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ALTERNATOR (Three phase synchronous generator)
Windings in Alternators: In case of three phase alternators the following types of windings areemployed.(i) Lap winding,(ii) wave winding and(iii) mush winding.Based on pitch of the coil(i) full pitched(ii) short pitched windingsBased on number of layers(i) Single layer(ii) Double layer
Short-pitched coils are deliberately used because of the following advantages:1. They save copper of end connections.2. They improve the wave-form of the generated e.m.f. i.e. the generated e.m.f. can be made to approximate to a sine wave more easily and the distorting harmonics can be reduced or totally eliminated.3. Due to elimination of high frequency harmonics, eddy current and hysteresis losses are reduced thereby increasing the efficiency.
But the disadvantage of using short-pitched coils is that the total voltage around the coils is somewhat reduced.
Hence the resultant coil emf is given by Ec = 2E1 cos /2 = 2E cos /2 volts.
• Hence the resultant emf in the short pitched coils is dependant on chording angle . Now the factor by which the emf induced in a short pitched coil gets reduced is called pitch factor
• Pitch factor Kp= emf induced in a short pitched coil/ emf induced in a full pitched coil
α =chording angle
• Distribution Factor: In the 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 less than that in the concentrated winding.
• Distribution factor Kd = emf induced in a distributed winding/ emf induced in a concentrated winding
• = vector sum of the emf/ arithmetic sum of the emf
• Let• E = emf induced per coil side• m = number of slots per pole per phase,• n = number of slots per pole• β= slot angle = 180/n• The emf induced in concentrated winding with
m slots per pole per phase = mE volts.
• In practical machines the windings will be generally short pitched and distributed over the periphery of
• the machine. Hence in deducing the emf equation both pitch factor and distribution factor has to be
• considered.• Hence the general emf equation including pitch
factor and distribution factor can be given as• EMF induced per phase = 4.44 f ɸ Tph x KpKd
volts• Eph = 4.44 KpKd f ɸ Tph vlolts
• Synchronization of alternator means connecting an alternator into grid in parallel with many other alternators, that is in a live system of constant voltage and constant frequency. Many alternators and loads are connected into a grid, and all the alternators in grid are having same output voltage and frequency (whatever may be the power). It is also said that the alternator is connected to infinite bus-bar.A stationary alternator is never connected to live bus-bars, because it will result in short circuit in the stator winding (since there is no generated emf yet). Before connecting an alternator into grid, following conditions must be satisfied:
• Equal voltage: The terminal voltage of incoming alternator must be equal to the bus-bar voltage.
• Similar frequency: The frequency of generated voltage must be equal to the frequency of the bus-bar voltage.
• Phase sequence: The phase sequence of the three phases of alternator must be similar to that of the grid or bus-bars.
• Phase angle: The phase angle between the generated voltage and the voltage of grid must be zero.
• In a 3 phase star connected alternator, there are 2 coil sides per slot and 16 turns per coil. The stator has 288 slots. When run at 250 rpm the line voltage is 6600 volts at 50 Hz. The coils are shot pitched by 2 slots. Calculate the flux per pole.
Solution• Distribution factor kd = (sin mβ/2) / (m sin β /2)= 0.9576• Hence pitch factor kp = cos /2 = cos 30/2 = 0.9659• Turns per phase = 1536• ɸ= 0.012 wb
