Let us consider the case of starting of a 3 phase inductor motor. Here, we apply a three-phase supply across the motor having three-phase stator winding. We arrange the stator winding in such a way that each phase is 120o separate from each other. This arrangement produces a revolving magnetic field in the stator due to applied three phase supply. The rotor remains stationary just after switching ON the supply. The change in flux linkage to the rotor is maximum therefore emf will be induced in rotor conductor causing large current to flow through it. This current is called starting current which is multiple times of full load current.
Hence now the rotor is acting like current carrying conductor placed in revolving magnetic field. Hence, the rotor conductors now experience mechanical force in the direction same as direction of the revolving magnetic field, and hence the rotor starts rotating and attends a speed given as
Slip Speed = Ns – N
Where Ns is the synchronous speed that is the speed of revolving magnetic field present at stator winding and N is Rotor speed.
That is from the above equation the rotor always rotates with speed less than the speed of revolving magnetic field. If rotor rotates at a speed of revolving magnetic field then there will not be any flux cutting action by conductor and field, and hence there will not be emf induced phenomena in conductor, and hence motor will be decelerated rotate and that is why the induction motor never runs at synchronous speed. But here the important point is the large current flowing initially just after switching ON the supply causes large voltage drops across various elements of motor which greatly affects the functioning of motor.
To avoid this problem starters comes into the picture which helps to reduce that initial large current flowing just after switching supply ON and it ensures smooth switching of a motor. There are different types of starters for 3 phase induction motor such as star delta starter, direct online starter, auto transformer etc.
Now, let us have a look at the contribution of starters in starting of an induction motor.
DOL Starter – It is a direct online starter consists of one-way switch simultaneously operates on each phase of the three-phase stator winding. The overload release of this switch protects the motor against overcurrent and No-volt release of this switch protects the motor from sudden three phase supply failure. DOL starters can be used to start motors up to 5 H.P.
Star Delta Starter – The stator winding is connected in star fashion so that voltage at each phase is
Where, VL is line voltage. Therefore voltage is reduced at each phase at starting and hence current is reduced. As soon as motor achieves certain speed, motor winding is connected in delta fashion such that line voltage is equal to phase voltage with the help of two way switch. The star delta starter is used for starting of induction motor above 5 H.P.
Auto Transformer – The necessity of reduction of starting current to avoid failure of motor can be effectively done by auto transformer which consists of a single winding. The number of turns of the winding can be varied manually by moving the circular slider provided on it such that limited current will flow through the stator winding for an initial duration of time.