The main purpose of any starter is to reduce the requirement of high starting current. Normally the starting current of an induction motor is 6 to 7 times of the full load current. If one has an induction motor with a DOL starter, drawing a high current from the line, which is higher than the current for which this line is designed. This will cause a drop in the line voltage, all along the line, both for the consumers between the substation and this consumer, and those, who are in the line after this consumer. This is the reason for which a starter is to be used.

In a squirrel cage induction motor, the starter is used only to decrease the input voltage to the motor so as to decrease the starting current.

It is T.P.D.T switch used to first start the motor with the winding connected in star and then switch for delta connection in running position. TPDT stands for Triple Pole, Double Throw.

Why Star Delta Starter Preferred in Induction Motor?

If the winding of Induction Motor is connected in star, the voltage per phase supplied to each winding is reduced by 0.577.In general the voltage per phase in delta connection is Vs, the phase current in each stator winding is (Vs/Zs), where Zs represents the impedance per phase of the motor at standstill or start.

The line current or the input current to the motor is Ist (starting current) = (1.732*Vs)/Zs which is the current when it has to be started by DOL starter.

Now, if the stator winding is connected as star, the phase or line current drawn from supply at start (standstill)  = (Vs/Zs)/(1.732)

which is 0.333= (0.577*0.577) of the starting current, if DOL starter is used.

The voltage per phase in each stator winding is now Vs/√3. So the starting current is reduced by 33.3%. Because of the reduction in starting current, starting torque reduces.

Therefore we can conclude that by using Star Delta starter, the starting current is reduced to approximately two-thirds. Since starting current is reduced, the voltage drops during the starting of motor in systems are reduced. 

Thank you!

6 thoughts on “Why Star Delta Starter Preferred in Induction Motor?

  1. ANIL SAINI says:

    IF we can use the winding that will bear the high starting current then can we use DOL ?
    As per my knowledge one of the reasion for using starter is "if we use DOL the it will carry high current that will cause the voltage dip in the line and the distribution authorities don't want the V dip in line that's they forces the company to use starter.otherwise why would companies buy such costly starter.

  2. Aditya Kumar says:

    Thank you for adding! You are correct and that is what mentioned in the post.

  3. Che Kuan Yau says:

    1. "The main purpose of any starter is to reduce the requirement of high starting current." This statement is NOT correct. [The main purpose of any starter is to switch-on (=start) and switch-off (=stop) of a motor] This is evident, for example a DOL starter does NOT reduce the requirement of high starting current.
    2. In general, ALL motors are manufactured with the windings that CAN bear the high DOL starting current. That is to say ALL motors are designed suitable for DOL starting.
    3. The use of SD, AT, ST, VFD/VSD etc. starters are NOT due to the winding that can not bear the high DOL staring current. Their use is to LOWER the starting-current (during starting only).
    There is NO difference in the full-load current, irrespective of the type of starter used.
    4. Voltage-dip/voltage-drop is proportional to the value of the CURRENT. The length/resistance of the wire DOES NOT change, irrespective of the type of starter used.
    5. DOL starting draws the HIGHEST starting-current, therefore causes the MOST sever voltage-dip/voltage-drop.
    Che Kuan Yau (Singapore)

  4. Aditya Kumar says:

    Thank you very much Che Kuan Yau for value addition.

  5. Che Kuan Yau says:

    ” It is T.P.D.T switch used to first start the motor with the winding connected in star and then switch for delta connection in running position. TPDT stands for Triple Pole, Double Throw.” The T.P.D.T switch method (was) used more than 60+ years ago. The current practice is by using 2 (but better 3) numbers of [3-pole contactors] in place of the TPDT switch. Obsoleted equipment used or practice should be replaced by current in the market.

    1. admin says:

      Thank you very much for sharing so valuable information.

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