Dc Motor

Step 1: Determine total resistance required at minimum current.
Minimum desired current is

Using Ohm’s law,

Step 2: Calculate maximum rheostat resistance.
Load resistance is

Hence, rheostat resistance required is

Step 3: Determine total resistance required at maximum current.
Maximum desired current is


Step 4: Check rheostat range.
At maximum current, rheostat resistance is

Thus, the rheostat must be capable of varying resistance approximately between and .
Step 5: Select nearest standard option.
From the given choices, the suitable rheostat is

Step 6: Current rating justification.
The rheostat must safely carry the maximum current of , hence a rated rheostat is appropriate.

Step 1: Write the emf equation of a DC generator.
For a DC generator, the generated emf is directly proportional to flux per pole and speed.

Step 2: Form the ratio of emfs under two operating conditions.

Step 3: Substitute the given values.

Step 4: Simplify the equation.


Step 5: Calculate percentage increase in flux.

Step 6: Conclusion.
The flux per pole must be increased by

Step 1: Calculate the back emf of the motor.


Step 2: Convert speed from rev/s to rad/s.

Step 3: Calculate the mechanical power developed.

Step 4: Calculate the torque developed.

Step 5: Select the nearest correct option considering rounding and practical motor losses.