Why DC series motor should never be started without some external load?

Forums Common electrical queries Why DC series motor should never be started without some external load?

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  • #454

    Rishi
    0 votes

    Why it is advised not to start a dc series motor on No-load (i.e. without any external load connected to it). What happens if dc series motor is started without any external load connected?

    #461
    votes +2
    Profile photo of Dnyanesh
    Dnyanesh
    Moderator
    8 votes
    Points: 284
    Query Resolver

    On NO-LOAD condition the current drawn by the motor is small.

    and

    θ ∝ I

    As a result flux produced is also small

    And as we know from emf equation of dc motor-
    Eb= ΦP NZ/60A
    N= 60AEb/ΦPZ
    As we keep Econstant
    N ∝ 1/ Φ
    So on NO-LOAD condition speed of dc series motor will increase dangerously, which will give mechanical harm to motor.
    So its advised to run dc motor on LOAD.

    #519
    votes +1
    Profile photo of Aman Varshney
    Aman Varshney
    Participant
    1 votes
    Points: 30

    Speed of the dc motor is inversaly proportional to the field flux per pole, that is proportional to the field winding current.

    In case of Dc series motor, Field winding is in series with the Armature wdg., So same current flows through both the windings.

    At no load conditions, load torque is zero thus the armature current and hence the field current is also zero.

    As speed inversaly varies with field then at zero field speed becomes infinite or dangerously high…

    That’s why dc series motor is always started at heavy load to limit the operating speed within the prescribe limits..

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