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## 07/10/2020

### 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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