A 15 kW, 250 V, 80A, 1000 rpm d.c series motor has the following full load losses 

Armature circuit ohmic loss (including brush loss) = 600 W

Rotational Loss = 400 W

Field ohmic loss = 400 W

Neglect armature reaction and saturation and assume the rotational losses to remain constant. If the motor draws three forth of the rated current at rated voltage then the speed of the motor in rpm is____