Operating Characteristics of DC Generators

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The operating characteristics represent the graphical relationships between different basic quantities related to operation of generator. These quantities include the generator terminal voltage V_t, generated emf E_g, armature current I_a, field current I_f, line current I_L and speed N.

There three important characteristics of DC generator as given under.

  1. Magnetization
characteristic

      a. No-load magnetization characteristic
      b. Load magnetization characteristic
  2. Internal characteristic
  3. External characteristic

Magnetization characteristic

No-load magnetization characteristic (E_g Vs I_f)

This characteristics gives the relationship of the field or excitation current with generated voltage E_g in armature (i.e. E_g=f(I_f)) at no load and constant speed N. The shape of the characteristics is generally the same for all types of DC generators. It is also known as saturation curve, open circuit characteristics (OCC), or simply magnetization characteristic.

Load Magnetization characteristic (V_t Vs I_f)

This characteristics gives the relationship of the field or excitation current with the terminal voltage V_t of generator (i.e. V_t=f(I_f)) when the generator is loaded and running at constant speed N. This curve is also called as load saturation curve or load curve.

Internal characteristic (E Vs I_a)

The internal characteristic give the relation between the emf induced in armature E after allowing for armature reaction and the armature current I_a.

External characteristic (V_t Vs I_L)

This characteristics gives the relationship between line current (or load current) with and terminal voltage V_t of generator at constant speed N and constant field current I_f. This characteristic is also called as performance characteristic or also voltage regulating curve.