Starting Methods of Induction Motor

 A three Phase induction motor is self starting.When the supply is connected to the stator of a three-phase induction motor, rotating magnetic field is produced.and the rotor starts rotating and the induction motor starts.At the time of starting the starting current is very high.Due to this high  current, we need some soft start for motor.
Soft Starter 

        A motor soft starter is a device used with Ac electrical motors to temporarily reduce the load and torque in the power train and electric current surge of the motor during start up.
         Motor often require large amounts of energy when quickly accelerating to full speed. Soft starters and variable frequency drives can both be used to reduce inrush current and limit torque protecting equipment and extending the life of a motor by reducing motor heating caused by frequent starts and stops.But there are a number of differences between a soft starter and a variable frequency drive.
Direct on line (DOL) Starter
        
        The simplest form of motor starter for the induction motor is the Direct on line (DOL) starter.The DOL starter consist of MCCB or circuit Breaker,contactor and an over load relay for protection.Electromagnetic contactor which can be opened by the thermal over-load relay under fault condition.
        Because Direct on line starter have a maximum start current and maximum start torque so, Normally we can use DOL starter only for less than 5HP motors.
Advantages of DOL Starter
  •   Most Economical and cheapest starter.
  •   Simple to establish, operate and maintain.
  •   Simple Control circuitry.
  •    Easy to understand and trouble shoot.
  •    Only one set of cable is required from starter to motor.
  •     It provide 100% torque at starting.
  •     Motor is connected in delta at motor terminal.
Disadvantages of DOL Starter   
  •     It does not reduce the starting current of motor.
  •        There is a big dip in the electrical installation.
  •        Unnecessary high starting toque.
  •        Thermal stress on the motor.
Star-Delta Motor Starter
         
         Most induction motors are started directly on line ,but when large motors are started that way they cause a disturbance of voltage on the supply line due to large starting current surges. To limit the starting current surge, large induction motors are started at reduced voltages and then have full voltage reconnected when they run up to near rotated speed, we used star-delta starting method.
           This is the reduced voltage starting method.During starting the motor windings are connected in the star configuration and this reduces the voltage across each winding.This also reduces the torque by a factor of three.After a period of time the winding are reconfigured as delta and the motor runs normally.
Components of Star-Delta Starter 

  • Contactors( Main, Star and delta contactors) 3No.
  • Time relay (Pull-in delayed) 1No.
  • Three pole thermal over current release. 1No.
  •  Fuse elements or automatic cut-outs for the main circuit 3No.
  •  Fuse element or automatic cut-out for the control circuit 1No.
Motor Starting Characteristics 
  • Available starting current 33% full load current,
  • Peak starting current 1.3 to 2.6 full load current.
  • Peak starting torque 33% of full load torque.
Advantages of star-delta starter
  • The operation of the star-delta method is simple and rugged.
  • It is relatively cheap compared to other reduced voltage methods.
  • Good torque current performance.
  • It draws 2 times starting current of the full load ampere of the motor connected.
Disadvantages of star-delta starter
  •  Low starting torque ( Torque= (sq. of voltage) is also reduce).
  •  Break in supply - possible transients.
  •  Six terminal motor required (Delta connected).
  •  It requires 2 set of cables from starter to motor.
  •  In this method of starting initially motor is connected to star and then after change over the motor is connected in delta.The delta of motor is formed in starter and not on motor terminals.
  •  It provides only 33% starting torque and if the load connected requires higher starting torque at the time of starting then very heavy transients and stresses are produced while changing from star to delta connections and because of these transients and stresses many electrical and mechanical break down occurs.
  
        

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