  4. strantor

    AAC Fanatic!

    Oct 3, 2010
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    You need the COMPLETELY other solution. What you have will not work AT ALL.
    Your "250Vrms" module is only safety rated for 250Vrms. It's working voltage is only +/- 60V. And even if you used a voltage divider to scale it down, its sample rate is only 100 samples/second. That isn't even close to fast enough to measure a 50Hz wave.
    ni9219.png

    Look at the module that is designed for AC input, to get an idea what you need:
    ni9244.png
    50,000 samples/sec. (500 times faster than what you have)
    400V phase>neutral, 800V phase>phase (13 times higher than what you have)

    And then to measure the current to get the power factor, you're going to need something just as fast as the voltage input (i.e. about 50,000 samples/sec). As we know, your NI9219 is only 100S/s, and your NI9207 is 500S/s; that one too, is too slow, by a factor of 100. AND the NI9207 is not a current input module! Current is usually measured by a current transformer; these toroidal transformers typically output 5A for their rated max input amps. So you need a current input module good for 5A. Like this one:

    ni9246.png

    In summary, sell your 9219 and you 9207, and buy a 9244 and a 9246.
     
    kokkie_d likes this.
  5. Reloadron

    Active Member

    Jan 15, 2015
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    232
    Yes, they are Active O'Scope probes which allow for high voltage differential inputs. The ones I have used for 3 phase circuits were manufactured by Tektronix and allowed for attenuations of 100X as well as other values with the push of a button. When connected to the right scopes they were scope powered but couls also be used with an external power supply. They don't come cheap and the ones I used were roughly $1,000 per probe as I recall. If you plan to measure phase to phase you need isolated differential type probes, you can't use anything single ended. A Google of "Test Probes ACTIVE DIFF PROBE" should get you some results and maybe an economical solution.

    Ron
     
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  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,516
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    An oscilloscope and a multimeter have very different input stages, and a scope probe will not deliver its rated performance unless it is used with a scope.

    ak
     
    kokkie_d likes this.
  7. kokkie_d

    Thread Starter Active Member

    Jan 12, 2009
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    0
    Thank you very much for the good advice.
    I was planning on using a hall sensor for the current measurement so I would not have to worry about the current input. But the sample speed advice is well noted.

    Thanks for the advice on the probes.

    I cant sell my other modules so I might have to buy new or buy a probe for my scope. price wise there doesnt seem to be much in it.

    thank you all
     
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