Electrical Engineering Electrical Machines Lab Experiments

Load Test On Seperately Excited DC Motor

Procedure

  1. Close the DPST (Double Pole Single Throw) switch connecting supply to field winding of the Separately Excited DC motor.
  2. Increase the Separately Excited DC motor field voltage to rated value (220V).
  3. Use variable voltage device to increase or decrease the supply
  4. Close the DPST switch connecting supply to armature winding of the Separately Excited DC motor.
  5. Increase the motor armature voltage to rated value (440V).
  6. As Separately Excited DC motor is connected mechanically to Separately Excited DC generator, So both machines rotate with same speed. Therefore we can observe speed of generator in digital tachometer.
  7. Now increase the excitation of separately excited generator such that o/p of generator will be equal to rated value(220V).
  8. Now connect the load by closing the switch as shown in the figure
  9. Note the decrease in voltage of generator and increase in current of generator.As the load goes on increasing speed and armature voltage of the generator decreases.
  10. Store this data by clicking "Start Storing Data"
  11. So to keep the voltage of generator constant at load condition adjust the field excitation of generator.
  12. Now again increase the load by closing another load switch.
  13. Store this data by clicking "Start Storing Data"
  14. Go on repeating this procedure till all loads are connected and data is stored.
  15. Now display the data by clicking "Show data". For Ex
  • Vf=field voltage of DC motor
  • If=field current of DC motor
  • Va=armature voltage of DC motor
  • Ia= armature current of DC motor
  • Vfg= field voltage of generator
  • Ifg=field current of generator
  • Vt=terminal voltage
  • Iag=armature current of generator
  1. Now select one of the graphs from available graphs and use "Generate graph" to plot the graph.For Ex
  2. While switching off, reduce the generator excitation to some lower value and then remove the electrical load of generator.
  3. Make motor excitation equal to zero and then reduce motor armature voltage to zero.
  4. Speed of motor goes on decreasing and eventually becomes zero, then remove field excitation of motor.