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Synchronous Machines - puneet000888 - 10-04-2017 Synchronous Machines Synchronous generators or alternators are used to convert mechanical power derived from steam, gas, or hydraulic-turbine to ac electric power Synchronous generators are the primary source of electrical energy we consume today Large ac power networks rely almost exclusively on synchronous generators 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 Operation Principle The rotor of the generator is driven by a prime-mover A dc current is flowing in the rotor winding which produces a rotating magnetic field within the machine The rotating magnetic field induces a three-phase voltage in the stator winding of the generator Equivalent Circuit_1 The internal voltage Ef produced in a machine is not usually the voltage that appears at the terminals of the generator. The only time Ef is same as the output voltage of a phase is when there is no armature current flowing in the machine. There are a number of factors that cause the difference between Ef and Vt: The distortion of the air-gap magnetic field by the current flowing in the stator, called the armature reaction The self-inductance of the armature coils. The resistance of the armature coils. The effect of salient-pole rotor shapes. Three-phase equivalent circuit of a cylindrical-rotor synchronous machine The voltages and currents of the three phases are 120o apart in angle, but otherwise the three phases are identical. Determination of the parameters of the equivalent circuit from test data The equivalent circuit of a synchronous generator that has been derived contains three quantities that must be determined in order to completely describe the behaviour of a real synchronous generator: The saturation characteristic: relationship between If and f (and therefore between If and Ef) The synchronous reactance, Xs The armature resistance, Ra The above three quantities could be determined by performing the following three tests: Open-circuit test Short-circuit test DC test Open-circuit test The generator is turned at the rated speed The terminals are disconnected from all loads, and the field current is set to zero. Then the field current is gradually increased in steps, and the terminal voltage is measured at each step along the way. It is thus possible to obtain an open-circuit characteristic of a generator (Ef or Vt versus If) from this information Short-circuit test Adjust the field current to zero and short-circuit the terminals of the generator through a set of ammeters. Record the armature current Isc as the field current is increased. Such a plot is called short-circuit characteristic. DC Test The purpose of the DC test is to determine Ra. A variable DC voltage source is connected between two stator terminals. The DC source is adjusted to provide approximately rated stator current, and the resistance between the two stator leads is determined from the voltmeter and ammeter readings Determination of Xs For a particular field current IfA, the internal voltage Ef (=VA) could be found from the occ and the short-circuit current flow Isc,A could be found from the scc. Then the synchronous reactance Xs could be obtained using Short-circuit Ratio Another parameter used to describe synchronous generators is the short-circuit ratio (SCR). The SCR of a generator defined as the ratio of the field current required for the rated voltage at open circuit to the field current required for the rated armature current at short circuit. SCR is just the reciprocal of the per unit value of the saturated synchronous reactance calculated by Parallel operation of synchronous generators There are several major advantages to operate generators in parallel: Several generators can supply a bigger load than one machine by itself. Having many generators increases the reliability of the power system. It allows one or more generators to be removed for shutdown or preventive maintenance. Synchronous Machines - Electronic Chip - 10-04-2017 Synchronous Machines [attachment=17355] introduction The synchronous machine is an important electromechanical energy converter. Synchronous generators usually operate together (or in parallel), forming a large power system supplying electrical energy to the loads or consumers. For these applications synchronous machines are built in large units, their rating ranging from tens to hundreds of megawatts. For high-speed machines, the prime movers are usually steam turbines employing fossil or nuclear energy resources. Low-speed machines are often driven by hydro-turbines that employ water power for generation. Smaller synchronous machines are sometimes used for private generation and as standby units, with diesel engines or gas turbines as prime movers. Synchronous machines can also be used as motors, but they are usually built in very large sizes. The synchronous motor operates at a precise synchronous speed, and hence is a constant-speed motor. Unlike the induction motor, whose operation always involves a lagging power factor, the synchronous motor possesses a variable-power-factor characteristic, and hence is suitable for power-factor correction applications. Encyclopedia of Life Support Systems (EOLSS) UNESCO - EOLSS SAMPLE CHAPTER ELECTRICAL ENGINEERING Synchronous Machines - Tze-Fun Chan A synchronous motor operating without mechanical load is called a compensator. It behaves as a variable capacitor when the field is overexcited, and as a variable inductor when the field is underexcited. It is often used in critical positions in a power system for reactive power control. 2. Types of Synchronous Machine According to the arrangement of the field and armature windings, synchronous machines may be classified as rotating-armature type or rotating-field type. 2.1. Rotating-Armature Type The armature winding is on the rotor and the field system is on the stator. The generated current is brought out to the load via three (or four) slip-rings. Insulation problems, and the difficulty involved in transmitting large currents via the brushes, limit the maximum power output and the generated electromagnetic field (emf). This type is only used in small units, and its main application is as the main exciter in large alternators with brushless excitation systems. 2.2. Rotating-Field Type The armature winding is on the stator and the field system is on the rotor. Field current is supplied from the exciter via two slip-rings, while the armature current is directly supplied to the load. This type is employed universally since very high power can be delivered. Unless otherwise stated, the subsequent discussion refers specifically to rotating-field type synchronous machines. According to the shape of the field, synchronous machines may be classified as cylindrical-rotor (non-salient pole) machines (Figure 1) and salient-pole machines (Figure 2). The cylindrical-rotor construction is used in generators that operate at high speeds, such as steam-turbine generators (usually two-pole machines). This type of machine usually has a small diameter-to-length ratio, in order to avoid excessive mechanical stress on the rotor due to the large centrifugal forces. Synchronous Machines - paul - 10-04-2017 [attachment=15005] Synchronous Machines Synchronous generators or alternators are used to convert mechanical power derived from steam, gas, or hydraulic-turbine to ac electric power Synchronous generators are the primary source of electrical energy we consume today Large ac power networks rely almost exclusively on synchronous generators 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 Construction Various Types Salient-Pole Synchronous Generator Cylindrical-Rotor Synchronous Generator Operation Principle The rotor of the generator is driven by a prime-mover A dc current is flowing in the rotor winding which produces a rotating magnetic field within the machine The rotating magnetic field induces a three-phase voltage in the stator winding of the generator Electrical Frequency Generated Voltage Equivalent Circuit_1 The internal voltage Ef produced in a machine is not usually the voltage that appears at the terminals of the generator. The only time Ef is same as the output voltage of a phase is when there is no armature current flowing in the machine. There are a number of factors that cause the difference between Ef and Vt: The distortion of the air-gap magnetic field by the current flowing in the stator, called the armature reaction The self-inductance of the armature coils. The resistance of the armature coils. The effect of salient-pole rotor shapes. |