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Current Transformers on each core of a 3 phase line

  1. Sep 13, 2012 #1

    J_B

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    Hi folks,

    I'm a Mech Eng who is trying his hand at some Electrical design so please forgive my ignorance.

    My set-up is as follows: I have a 3 phase source powering a 3 phase induction motor. If I then went half way along my power cable and put a separate current transformer on each of the phases, could I drive another secondary 3 phase motor from them? Obviously a lot smaller than the primary motor.

    I don't even know if the theory is correct. If it is correct then obviously it depends on the current in the primary power line and the size/load of the secondary 3 phase motor.

    If I switch 2 of the phases at the supply, it would drive my primary motor in the opposite direction. Would it also drive my secondary motor in the opposite direction?

    Any advice/info greatly appreciated.

    JB
     
  2. jcsd
  3. Sep 13, 2012 #2
    Hello JB -

    A current transformer (CT) is meant for sensing current ( for measurement, control or protection) but not (in any real case I have seen) used for actual Power transformation... i.e. run a motor.

    The CTs typicality have a VA rating - referring to Volts x Amps. So if the secondary of the CT is rated for 1 A and the VA rating is 50 - the highest Voltage you will see is 50V, and that is only when the primary of the CT is at rated current 50:1 CT you need 50 A in the primary to get the 1 A in the secondary, more current that that CT core will saturate. The scenario you describe is just not what they are made for.

    As for swapping 2 of the 3 legs of the primary - you are correct this will revers a typical motor. If you are able to get a motor to run on the secondary of the CTs - it will also reverse direction.

    So your theory is correct - but not a practical system.
     
    Last edited: Sep 13, 2012
  4. Sep 13, 2012 #3

    J_B

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    Windadct,

    Thanks a lot for this. Would it even work for a particularly high power primary motor driving a very small secondary micro motor?
     
  5. Sep 13, 2012 #4

    dlgoff

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  6. Sep 14, 2012 #5
    Hello Again J B - yes it is possible, as dlgoff points out - it is technically how the disc rotates in your KWH Meter.

    Other than metering - it is the why would you - that I can not answer.
     
  7. Sep 17, 2012 #6

    J_B

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    Thanks for the info guys. Could you point me in the direction of some calculations that would allow me to work out the power available to the secondary motor given that I know the voltage and current of my primary lines.

    In a 3 phase induction motor, are the phases connected at the motor in order to "complete" the circuit. I'm thinking back to my school physics days and know that a circuit must be complete for electricity to flow at all. Or is does it return down one of the other phases?
     
  8. Sep 17, 2012 #7

    NascentOxygen

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    Hi J_B. Perhaps you should explain what you are wanting to achieve, because the arrangement you discuss here is almost certainly not what anyone would want (in all reality).
     
  9. Sep 17, 2012 #8

    NascentOxygen

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    Staff: Mentor

    Three currents of equal magnitude but each differing from the next by 120° exactly cancel; but it is easier if you picture the current in one phase returning via the other two lines. You only need the three lines. It is best that the load is balanced so the phase currents are all equal.
     
  10. Sep 17, 2012 #9
    J-B :

    Can you provide the details - particularly of the CT?

    I am assuming that for a balanced 3Ph load - the standard power equation applies.

    √3* V * I * PF = Power

    ... the big unknown is the CT how much inductance it will add to the circuit - causing a phase shift between the V and I - since the CT is not designed to output V it may be allowed to shift considerably from the current.

    The main functions for the CT are current ratio and Phase angle if the Primary to Secondary current. As long as those to things are OK it has done its job. Although the more I think about it the V to I phase shift is probably not an issue...but I have never doe this.
     
  11. Sep 17, 2012 #10

    J_B

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    If 2 of the lines have a break in them, but the source is "on" what will happen at the motor? Nothing because current can't flow in one phase only? Or would the one phase that is still good burn out?

    I can't go into too much detail about what I want to achieve at this stage as I want to find out if the concept is achievable and then protect the design before I disclose it in a public domain.

    Thanks for your help.
     
  12. Sep 17, 2012 #11

    J_B

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    Windadct,

    I hadn't seen your last post before replying my last post (I'm on my phone).

    I don't actually have details of the CT yet as I haven't begun designing it. But I have the details of the primary 3 phase circuit.
     
  13. Sep 17, 2012 #12
    J B if you don't want to disclose you idea on a public forum, why don't you send a personal message to one of the responders (or myself) describing your idea.
     
  14. Sep 17, 2012 #13

    psparky

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    You could do this....and it would power another motor....but the power is anything but free.

    If your extra motor puts out 100 watts of power at the shaft....then the power company will need an extra 100 watts....or more relastically.....130 VA worth of extra power.

    AS soon as you put extra drag on the wires via Current transformer.....you power load just went up.

    Not sure this is what you are asking...but if so.....nope, you can't make free power.

    IF you are asking if a current induces a rotating magnetic field or visa versa...then the answer is simply..."yes".
     
  15. Sep 17, 2012 #14

    J_B

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    Psparky,

    Thanks for your input. I'm not trying to get free power - I just want to know if it's feasible and worth looking into.

    I'd like to drive a micro linear actuator from a 5kv, 140 amps 3 phase circuit via 3 current transformers.

    Something like this: http://schumo.ch/products_motors.htm
     
  16. Sep 17, 2012 #15

    NascentOxygen

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    Staff: Mentor

    Nothing. With only 3 power lines, current has no where to go. (If neutral was connected, as a 4th line, then there could be single phase current in that phase.)
     
  17. Sep 19, 2012 #16

    J_B

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    If I had a CT on 2 of the phases only driving a 2 phase AC micro motor, would it work given that the primary source is 3 phase? They would be 240 degrees out and the 120 degrees out I suppose?
     
  18. Sep 20, 2012 #17

    NascentOxygen

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    I expect it would, or at least expect that it could be made to. May not be really efficient, but let's assume it can run ...... now what?
     
  19. Sep 20, 2012 #18
    J_B

    The configuration is called "open delta"- and can be used to transform 3 phase from 2 transformers. Use Google Images to to see a number of examples.
     
  20. Sep 21, 2012 #19

    J_B

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    NascentOxygen,

    Now what? Now I will try to calculate what torque I can achieve from this secondary micromotor to see if I can do anything with it. Or what force I can generate from a linear actuator (lead screw torque calc)

    Windadct,

    Thanks for this - good info.

    J_B
     
  21. Sep 22, 2012 #20
    "Something like this: http://schumo.ch/products_motors.htm" [Broken] is for single phase actuators.
    One current transformer on one line would kinda work. When the line current got above some value, the actuator would activate and when the line current got below some level the actuator would de-activate.
    Of course when the line current got too high, the actuator would burn up.
    What are you trying to do?
     
    Last edited by a moderator: May 6, 2017
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