Ac Generators or Alternators

Basic Principle
       
       Ac generators or alternators operate on the principles of electromagnetic induction.
There is one important difference between A.C generator and D.C generator the armature rotates and the field system is stationary where as in A.C generator it is just the reverse of it.In their case standard construction consists of armature winding mounted on a stationary element called stator and field winding on a rotating element called rotor.
Speed and Frequency
  
       In an alternator there exists a definite relationship between the rotational speed (N) of the rotor the frequency (f) of the generated e.m.f and the number of poles P.

P= 120f / N

       It is clear from the above equation that because of slow relative speeds of engine driven alternators their number of poles is must greater as compared to that of the turbo generators which run at very high speeds.
Alternator on Load

       as the load on an alternator is varied its terminal voltage is also found to vary.This variation in terminal voltage V is due to the following reasons.
  1. Voltage drop due to armature resistance Ra.
  2. Voltage drop due to armature leakage reactant XL.
  3. Voltage drop due to armature reaction.
Voltage Regulation

       With change in load there is a change in terminal voltage of an alternator . The magnitude of this change depends not only on the load but also on the load power factor.
        The voltage regulation of an alternator as defined as " the rise in voltage when full load is removed divided by the rated terminal voltage".
Parallel Operation of Alternators

        The operation of connecting an alternator in parallel with another alternator or with common bus bar is known as synchronizing. 
Generally alternators are used in a power system where they are in parallel with many other alternators .It means that the alternator is connected to a live system of constant voltage and constant frequency.Following three conditions must be satisfied for connected alternators in parallel.
  1. The terminal voltage of the incoming alternator must be the same as bus bar voltage.
  2. The speed of the incoming machine must be such that its frequency equals to bus bar frequency.
  3. The phase of the alternator voltage must be identical with the phase of the bus-bar voltage.
          It is never advisable to connect a stationary alternator to live bus-bar because stator induced e.m.f being zero a short circuit will results.
Factors Effecting the generator performance

Over load 

        If an alternator is operated above its designed base continuous rating the operating temperature will increases to a level that will degrade and so shorten the life of the wound components of the insulation system.
Effects of Harmonics on generator

        Generators and motors are adversely affected by harmonics in the networks to which they are connected.
  1. Increased heating due to iron and copper losses at the harmonic frequencies.
  2. Higher audible noise emission as compared with sinusoidal excitation.
  3. Harmonic current in the rotor.
Unbalance load to generator

        In balance and unbalance load condition the value of generator vibration is tend to fluctuate.If the load of the generator increased the generator vibration value increased by 20% in each balance and unbalance conditions.
Losses in generator
 Electrical Losses.
  • Stator winding loss
  • Rotor winding loss
  • Rectifier diode voltage drop loss
  • Regulator out put device loss
  • Brush Drop 
Magnetic Losses.
  • Eddy current loss
  • Hysteresis loss
  • Stray loss
Mechanical Losses.
  • Bearing friction loss
  • Wind-age loss

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