- 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.
Starting a 3 phase inductor motor means connecting the three-phase stator, with phases 120 degrees apart, to the supply. That winding set produces a revolving magnetic field. At the instant of switch-on the rotor is still. Slip is then 1, so the flux change in each rotor conductor is a maximum and the starting current is several times full-load current, often about 5 to 7 times on a Design B machine.
The rotor then behaves as a current carrying conductor in a revolving magnetic field. Force on the bars is in the field direction, so the rotor accelerates. The remaining speed difference is
Slip Speed = Ns – N
where Ns is the synchronous speed of the stator field and N is rotor speed.
The rotor therefore always runs slower than the field. If it ever reached the field speed, flux cutting would stop, emf in the conductor would fall and torque would collapse, so the machine would slow. An induction motor therefore runs below synchronous speed under load. The high starting current also causes large voltage drops on a weak supply.
Starters limit that first current so the supply voltage does not collapse. Common starters for three-phase induction motors include the star delta starter, Direct Online (DOL) starters and the auto transformer.
A DOL starter is a line contactor on each phase of the stator, with overload protection against overcurrent and no-volt release against a supply failure.
DOL is common on small motors, often quoted up to about 5 horsepower (HP) on a stiff local supply. Larger machines need a reduced-voltage method or a drive if the feeder cannot accept the inrush.
Star Delta Starter – The stator winding is first connected in star so that voltage at each phase is
where VL is line voltage. Phase voltage and line current at start are then about 1/√3 of the delta values, and starting torque falls to about one third. After the motor reaches a set speed, changeover contactors reconnect the winding in delta so line voltage equals phase voltage. Star-delta is a common reduced-voltage method for an induction motor above about 5 HP when six stator leads are available.
Auto Transformer – Starting current can also be cut by an auto transformer starter. The unit has a single winding with fixed taps, commonly about 50%, 65% or 80% of line voltage, switched by contactors for a short start interval. Motor starters use those taps, not a laboratory slider. After the motor has accelerated, the autotransformer is bypassed and full voltage is applied.
