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

                                      E_L =  – L ( di / dt )

                                      E_L =  – L ( 2I_a / T )

Where

L = Self inductance of the coil and

2I_a = Change in current during commutation

This emf E_L 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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