Advertisement
If you have a new account but are having problems posting or verifying your account, please email us on hello@boards.ie for help. Thanks :)
Hello all! Please ensure that you are posting a new thread or question in the appropriate forum. The Feedback forum is overwhelmed with questions that are having to be moved elsewhere. If you need help to verify your account contact hello@boards.ie

3Phase Motor cable calc.

Options
13»

Comments

  • Options
    #62
    Tuco88
    Registered Users Posts: 1,051 ✭✭✭Tuco88


    Hi lads, thanks for all the feedback nice to see other views on this. Its 55Kw on the input, I will be placing the over-load on the main KM of the delta loop set at 0.58 F.L.C ill stick to the way I have done it before, there is thermistor for protection also. IMO a Star delta start is only good for the switch gear supplier... I shall let you know how it all works out.

    Just a little clarity on the current for my own understanding, is my view on it correct. Taking this to be the ideal motor excluding wind friction loss, load changes and all the rest of that good stuff.

    Motor Current starting in Star. They way I always seen it was 1/1.732 x F.L.C so 0.57735 x 93 = 53 amps per phase. (1/1.732 stands for the same loss in Star to the watts/Torque)

    Motor Current running in Delta. Root 3 x IP 1.732 x 53 = 93 Amps per phase.

    Now its clear on start up there will be a large in-rush current in star but should settle to around 53 Amps approx?

    Is fair to say the Overload has a time delay for such large inrush currents on start up in star, and the correct overload protection for such effects is to be selected (I.e C/D type mcb)


    We worked out I(L) or the design current to be 93 Amps, but if you remember the formula for star I(L) = I(p) it would me 93 amps for star its what I do not understand? So is the 93 Amps for the motor in Delta?


    Thanks again, sorry for all the silly question's but you are after clearing a ton up for me that I must have forgot/misunderstood. You would be surprise how many people have different views on this subject.


  • Options
    #63
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    The running current in star will be a third of that in delta, so 31 amps.

    The 55kw motor will be 18.333kw in star

    18333/(400 x 1.73 x 0.85) = 31 amps or there abouts. That`s 31 amps on each of the 3 supply cables.


  • Options
    #64
    arse..biscuits
    Registered Users Posts: 1,878 ✭✭✭arse..biscuits


    Bruthal is correct. When connected in Star, the motor will only be one third as powerful and line current will be one third of that in delta. So line current in star will be 93/3=31A (again on each cable- hopefully that confusion is put to bed)

    The start up current however can be multiples of the running current. But as one already said, this should be over come by selecting the correct over-current protection. (Type C MCB)

    If connecting your overloads in the Delta loop, you should set them to the phase current for delta which is 53.9A


  • Options
    #65
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Just to add to the last post tuco, the current is 31 amps per phase supply in star as said. And that is also the current in each winding.

    That is 1.73 times lower than the winding current in delta, which has been shown to be 53 amps in each winding.

    And the voltage is 230v across each winding in star as well. Compared to 400v across each winding in delta.

    So both the voltage, and current are reduced by a factor of 1.73

    And that is how the motor is 1/3rd the power in star that it is in delta.


  • Options
    #66
    Tuco88
    Registered Users Posts: 1,051 ✭✭✭Tuco88


    Thanks for clearing that up lads, and the expert advise. Once again the gang from boards to the rescue...


  • Advertisement
  • Options
    #67
    Borzoi
    Registered Users Posts: 2,876 ✭✭✭Borzoi


    Tuco88 wrote: »
    Thanks for clearing that up lads, and the expert advise. Once again the gang from boards to the rescue...

    Can I muddy the waters a bit....a lot of these large fridge motors aren't star-delta but are part wind start, typically 66/33% ( but by no means always). Unfortunately it's difficult to identify this without proper labelling as there's just a standard set of 6 terminals.

    On another note fridge systems rarely run flat out, so don't go overboard on the MIC sizing


  • Options
    #68
    arse..biscuits
    Registered Users Posts: 1,878 ✭✭✭arse..biscuits


    Borzoi wrote: »
    Can I muddy the waters a bit....a lot of these large fridge motors aren't star-delta but are part wind start, typically 66/33% ( but by no means always). Unfortunately it's difficult to identify this without proper labelling as there's just a standard set of 6 terminals.

    On another note fridge systems rarely run flat out, so don't go overboard on the MIC sizing

    It's very easy to tell if it's star/delta, there are 3 contactors and a timer all part of the circuit. Also I don't remember him saying it is a fridge motor, he said it was a motor, part of a chilling process. There is a difference.


  • Options
    #69
    Borzoi
    Registered Users Posts: 2,876 ✭✭✭Borzoi


    arse..biscuits wrote: »
    It's very easy to tell if it's star/delta, there are 3 contactors and a timer all part of the circuit. Also I don't remember him saying it is a fridge motor, he said it was a motor, part of a chilling process. There is a difference.

    Very helpful, thanks


Advertisement