07/10/2020

What is meant by Reactance Voltage?


  • The rate of change of current during commutation period is from + Ia to – Ia. 
  • We have taken it + 25 Amp to – 25 Amp.
  • The rapid reversal of current in the armature coil is delayed as a result coil undergoing commutation do not in a position to attain full value of current by the end of short circuit.   
  • The value of self induced emf in the coil during commutation is given by

                                      EL =  – L ( di / dt )

                                      EL =  – L ( 2Ia / T )

Where

L = Self inductance of the coil and

2Ia = Change in current during commutation

This emf EL is called as reactance voltage.

Type of DC Machines

Average Reactance voltage

Non – inter polar machines

< 2 to 3 Voltage

Inter polar machine

< 15 V

 

Effect of reactance voltage

  • Delay change in current from positive to negative in the short circuit coil
  • Current will not reach its ideal value ( - 25 A ) at the end of short circuit period and its value may be say 20 A therefore current short fall by 25A  – 20 A , when trailing edge of the brush leaves its segment which result in sparking at the commutator surface.
  • Current versus time graph is not linear but curve as shown in the figure.





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