Current Monitoring Using two CT's but one Ammeter

Discussion in 'Test & Measurement Forum' started by EJCE, Apr 16, 2018 at 5:34 AM.

  1. EJCE

    Thread Starter New Member

    Monday
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    0
    I am currently designing a system and have run into a few hurdles. The system includes two motors. One 0.5kW and one 7.5kW. I need to monitor the current for each one and display the current on the enclosure. Normally this would not be an issue. However the display window is extremely limited and I only have enough room for one ammeter. My plan is to use two current transformers feeding the one ammeter but have a switch in line to allow switching between the two current transformer feeds. In theory this would allow monitoring of both currents without interrupting the main supply and allow me to use one ammeter.

    I am having difficulty sourcing an appropriate CT for the 0.5kW motor. The current rating will be between 0 and 2 amps. This is fairly low. I also have the issue of potential losses or inaccuracy due to the switching. My cable is 2.5mmSQ.

    Does anyone have any suggestions or recommendations? The main goals is to be able to monitor two separate currents but only using one ammeter.

    Thanks
     
    Last edited: Apr 16, 2018 at 5:39 AM
  2. danadak

    Member

    Mar 10, 2018
    299
    63
    Bit of a trade off but an OLED small graphic display would fit
    into a traditional panel meter radius. With processor, here are
    example boards -

    https://www.banggood.com/search/oled.html

    This would allow you to do either a single large character display or
    a scrolling display of both readings or a two line display, smaller char-
    acters. Many possibilities.

    Also processor would allow you to condition low output sensor, possibly.

    Regards, Dana.
     
  3. EJCE

    Thread Starter New Member

    Monday
    3
    0
    Hi Dana,

    Thank you for your reply. It is definitely a more interesting approach, however I have since sourced a range of ammeters which are small enough to fit within the given window.

    Thanks again, Euan
     
  4. Reloadron

    Distinguished Member

    Jan 15, 2015
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    How small is the display window? Panel meters are normally expressed in DIN size, so for example you have a 1/8 DIN meter which is 50 mm x 97 mm). Have you selected a panel meter yet and if so which meter?
    Most current transformers have a secondary current of 1 or 5 Amps. Generally currents less than 5 amps are read directly. I doubt you will find a CT for such a low current.
    Rather than a CT I would think about using a few AC current transducers which are fairly common. What is your anticipated current on the larger motor? Also are you good with the RMS current being average responding or do you need true RMS responding? Examples of basic AC current transducers.
    Also something to keep in mind using a CT is you can switch between CTs but you really do not want to leave a CT secondary output open.

    Ron
     
  5. EJCE

    Thread Starter New Member

    Monday
    3
    0
    Hi Ron, the display window is 220mm x 80mm. I also need have an hour run meter and VSD display visible within the window, hence the tight space. I found a range of meters than may be suitable. They are Murata ACA-20PC-1-AC1-RL-C and ACA-20PC-2-AC1-RL-C. The larger motor will have a current of 13-18A. The average RMS is acceptable.

    Thank you for the advise on the CT. This is my first project using a CT.
     
  6. Reloadron

    Distinguished Member

    Jan 15, 2015
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    Here is how I would likely go about doing this. I have used CTs in the past but for newer applications I like the AC current transducers. I have seen these units marketed under a half dozen different names and they are all the same. I have one sitting here labeled Sentry 200-1. They are nice in that they have a jumper select range. To cover both motors scale a current transducer at 0 to 20 AAC. They output a 4 - 20 mA and this way you have two current sensors which are the same easily switchable into a single display. Murata and a dozen others make meters designed for use with a 4 to 20 mA loop, designed for scaling with 4 to 20 mA devices. Just add a simple SPDT switch to select which motor you are reading. You will need a 24 VDC supply to power the current sensors.

    I am not saying don't use current transformers, I am saying using current transducers makes for a simpler cleaner solution.

    Ron
     
  7. dendad

    Well-Known Member

    Feb 20, 2016
    1,386
    341
    If you need more output from the current transformer, run a couple of turns (or more) through it instead of just one.
    Try that and see if it will work for you.
     
    Reloadron likes this.
  8. Reloadron

    Distinguished Member

    Jan 15, 2015
    2,988
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    With the above in mind here is an example of a 60:5 CT where I wrapped 3 turns through the primary making it now a 20:5 CT.

    60 to 5 CT 3 Turn Primary.png

    Ron
     
    MaxHeadRoom likes this.
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