2 AC MachineAlternating 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 machineAC 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 GeneratorThis 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
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.
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 MachineSynchronous 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 drivesApplication 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 polesfe 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 storedEither 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 MachineConstructionDC field windings are mounted on the (rotating) rotor - which is thus a rotating electromagnetAC windings are mounted on the (stationary) stator resulting in three-phase AC stator voltages and currentsThe main part in the synchronous machines arei) Rotorii) Stator
15 Synchronous Machine Rotor There are two types of rotors used in synchronous machines:i) cylindrical (or round) rotorsii) salient pole rotorsMachines 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 TurbineD » 1 mL » 10 mSteamd-axisq-axisStator winding High speed3600 r/min Þ 2-pole1800 r/min Þ 4-poleî Direct-conductor cooling (using hydrogen or water as coolant)î Rating up to 2000 MVANUniform air-gapStatorRotor windingRotorSTurbogenerator
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 rotorTurbineHydro (water)D » 10 mNon-uniform air-gapNSd-axisq-axisHydrogenerator
19 Synchronous Machine – Salient Pole StatorSalient-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.
24 Synchronous MachineMagnetomotive Forces (MMF’s) and Fluxes Due to Armature and Field WindingsFlux produced by a stator winding
25 Synchronous MachineMagnetomotive Forces (MMF’s) and Fluxes Due to Armature and Field Windings
26 Synchronous MachineMagnetomotive Forces (MMF’s) and Fluxes Due to Armature and Field WindingsTwo 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 generatorThe 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)
31 Synchronous Generator Power and TorqueIn generators, not all the mechanical power going into a synchronous generator becomes electric power out of the machineThe power losses in generator are represented by difference between output power and input power shown in power flow diagram below
32 Synchronous Generator LossesRotor- resistance; iron parts moving in a magnetic field causingcurrents 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 ringsStray 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 byThe real electric output power of the synchronous generator can be expressed in line and phase quantities asand 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
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 at0.8 power-factor lagging, and0.8 power-factor leading.