Generator Action:
If an induction motor is forced to run at speeds in excess of the synchronous speed, the load torque exceeds the machine torque and the slip is negative, reversing the rotor induced EMF and rotor current. In this situation the machine will act as a generator with energy being returned to the supply. If the AC supply voltage to the stator excitation is simply removed, no generation is possible because there can be no induced current in the rotor.Regenerative braking:
Thus in traction applications, regenerative braking is not possible below synchronous speed in a machine fed with a fixed frequency supply. If however the motor is fed by a variable frequency inverter then regenerative braking is possible by reducing the supply frequency so that the synchronous speed becomes less than the motor speed. AC motors can be microprocessor controlled to a fine degree and can regenerate current down to almost a stop whereas DC regeneration fades quickly at low speeds.

Dynamic Braking:
Induction motors can be brought rapidly to a stop (and / or reversed) by reversing one pair of leads which has the effect of reversing the rotating wave. This is known as "plugging". The motor can also be stopped quickly by cutting the AC supply and feeding the stator windings instead with a DC (zero frequency) supply. With both of these methods, energy is not returned to the supply but is dissipated as heat in the motor. These techniques are known as dynamic braking. Starting Three phase induction motors and some synchronous motors are not self starting but design modifications such as auxiliary or "damper" windings on the rotor are incorporated to overcome this problem. Usually an induction motor draws 5 to 7 times its rated current during starting before the speed builds up and the current is modified by the back EMF. In wound rotor motors the starting current can be limited by increasing the resistance in series with the rotor windings.In squirrel cage designs, electronic control systems are used to control the current to prevent damage to the motor or to its power supply. Even with current control the motor can still overheat because, although the current can be limited, the speed build up is slower and the inrush current, though reduced, is maintained for a longer period.

Power Factor:
The current drawn by an induction motor has two components, the current in phase with the voltage which governs the power transfer to the load and the inductive component, representing the magnetising current in the magnetic circuit, which lags 90° behind the load current. The power factor is defined as cosF where F is the net lag of the current behind the applied voltage due to the in phase and out of phase current components. The net power delivered to the load is VAcosF where V is the applied voltage, A is the current which flows. Various methods of power factor correction are used to reduce the current lag in order to avoid losses due to poor power factor. The simplest is to connect a capacitor of suitable size across the motor terminals. Since the current through a capacitor leads the voltage, the effect of the capacitor is to counter-balance the inductive element in the motor canceling out the current lag. Power factor correction can also be accomplished in the motor controller. Characteristics:
One of the major advantages of the induction motor is that it does not need a commutator. Induction motors are therefore simple, robust, reliable, maintenance free and relatively low cost.They are normally constant speed devices whose speed is proportional to the mains frequency.Variable speed motors are also possible by using motor controllers which provide a variable frequency output.

Applications:
Three phase induction motors are used wherever the application depends on AC power from the national grid. Because they don't need commutators they are particularly suitable for high power applications. They are available with power handling capacities ranging from a few Watts to more than 10 MegaWatts. They are mainly used for heavy industrial applications and for machine tools. The availability of solid state inverters in recent years means that induction motors can now be run from a DC source. They are now finding use in automotive applications for electric and hybrid electric vehicles. Nevertheless, the induction motor is ill-suited for most automotive applications because of the difficulties associated with extracting heat from the rotor, efficiency problems over wide speed and power ranges, and a more expensive manufacturing process due to distributed windings. Permanent magnet and reluctance motors offer better solutions for these applications. Wound Rotor:
Induction MotorNow of historic interest only, these motors were designed to permit control of the speed - torque characteristics of the machine. They used conventional windings on the rotor which were accessible through slip rings. The rotor windings were not connected to the supply line but current through the windings could be controlled by external rheostats connected in series with the windings. Modern electronic controls have made these designs obsolete.