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Synchronous motors

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Synchronous motors

  1. 1. •A synchronous machine is a most important type of electric machine. •Synchronous Machine used at generating stations are known as Synchronous Generators or Alternators •Synchronous motors are widely used in Industries and are well known for their const. speed operation •Electromechanical energy conversion occurs whenever a change in flux is associated with mechanical motion.
  2. 2. It consists of: Stator Rotor Field Windings (On rotor) Armature Windings (On Stator)
  3. 3. It is a stationary member It is the cylindrical portion inside which the rotor rotates An air gap is provided between the stator and the rotor Armature winding are 3 phased and are housed in the slots cut in the stator It consists of cast iron stator frame, a cylindrical laminated , a cylindrical laminated and uniformly slotted core.
  4. 4. Rotor is the rotating part of the machine Can be classified as: (a) Cylindrical Rotor and (b) Salient Pole rotor Large salient-pole rotors are made of laminated poles retaining the winding under the pole head.
  5. 5. Armature windings connected are 3-phase and are either star or delta connected The windings are 120 degrees apart and normally use distributed windings
  6. 6. The 3-phase armature winding is distributed in the slots along the armature air gap periphery For example: Consider that we have 18 slots, 2-pole 3- phase winding.. Hence we have 9 slots/per pole as shown figure..
  7. 7. The winding diagram of phase ‘a’ can be shown as: Similarly, phase ‘b’ and phase ‘c’ are distributed in same manner
  8. 8. This implies that per phase emf is getting divided in each phase. When all the 3-phase are connected then mmf-phase graph for each phase is displaced by 120 degrees The mmf-phase graph can be drawn as:
  9. 9. The field winding of a synchronous machine is always energized with direct current Under steady state condition, the field or exciting current is given Ir = Vf/Rf Vf = Direct voltage applied to the field winding Rf= Field winding Resistance
  10. 10. Mostly all the synchronous machines use Distributed winding Attempt is made to use all the slots available under a pole for the winding which makes the nature of the induced e.m.f. moe sinusoidal Consider a sync. Machine with 3-phase winding, Slots, s= 18 slots Poles=2 slots per pole, g= s/p= 9 slots/pole/phase= g/3 = 3
  11. 11. • Let E = Induced e.m.f. per coil and there are 3 coils per phase •In concentrated type all the coil sides will be placed in one slot under a pole. So induced e.m.f. in all the coils will achieve maxima and minima at the same time i.e. all of them will be in phase. Hence resultant e.m.f. after connecting coils in series will be algebraic sum of all the e.m.f.s. as all are in phase
  12. 12. As against this, in distributed type, coil sides will be distributed, one each in the 3 slots per phase available under a pole as shown in the Fig. Slot pitch , Y= ԓP/s = 20 deg.
  13. 13. In general, let there be Slots per pole = n slots/pole/phase= m Let turns/phase= Nph Turns per slot, N1 = Nph/ q Induce emf/turn =E1 = Ec / N Induced emf/ slot= E = E1 * N1 = Ec * Nph Nph q = Ec q
  14. 14. q – slots for each phase under one pole Since all the coils are connected in series, hence the resultant emf is phasor sum of all the emfs

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