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ELECTRICAL MACHINES - III

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Type: NoteInstitute: Jawaharlal nehru technological university anantapur college of engineering Offline Downloads: 158Views: 2958Uploaded: 9 months agoAdd to Favourite

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UNIT-I SYNCHRONOUS MACHINES & CHARACTERISTICS Introduction Synchronous machines are principally used as alternating current generators. They supply the electric power used by all sectors of modern society. Synchronous machine is an important electromechanical energy converter. Synchronous generators usually operate in parallel forming a large power system supplying electrical power to consumers or loads. For these applications the synchronous generators are built in large units, their rating ranging from tens to hundreds of Megawatts. These synchronous machines can also be run as synchronous motors. Synchronous machines are AC machines that have a field circuit supplied by an external DC source. Synchronous machines are having two major parts namely stationary part stator and a rotating field system called rotor. In a synchronous generator, a DC current is applied to the rotor winding producing a rotor magnetic field. The rotor is then driven by external means producing a rotating magnetic field, which induces a 3-phase voltage within the stator winding. Field windings are the windings producing the main magnetic field (rotor windings for synchronous machines); armature windings are the windings where the main voltage is induced (stator windings for synchronous machines). Types of synchronous machines According to the arrangement of armature and field winding, the synchronous machines are classified as rotating armature type or rotating field type. In rotating armature type the armature winding is on the rotor and the field winding is on the stator. The generated emf or current is brought to the load via the slip rings. These type of generators are built only in small units. In case of rotating field type generators field windings are on the rotor and the armature windings are on the stator. Here the field current is supplied through a pair of slip rings and the induced emf or current is supplied to the load via the stationary terminals. Based on the type of the prime movers employed the synchronous generators are classified as 1. 2. 3. Hydrogenerators : The generators which are driven by hydraulic turbines are called hydrogenerators. These are run at lower speeds less than 1000 rpm. Turbogenerators: These are the generators driven by steam turbines. These generators are run at very high speed of 1500rpm or above. Engine driven Generators: These are driven by IC engines. These are run at aspeed less than 1500 rpm. Construction of synchronous machines 1. 2. Salient pole Machines: These type of machines have salient pole or projecting poles with concentrated field windings. This type of construction is for the machines which are driven by hydraulic turbines or Diesel engines. Nonsalient pole or Cylindrical rotor or Round rotor Machines: These machines are having cylindrical smooth rotor construction with distributed field winding in slots. This type of rotor construction is employed for the machine driven by steam turbines. Stator core: The stator is the outer stationary part of the machine, which consists of • • The outer cylindrical frame called yoke, which is made either of welded sheet steel, cast iron. The magnetic path, which comprises a set of slotted steel laminations called stator core pressed into the cylindrical space inside the outer frame. The magnetic path is laminated to reduce eddy currents, reducing losses and heating. CRGO laminations of 0.5 mm thickness are used to reduce the iron losses. A set of insulated electrical windings are placed inside the slots of the laminated stator. The cross-sectional area of these windings must be large enough for the power rating of the machine. For a 3-phase generator, 3 sets of 1
windings are required, one for each phase connected in star. Fig. 1 shows one stator lamination of a synchronous generator. In case of generators where the diameter is too large stator lamination can not be punched in on circular piece. In such cases the laminations are punched in segments. A number of segments are assembled together to form one circular laminations. All the laminations are insulated from each other by a thin layer of varnish. Figure 1.1. Non Salient pole generator Figure: 1.2. Salient pole generator. 2
Rotor of water wheel generator consists of salient poles. Poles are built with thin silicon steel laminations of 0.5mm to 0.8 mm thickness to reduce eddy current laminations. The laminations are clamped by heavy end plates and secured by studs or rivets. For low speed rotors poles have the bolted on construction for the machines with little higher peripheral speed poles have dove tailed construction as shown in Figs. Generally rectangular or round pole constructions are used for such type of alternators. However the round poles have the advantages over rectangular poles. Generators driven by water wheel turbines are of either horizontal or vertical shaft type. Generators with fairly higher speeds are built with horizontal shaft and the generators with higher power ratings and low speeds are built with vertical shaft design. Vertical shaft generators are of two types of designs (i) Umbrella type where in the bearing is mounted below the rotor. (ii) Suspended type where in the bearing is mounted above the rotor. In case of turbo alternator the rotors are manufactured form solid steel forging. The rotor is slotted to accommodate the field winding. Normally two third of the rotor periphery is slotted to accommodate the winding and the remaining one third unslotted portion acts as the pole. Rectangular slots with tapering teeth are milled in the rotor. Generally rectangular aluminum or copper strips are employed for filed windings. The field windings and the overhangs of the field windings are secured in place by steel retaining rings to protect against high centrifugal forces. Hard composition insulation materials are used in the slots which can with stand high forces, stresses and temperatures. Perfect balancing of the rotor is done for such type of rotors. Damper windings are provided in the pole faces of salient pole alternators. Damper windings are nothing but the copper or aluminium bars housed in the slots of the pole faces. The ends of the damper bars are short circuited at the ends by short circuiting rings similar to end rings as in the case of squirrel cage rotors. These damper windings are serving the function of providing mechanical balance; provide damping effect, reduce the effect of over voltages and damp out hunting in case of alternators. In case of synchronous motors they act as rotor bars and help in self starting of the motor. Relation between Speed and Frequency: In the previous course on induction motors it is established that the relation between speed and frequency and number of poles is given by Frequency f = P x N /120 Hz Windings in Alternators: In case of three phase alternators the following types of windings are employed. (i) Lap winding, (ii) wave winding and (iii) Mush winding. Based on pitch of the coil (i) full pitched (ii) short pitched windings Based on number of layers (i) Single layer (ii) Double layer Figure: 1.3. Single layer winding 3
Figure: 1.4. Double layer winding EMF Equation of an alternator: Consider the following Φ = flux per pole in wb P = Number of poles Ns = Synchronous speed in rpm f = frequency of induced emf in Hz Z = total number of stator conductors Zph = conductors per phase connected in series Tph = Number of turns per phase Assuming concentrated winding, considering one conductor placed in a slot According to Faradays Law electromagnetic induction, The average value of emf induced per conductor in one revolution eavg = d /dt eavg = Change of Flux in one revolution/ Time taken for one revolution Change of Flux in one revolution = p x Time taken for one revolution = 60/Ns seconds Hence eavg = (p x ) / ( 60/Ns) = p x x Ns / 60 We know f = PNs /120 hence PNs /60 = 2f Hence eavg = 2 f volts Hence average emf per turn = 2 x 2 f volts = 4 f volts If there are Tph, number of turns per phase connected in series, then average emf induced in T ph turns is Eph, avg = Tph x eavg = 4 f ø Tph volts Hence RMS value of emf induced E = 1.11 x Eph, avg = 1.11 x 4 f ø Tph volts = 4.44 f ø Tph volts This is the general emf equation for the machine having concentrated and full pitched winding. In practice, alternators will have short pitched winding and hence coil span will not be 180 0, but on or two slots short than the full pitch. Pitch Factor: Pitch factor Kp= emf induced in a short pitched coil/ emf induced in a full pitched coil = (2E cos α/2 )/ 2E Kp = cos α/2 where α is called chording angle. Distribution Factor: Even though we assumed concentrated winding in deriving emf equation, in practice an attempt is made to distribute the winding in all the slots coming under a pole. Such a winding is called distributed winding. In concentrated winding the emf induced in all the coil sides will be same in magnitude and in phase with each other. In case of distributed winding the magnitude of emf will be same but the emfs induced in each coil side will not be in phase with each other as they are distributed in the slots under a pole. Hence the total emf will not be same as that in concentrated winding but will be equal to the vector sum of the emfs induced. Hence it will be 4

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