Troubleshooting of Star-Delta Starter

Star Delta starters are widely used in electrical installation for starting of large rating squirrel cage induction motor to limit the starting current of the motor.Every electrician and engineer must know about what are the probable problems which frequently occur in the starter or motor connected to it. We will discuss all the problems related to star delta starter in this post. First, let us list out the problems.


  1. Motor takes high current at start and trips

  2. Motor keep on running in star and does not come in delta

  3. The fuses get blown during changeover from star to delta

  4. Motor over-current trip during star to delta changeover

  5. Motor current hunting


Now, Let us discuss all the reasons in details and their causes and solution. The circuit diagram of the star delta starter is as follows.

KM1 - STAR CONTACTOR

KM2 - DELTA CONTACTOR

KM3 - MAIN CONTACTOR


Motor takes high current at starts and trips


Generally, squirrel cage induction motor draws 5 to 7 times current of its full load current(FLC) when we start it from rest position. If the overload relay trips the main contactor during motor running in star configuration, the overload relay setting must be checked. If relay setting is correct check the trip class of the relay. If motor is driving fan then the trip class setting of the relay must be 25 to 30.


Also, Check the followings to ensure that there is no single phasing.

  1. Main Contactor - Check the power contacts and Aux. contact

  2. Star Contactor - Check the power contacts and auxiliary contacts

  3. Motor Cable - All the six cores of the motor should be checked

  4. Motor winding - Check the winding resistance

Motor Keeps on Running in Star and does not come in Delta


The timer is used in the star delta starter for doing changeover from star to delta. If timer is defective, the motor will keep on running in star mode if the load permits. Here load permits means if the torque requirement is less than the motor may keep on running else the motor will trip with overload. Pl note that the motor in star mode delivers only 58.7 % of the motor rated torque.


Fuses gets blown during star to delta changeover


This is very interesting problem. One may come across this problem when the existing faulty motor is replaced. One must check the proper configuration of the delta connection formed in the starter or feeder of newly installed motor. The proper delta may be checked by measuring the inductance of the motor. The inductance of all the three phases will be balanced if winding is correctly connected in delta.


If delta is not correctly formed, line to line short circuit will take place at the instant when motor winding configuration is changed from star to delta. Though there may be many reasons of short circuit and blowing of fuses, but this is one of the reason of blowing of fuses during star to delta changeover.


Motor over-current trip during star to delta changeover


It is important to correct set the timing of timer. When motor starts accelerating in star it current gets decrease as motor speeds up. But it is only possible if the load on the motor is less compared to motor torque. When the motor is changed from star to delta motor may draw high current if the motor has not attained the sufficient speed when running in star. Therefore, according to to load and motor speed torque characteristics the timing of the timer must be set to have minimum current shift during transition from star to delta.


Motor Current Hunting


If the fluctuation in motor current is observed, the motor must be immediately stopped if the protection relay does not have negative sequence protection element(I2). The motor current hunts when there is loose connection in the rotor or single phasing. Single phasing is the extreme case of negative phase sequence. The negative phase sequence current produce counter torque in the motor and, winding starts heating up and eventually leads insulation failure. The following must be checked if hunting in the motor current is observed.

  1. Motor winding

  2. Delta connection circuit- Main contactor and delta contactor

  3. Cable

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