Electrical Machines
A 240 V DC series motor takes 40 A when giving its rated output at 1500 rpm. Its resistance is 0.3 ohms. The value of resistance which must be added to obtain rated torque at 1000 rpm is

 6 ohms.
 5.7 ohms.
 1.9 ohms.
 2.2 ohms.

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Electrical Machines
The speed of a DC series motor is

 Proportional to field current.
 Proportional to the armature current.
 Proportional to the square of the armature current.
 Inversely proportional to the armature current.

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