A DC series motor with negligible series resistance is running at a certain speed driving a load, where the load torque varies as cube of the speed. The motor is fed from a 400 V DC source and draws 40 A armature current. Assume linear magnetic circuit. The external resistance, in , that must be connected in series with the armature to reduce the speed of the motor by half, is closest to

A  DC shunt motor has  armature resistance including brushes. The motor draws a no-load current of . The field current is constant at . Assuming that the core and rotational losses are constant and independent of the load, the current (in amperes) drawn by the motor while delivering the rated load, for the best possible efficiency, is ________ (rounded off to 2 decimal places).

The four stator conductors  and  of a rotating machine are carrying  currents of the same value, the directions of which are shown in the figure (i). The rotor coils  -  ' and  -  ' are formed by connecting the back ends of conductors '  ' and '  ' and '  ' and '  ', respectively, as shown in figure (ii). The e.m.f. induced in coil  -  ' and coil  -  are denoted by  and , respectively. If the rotor is rotated at uniform angular speed  in the clockwise direction then which of the following correctly describes the  and  ?

fig. (i) cross-section view fig. (ii) rotor winding connection diagram

A separately excited DC motor rated  A, 1500 RPM drives a constant torque load at rated speed operating from  supply drawing rated current. The armature resistance is 1.2Ω . If the supply voltage drops by  with field current unaltered then the resultant speed of the motor in RPM is __________ (Round off to the nearest integer).