- Three-Phase Induction Motor Definition: A three-phase induction motor is a type of motor that operates using a three-phase power supply and a three-phase stator winding.
- Revolving Magnetic Field: The stator windings are arranged 120 degrees apart to create a revolving magnetic field that induces current in the rotor.
- Slip Speed: Slip speed is the difference between the synchronous speed of the stator’s magnetic field and the rotor speed, ensuring the motor does not run at synchronous speed.
- Starting Current and Voltage Drops: High starting current causes significant voltage drops, which can affect motor performance if not managed.
- Starting Methods of Three Phase Induction Motor: Different methods, like DOL, Star Delta, and Auto Transformer starters, are used to reduce starting current and ensure smooth motor operation.
Let’s consider how a 3 phase inductor motor starts. We apply a three-phase supply to the motor’s stator winding, arranged so each phase is 120 degrees apart. This setup creates a revolving magnetic field in the stator. When we switch on the supply, the rotor remains stationary at first. The maximum change in flux linkage induces an emf in the rotor conductor, causing a large starting current that is several times higher than the 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.
From the formula, we see that the rotor always rotates slower than the revolving magnetic field. If the rotor moved at the same speed as the field, there would be no flux cutting action and no emf induced in the conductor, causing the motor to slow down. This is why induction motor never run at synchronous speed. However, the high starting current initially causes large voltage drops, affecting the motor’s performance.
Starters help reduce the initial large current when the supply is switched on, ensuring smooth motor operation. There are different types of starters for three-phase induction motors, including star delta starter, Direct Online (DOL) starters, and auto transformer.
Now, let us have a look at the contribution of starters in starting of an induction motor.
DOL Starter: A Direct Online (DOL) starter uses a one-way switch that operates on each phase of the three-phase stator winding. It has an overload release to protect the motor from overcurrent and a no-volt release to protect against sudden supply failure. DOL starters are used for motors up to 5 horsepower (HP).
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.
