15-01-2013, 02:22 PM
Induction Generators
[attachment=46982]
Same basic construction as squirrel-cage induction motors
Drive at a speed greater than the synchronous speed
Not started as a motor
Operated by wind turbines, steam turbines, etc.
Operation as an Induction-Generator continued
When the speed approaches synchronous speed, the slip = 0, Rs/s becomes infinite, rotor current Ir = 0, and no motor torque is developed. (The motor is neither a motor or a generator – it is “floating” on the bus. The only stator current is the exciting current to supply the rotating magnetic field and the iron losses.
The speed of the rotating flux is independent of the rotor speed – only a function of the number of poles and the frequency of the applied voltage. Increasing the rotor speed above the synchronous speed causes the slip [(ns – nr)/ns] to become negative! The gap power, Pgap = Prcl/s becomes negative, now supplying power to the system!
Developed torque and line current vs. rotor speed
The interaction of the magnetic flux of the stator and the magnetic flux of the rotor produce a “countertorque” that opposes the driving torque of the prime mover. Increasing the speed of the rotor increases the countertorque and the power delivered to the system by the generator. The maximum value of the countertorque is called the “pushover” torque.
[attachment=46982]
Same basic construction as squirrel-cage induction motors
Drive at a speed greater than the synchronous speed
Not started as a motor
Operated by wind turbines, steam turbines, etc.
Operation as an Induction-Generator continued
When the speed approaches synchronous speed, the slip = 0, Rs/s becomes infinite, rotor current Ir = 0, and no motor torque is developed. (The motor is neither a motor or a generator – it is “floating” on the bus. The only stator current is the exciting current to supply the rotating magnetic field and the iron losses.
The speed of the rotating flux is independent of the rotor speed – only a function of the number of poles and the frequency of the applied voltage. Increasing the rotor speed above the synchronous speed causes the slip [(ns – nr)/ns] to become negative! The gap power, Pgap = Prcl/s becomes negative, now supplying power to the system!
Developed torque and line current vs. rotor speed
The interaction of the magnetic flux of the stator and the magnetic flux of the rotor produce a “countertorque” that opposes the driving torque of the prime mover. Increasing the speed of the rotor increases the countertorque and the power delivered to the system by the generator. The maximum value of the countertorque is called the “pushover” torque.