How Induction motor start ??

The simplest way to start a three-phase induction motor is to connect its terminals to the line. This method is often called "direct on line" and abbreviated DOL.

In an induction motor, the magnitude of the induced emf in the rotor circuit is proportional to the stator field and the slip speed (the difference between synchronous and rotor speeds) of the motor, and the rotor current depends on this emf. When the motor is started, the rotor speed is zero. The synchronous speed is constant, based on the frequency of the supplied AC voltage. So the slip speed is equal to the synchronous speed, the slip ratio is 1, and the induced emf in the rotor is large. As a result, a very high current flows through the rotor. This is similar to a transformer with the secondary coil short circuited, which causes the primary coil to draw a high current from the mains.

When an induction motor starts DOL, a very high current is drawn by the stator, in the order of 5 to 9 times the full load current. This high current can, in some motors, damage the windings; in addition, because it causes heavy line voltage drop, other appliances connected to the same line may be affected by the voltage fluctuation. To avoid such effects, several other strategies are employed for starting motors.
[edit] Star-delta starters

An induction motor's windings can be connected to a 3-phase AC line in two different ways:

* wye (star in Europe), where the windings are connected from phases of the supply to the neutral;
* delta (sometimes mesh in Europe), where the windings are connected between phases of the supply.

A delta connection of the machine winding results in a higher voltage at each winding compared to a wye connection (the factor is \sqrt{3}). A star-delta starter initially connects the motor in wye, which produces a lower starting current than delta, then switches to delta when the motor has reached a set speed. Disadvantages of this method over DOL starting are:

* Lower starting torque, which may be a serious issue with pumps or any devices with significant breakaway torque
* Increased complexity, as more contactors and some sort of speed switch or timers are needed
* Two shocks to the motor (one for the initial start and another when the motor switches from wye to delta)

[edit] Variable-frequency drives

Variable-frequency drives (VFD) can be of considerable use in starting as well as running motors. A VFD can easily start a motor at a lower frequency than the AC line, as well as a lower voltage, so that the motor starts with full rated torque and with no inrush of current. The rotor circuit's impedance increases with slip frequency, which is equal to supply frequency for a stationary rotor, so running at a lower frequency actually increases torque.
[edit] Resistance starters
A resistance starter and its 4MW / 11kV induction motor, driving a ball mill.

This method is used with slip ring motors where the rotor poles can be accessed byway of the slip rings. Using brushes, variable power resistors are connected in series with the poles. During start-up the resistance is large and then reduced to zero at full speed.

At start-up the resistance results in the stator's field strength being weakened less. As a result, the inrush current is reduced. Another important advantage is higher start-up torque. As well, the resistors generate a phase shift in the field resulting in the magnetic force acting on the rotor having a favorable angle[citation needed].
[edit] Autotransformer starters

such starters are called as auto starters or compensators, consists of an auto-transformer.
[edit] Series Reactor starters

In series reactor starter technology, an impedance in the form of a reactor is introduced in series with the motor terminals, which as a result reduces the motor terminal voltage resulting in a reduction of the starting current; the impedance of the reactor, a function of the current passing through it, gradually reduces as the motor accelerates, and at 95 % speed the reactors are bypassed by a suitable bypass method which enables the motor to run at full voltage and full speed. Air core series reactor starters or a series reactor soft starter is the most common and recommended method for fixed speed motor starting. The applicable standards are [IEC 289] AND [IS 5553 (PART 3) ]
[edit] Single Phase

In a single phase induction motor, it is necessary to provide a starting circuit to start rotation of the rotor. If this is not done, rotation may be commenced by manually giving a slight turn to the rotor. The single phase induction motor may rotate in either direction and it is only the starting circuit which determines rotational direction.