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AC Machine.

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Presentation on theme: "AC Machine."— Presentation transcript:

1 AC Machine

2 AC Machine Alternating current (ac) is the primary source of electrical energy. It is less expensive to produce and transmit than direct current. For this reason, and because ac voltage is induced into the armature of all generators, ac machines are generally more practical. May function as a generator (mechanical to electrical) or a motor (electrical to mechanical)

3 DC Machine & AC machine AC motor - no need rectification, so don't need split rings. DC motor - ends of the coil connect to a split ring to 'rectify' the emf produced

4 AC Motor As in the DC motor case,
a current is passed through the coil, generating a torque on the coil. Since the current is alternating, the motor will run smoothly only at the frequency of the sine wave.

5 AC Generator This process can be described in terms of Faraday's law when you see that the rotation of the coil continually changes the magnetic flux through the coil and therefore generates a voltage

6 Generator and Motor

7 How Does an Electric Generator Work?

8 Classification of AC Machines
Two major classes of machines; i) Synchronous Machines: • Synchronous Generators: A primary source of electrical energy. • Synchronous Motors: Used as motors as well as power factor compensators (synchronous condensers). ii) Asynchronous (Induction) Machines: • Induction Motors: Most widely used electrical motors in both domestic and industrial applications. • Induction Generators: Due to lack of a separate field excitation, these machines are rarely used as generators.

9 Synchronous Machine

10 Synchronous Machine Origin of name: syn = equal, chronos = time
Synchronous machines are called ‘synchronous’ because their mechanical shaft speed is directly related to the power system’s line frequency. the rotating air gap field and the rotor rotate at the same speed, called the synchronous speed. Synchronous machines are ac machine that have a field circuit supplied by an external dc source. DC field winding on the rotor, AC armature winding on the stator

11 Synchronous Machine Synchronous machines are used primarily as generators of electrical power, called synchronous generators or alternators. They are usually large machines generating electrical power at hydro, nuclear, or thermal power stations. Synchronous motors are built in large units compare to induction motors (Induction motors are cheaper for smaller ratings) and used for constant speed industrial drives Application as a motor: pumps in generating stations, electric clocks, timers, and so forth where constant speed is desired.

12 Synchronous Machine fe is the power line frequency.
The frequency of the induced voltage is related to the rotor speed by: where P is the number of magnetic poles fe is the power line frequency. Typical machines have two-poles, four-poles, and six-poles

13 Synchronous Machine Construction Energy is stored in the inductance
As the rotor moves, there is a change in the energy stored Either energy is extracted from the magnetic field (and becomes mechanical energy – motor) Or energy is stored in the magnetic field and eventually flows into the electrical circuit that powers the stator – generator

14 Synchronous Machine Construction DC field windings are mounted on the (rotating) rotor - which is thus a rotating electromagnet AC windings are mounted on the (stationary) stator resulting in three-phase AC stator voltages and currents The main part in the synchronous machines are i) Rotor ii) Stator

15 Synchronous Machine Rotor
There are two types of rotors used in synchronous machines: i) cylindrical (or round) rotors ii) salient pole rotors Machines with cylindrical rotors are typically found in higher speed higher power applications such as turbogenerators. Using 2 or 4 poles, these machines rotate at 3600 or 1800 rpm (with 60hz systems).  Salient pole machines are typically found in large (many MW), low mechanical speed applications, including hydrogenerators, or smaller higher speed machines (up to 1-2 MW). Salient pole rotors are less expensive than round rotors.

16 Synchronous Machine – Cylindrical rotor
Turbine D » 1 m L » 10 m Steam d-axis q-axis Stator winding     High speed 3600 r/min Þ 2-pole 1800 r/min Þ 4-pole î    Direct-conductor cooling (using hydrogen or water as coolant) î   Rating up to 2000 MVA N Uniform air-gap Stator Rotor winding Rotor S Turbogenerator

17 Synchronous Machine – Cylindrical rotor
Stator Cylindrical rotor

18 Synchronous Machine – Salient Pole
Most hydraulic turbines have to turn at low speeds (between 50 and 300 r/min) A large number of poles are required on the rotor Turbine Hydro (water) D » 10 m Non-uniform air-gap N S d-axis q-axis Hydrogenerator

19 Synchronous Machine – Salient Pole
Stator Salient-pole rotor

20 Synchronous machine rotors are simply rotating electromagnets built to have as many poles as are produced by the stator windings. Dc currents flowing in the field coils surrounding each pole magnetize the rotor poles. The magnetic field produced by the rotor poles locks in with a rotating stator field, so that the shaft and the stator field rotate in synchronism. Salient poles are too weak mechanically and develop too much wind resistance and noise to be used in large, high-speed generators driven by steam or gas turbines. For these big machines, the rotor must be a solid, cylindrical steel forging to provide the necessary strength.

21 Axial slots are cut in the surface of the cylinder to accommodate the field windings.
Since the rotor poles have constant polarity they must be supplied with direct current. This current may be provided by an external dc generator or by a rectifier. In this case the leads from the field winding are connected to insulated rings mounted concentrically on the shaft. Stationary contacts called brushes ride on these slip rings to carry current to the rotating field windings from the dc supply. The brushes are made of a carbon compound to provide a good contact with low mechanical friction. An external dc generator used to provide current is called a “ brushless exciter “.

22 Synchronous Machine Stator
The stator of a synchronous machine carries the armature or load winding which is a three-phase winding. The armature winding is formed by interconnecting various conductors in slots spread over the periphery of the machine’s stator. Often, more than one independent three phase winding is on the stator. An arrangement of a three-phase stator winding is shown in Figure below. Notice that the windings of the three-phases are displaced from each other in space.

23 Synchronous Machine Construction Stator

24 Synchronous Machine Magnetomotive Forces (MMF’s) and Fluxes Due to Armature and Field Windings Flux produced by a stator winding

25 Synchronous Machine Magnetomotive Forces (MMF’s) and Fluxes Due to Armature and Field Windings

26 Synchronous Machine Magnetomotive Forces (MMF’s) and Fluxes Due to Armature and Field Windings Two Cycles of mmf around the Stator

27 Synchronous Generator
Equivalent circuit model – synchronous generator

28 If the generator operates at a terminal voltage VT while supplying a load corresponding to an armature current Ia, then; In an actual synchronous machine, the reactance is much greater than the armature resistance, in which case; Among the steady-state characteristics of a synchronous generator, its voltage regulation and power-angle characteristics are the most important ones. As for transformers, the voltage regulation of a synchronous generator is defined at a given load as;

29 Synchronous Generator
Phasor diagram of a synchronous generator The phasor diagram is to shows the relationship among the voltages within a phase (Eφ,Vφ, jXSIA and RAIA) and the current IA in the phase. Unity P.F (1.0)

30 Synchronous Generator
Lagging P.F Leading P.F.

31 Synchronous Generator
Power and Torque In generators, not all the mechanical power going into a synchronous generator becomes electric power out of the machine The power losses in generator are represented by difference between output power and input power shown in power flow diagram below

32 Synchronous Generator
Losses Rotor - resistance; iron parts moving in a magnetic field causing currents to be generated in the rotor body - resistance of connections to the rotor (slip rings) Stator - resistance; magnetic losses (e.g., hysteresis) Mechanical - friction at bearings, friction at slip rings Stray load losses - due to non-uniform current distribution

33 Synchronous Generator
The input mechanical power is the shaft power in the generator given by equation: The power converted from mechanical to electrical form internally is given by The real electric output power of the synchronous generator can be expressed in line and phase quantities as and reactive output power

34 Synchronous Generator
In real synchronous machines of any size, the armature resistance RA is more than 10 times smaller than the synchronous reactance XS (Xs >> RA). Therefore, RA can be ignored

35 Synchronous Motor

36 Synchronous Motor Power Flow

37 Example : Synchronous Generator.
A three-phase, wye-connected 2500 kVA and 6.6 kV generator operates at full-load. The per-phase armature resistance Ra and the synchronous reactance, Xd, are (0.07+j10.4). Calculate the percent voltage regulation at 0.8 power-factor lagging, and 0.8 power-factor leading.

38 Solution.

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