Induction Motor

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                               How to smooth start a 3-phase induction motor ?

    Spir Georges GHALI

    Hamdullah said:

                               How to smooth start a 3-phase induction motor ?

    Dear ;


    When we want to smooth the starting of ” AC Induction Motor ” we can use the Electronic Starters where we have 2 possibilities, that are :

    – By ” Variable Speed Drive “,  or :

    – By ” Soft Starter “

    The main difference between them is :

    with ” Soft Starter ” we can only ” Run  &  Stop ” smootly the motor, but after running the motor will run absolutly at his nominal speed, where with ” Variable Speed Drive ” we have the same advantage, plus we can after running change the spped of the motor. So, if you don't need to change the speed the best solution is using the ” Soft Starter “, but if you should change the motor's speed you should use the ” Variable Speed Drive “.


    Noting that :

    – There's up to 630kW-415VAC  as ” Soft Starter ” or ” Variable Speed Drive “.

    – The Vriable Speed Drives generate the Harmonics.




    A variable speed drive can be achieved by variable-frequency control

    The emf induced in the stator of the induction motor is given by:


    If supply is changed E1 will also change to maintain the same air gap flux.If the stator voltage drop is neglected the terminal voltage is equal to E1.But in order to avoid saturation and to minimize losses,motor is operated at rated air gap flux by varying frequency in such a way such that V/f ratio remains constant.

    The variable frequency is obtained using a)Voiltage source inverter b)current source inverter c)cycloconverter

    soft starter generally make use of thyristor with a varying firing angle

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