Synchronous Machines

Synchronous machines come in a variety of different constructions and designs. The differences occur in the physical outlay of the rotor and in the way in which excitation flux is provided (if it is provided at all) in the machine. Regardless of the type however, all the synchronous machines have the same construction of the stator. Stator is of cylindrical crosssection, manufactured from laminated sheets of steel, and it carries a three-phase winding that is supplied with (in the motor case) or that produces (in the generator case) a system of threephase voltages....
Nội dung trích xuất từ tài liệu:
Synchronous MachinesENGNG 2024 Electrical Engineering SYNCHRONOUS MACHINES1. INTRODUCTION Synchronous machines come in a variety of different constructions and designs. Thedifferences occur in the physical outlay of the rotor and in the way in which excitation flux isprovided (if it is provided at all) in the machine. Regardless of the type however, all thesynchronous machines have the same construction of the stator. Stator is of cylindrical cross-section, manufactured from laminated sheets of steel, and it carries a three-phase winding thatis supplied with (in the motor case) or that produces (in the generator case) a system of three-phase voltages. The three windings that constitute the stator three-phase winding are displacedin space by 120 degrees around the circumference of the machine. Three-phase voltages havethe phase displacement of 120 degrees. Synchronous machines are the main work-horse of the electricity generation industry.They are used as generators in all the hydro, nuclear, coal-fired, gas-fired and oil-fired powerplants. This means that a synchronous generator is a standard machine used for conversion ofmechanical energy into electric energy in all the power plants that rely on conventional energysources. Rated powers of synchronous generators are typically from a few megawatts up to afew tens of megawatts, or even a several hundreds of megawatts. Synchronous machines areused as motors as well. In this case rated power of a synchronous motor is either relativelyvery low (up to few kilowatts) or is in the high power region, from around 150 kW to 15 MW.In between, induction motors are used as a rule, due to their numerous advantages in thispower region. The two types of synchronous machines that are relevant for the discussion that followsare: 1. Machines with uniform air-gap and excitation winding on rotor. 2. Machines with non-uniform air-gap and excitation winding on rotor. Rotor cross-section in these two types differs and leads to different mechanisms oftorque production. Both machine types are used for both generating and motoring application.In the machine with uniform air-gap torque is produced solely due to interaction between therotor and stator windings (fundamental torque component). In the machine with non-uniformair-gap, there are two torque components: fundamental torque, produced by the interactionbetween the stator and rotor windings, and reluctance torque component that is theconsequence of the non-cylindrical rotor cross-section. Since the rotor is not cylindrical, statorwindings see an air-gap of variable length as rotor rotates. Consequently, magnetic reluctanceis variable and all the inductances of the stator windings are functions of the rotor position.Reluctance torque component is therefore produced, in addition to the fundamental torquecomponent.Synchronous machine with uniform air-gap and an excitation winding on rotor: Rotor is of cylindrical construction, with an excitation winding placed on rotor. Theexcitation winding is supplied with a controllable DC current. Since the rotor rotates, this DC E Levi, 2000 1ENGNG 2024 Electrical Engineeringcurrent has to be supplied somehow from the stationary outside world. In the past, slip ringsand brushes were used to connect a stationary DC source to the rotating rotor winding.Nowadays, so-called brushless excitation systems (or just brushless exciters) are used (theseare beyond the scope of interest here). Synchronous machines with uniform air-gap andexcitation winding on rotor are often termed turbo-machines. They are used both for motoringapplication and for generation in coal-, oil-, and gas-fired plants and nuclear plants. Typicalnumber of magnetic pole pairs is one or two, so that the so-called synchronous speed ofrotation is 3000 rpm or 1500 rpm for 50 Hz stator frequency. Synchronous speed is defined as n s = 60 f / P ω s = 2πf / P (1)and this is the so-called mechanical synchronous speed of rotation (that is, actual speed ofrotation). Symbol P stands for the number of magnetic pole pairs (each pole pair consists oftwo magnetic poles, one north and one south). Rotor of a turbo-machine carries, apart from the excitation winding, a short-circuitedmulti-phase winding similar to the squirrel-cage winding of an induction motor. The existenceof this winding is crucial for motoring applications since, without it, the machine would not becapable of starting when connected to a three-phase supply on stator side. It is importanthowever to emphasise that this winding has no impact on steady-state operation, since nocurrents exist in the short-circuited rotor winding (often called damper winding) in steady-state. The damper winding is important in generation as well, since it has an important roleduring transients. Once again, however, it has no impact on steady-state operation and istherefore omitted from the schematic representation of a synchronous turbo-machine in Fig. 1,where only excitation winding is shown on rotor. Stator three phase winding is shown in Fig. 1with three concentric turns. It is important to stress once more that an electromagnetic torque in a synchronousmachine with uniform air-gap is produced purely due to the interaction between the statorcurrents and the rotor flux (that stems from the DC current in the rotor winding). The torqueconsist therefore entirely of the fundamental torque component, very much the same as thecase is with other uniform air-gap machines (DC machines, induction machines). According tothe condition of average torque existence, zero frequency in rotor means that the stator ...