A dc series motor driving and electric train faces a constant power load. It is running at rated speed and rated

A dc series motor driving and electric train faces a constant power load. It is running at rated speed and rated voltage. If the speed has to be brought down to 0.25 p.u. the supply voltage has to be approximately brought down to 

(A) 0.75 p.u (B) 0.5 p.u (C) 0.25 p.u (D) 0.125 p.u
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A dc series motor driving and electric train faces a constant power load. It is running at rated speed and rated voltage. If the speed has to be brought down to 0.25 p.u. the supply voltage has to be approximately brought down to 0.5 p.u

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