Underfrequency refers to a condition where the frequency of the AC supply drops to a value that is lower than its defined value such as 50 Hz or 60 Hz.  Underfrequency is usually caused by overloading a power source or problems with a prime mover such as engines or turbines.

The following are some of the effects of underfrequency

  • High flux density in electric machinery, thereby causing higher magnetizing currents.
  • High core loss and over-heating of the machines and possible failure.
  • Lower efficiency
  • Reduction in speed
  • The fault level increases due to reduced reactance

      Underfrequency situations can be avoided by the installation of underfrequency relays which isolate systems and machinery in the event of an underfrequency.