Asynchronous Generators Source: ABB 1/21
2. Asynchronous Generators 1. Induction generator with squirrel cage rotor 2. Induction generator with woed rotor Source: electricaleasy.com 2/21
2.1. Induction generator with squirrel cage rotor Cut through a stator of a small standard induction machine with squirrel cage rotor Source: Wikipedia.org 3/21
2.1. Induction machine with squirrel cage rotor, repetition: Stator of an asynchronous machine equals the one of a synchronous machine Squirrel cage rotor is very simple and robust: only iron sheets and short circuit winding (equals damper winding of a synchronous machine) Short circuit winding of a squirrel cage induction machine 4/21
2.1. Induction machine with squirrel cage rotor, repetition: The synchronous speed depends on the number of pair of poles (same as synchronous machines, see 1.4) Induction machine runs er load not with synchronous speed, but with slip. The induced rotor voltage V R is direct proportional to the slip. n S = f S p s = n S n R n S V R = s V R0 Stator Winding with 1 and 2 pair of poles 5/21
2.1. Induction machine with squirrel cage rotor, repetition: Exercise 2.1.1: Calculate the synchronous speed of a four pole machine at grid frequency of 50 and of 60 Hz? Beispiel 3.2, Blatt 6/21
2.1. Induction machine with squirrel cage rotor, repetition: Exercise 2.1.2: A four pole induction machine with a nominal power of 75 kw and a nominal speed of 1400 rpm is operated at a 50 Hz grid. Calculate rotor frequency and voltage, if the rotor voltage at standstill would be 120V? Beispiel 3,3 Blatt 7/21
2.1. Induction machine (both types), voltage equations: T-equivalent circuit of an induction machine (comparable to a transformer); stator quantities 1; rotor quantities 2; Voltage equations (still same frequency in rotor and stator): U 1 = I 1 R 1 + jx 1σ + I μ jx 1h U 2 = I 2 R 2 + jx 2σ + I μ jx 1h Induced voltage (in stator reference frame): U 1h = I μ jx 1h 8/21
2.1. Induction machine (both types), voltage equations: As speed is variable, rotor frequency is different to grid frequency and changes with slip: T-equivalent circuit of an induction machine Rotor voltage equation : U 2 = I 2 R 2 + jω 2 L 2σ + I μ jω 2 L 1h Rotor voltage equation, expressed in stator frequency quantities: U 2 = I 2 R 2 + sjx 2σ + I μ sjx 1h Induced voltage (in rotor reference frame): U 2h = I μ sjx 1h Results into (in stator reference frame): U 2h = s ü U 1h 9/21
2.1. Induction machine with squirrel cage rotor, voltage equations: As speed is variable, rotor frequency is different to grid frequency and changes with slip: T-equivalent circuit of a squirrel cage induction machine (comparable to a transformer with shorted secondary winding) Rotor equation: 10/21
2.1. Induction machine with squirrel cage rotor, power: Grid power: same as synchronous generator, see 1.3. Physical interpretation of R 2/s : Power delivered to the rotor, air-gap power Differentiation between rotor losses and shaft power: T-equivalent circuit of a squirrel cage induction machine with copper losses (R 1 + R 2) and mechanical Power (R mech ) 11/21
2.1. Induction machine with squirrel cage rotor, power: Differentiation between rotor losses and shaft power: Power delivered to the rotor, air-gap power Mechanical power (equals shaft power if no mechanical losses are assumed) Rotor losses (copper only, no friction) => P Cu,2 = s P δ P mech = (1 s) P δ 12/21
2.1. Induction machine, both types, losses: Mechanical losses for friction (bearings & air) and cooling (ventilation) Copper losses in rotor (R 2 ) and stator (R 1 ) Iron losses (magnetizing), mainly in stator due to low rotor frequency In motor mode: P shaft 13/21
2.1. Induction machine, both types, generator operation: In generator mode, the main power flow moves from shaft to grid: The prime mover turns the rotor over-synchronous with P shaft The bearing and friction losses need to be deducted to achieve P mech, The mechanical power P mech is diverted into rotor losses and air-gap power The air-gap power minus iron and stator losses gives the power delivered to the grid η = P grid P shaft = P shaft P friction P copper P iron P shaft Important: The (squirrel cage) induction machine needs to get excited through the stator Thus the stator needs to be connected to the grid to get reactive inductive power 14/21
2.1. Induction machine with squirrel cage rotor, excercise: Exercise 2.1.3: Induction generator in bio gas plant: 500 kw gas engine output power, grid voltage 400V, 50 Hz, Generator operated at grid current 746.2 A, cos φ = 0.9, speed = 1550 rpm, Iron losses 420 W, friction losses 1500 W, stator resistance 0,01 Ω, Calculate the losses of the generator and the power delivered to the grid. Efficiency factor? Beispiel 3.2, 3,3 Blatt 15/21
2.1. Induction machine with squirrel cage rotor, Complete torque speed characteristic: 16/21
2.1. Induction machine with squirrel cage rotor, Generator operation: Squirrel cage machines are always excited from the grid: they consume inductive reactive power Thus they are not applicable for island operation, and not preferred by grid operators (for big units, as the grid needs controllable reactive power) BUT Much less expensive than synchronous generator No need of DC excitation systems Automatically synchronizes to the grid: simpler control, no synchronization system needed Very often directly connected to capacitor banks to compensate inductive reactive power For power generation purpose: only used for small hydro, wind and combustion engine (Diesel, Gas) application (up to some MW). 17/21
2.2. Induction generator with woed rotor 18/21
2.2. Induction generator with woed rotor Stator of an asynchronous machine equals the one of a synchronous machine Woed rotor with 3-phase windings and slip rings End windings and slip ring of a woed rotor 15 KW 19/21
2.2. Induction generator with woed rotor 3-phase electrical diagram of an asynchronous generator with woed rotor, slip-rings and external rotor resistance 20/21
2.2. Induction generator with woed rotor, general observations: At standstill, this machine can be used like a transformer with variable phase shift between primary and secondary winding by changing rotor angle In conventional applications, this machine operates like an induction machine (see 2.1.), but with variable rotor resistance R 2, as external resistors can be connected to the rotor. Thus the starting behavior can be optimized. For power generation means, this machine is not used without converter (see 3.4.) 21/21