Electrical Machines
A DC shunt machine generates 250 V on open circuit at 1000 RPM. Armature resistance is 0.5 Ω, field resistance 250 Ω. When running as a motor takes 4 A at 250 V. Calculate the efficiency when running as a motor taking 40 A at 250 V. Armature reaction weakens the field by 4%.

 67.5%
 78.5%
 82.5%
 72.5%

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Electrical Machines
Armature reaction in a DC machine

  makes an angle of 90° with the main pole axis.
 makes an angle with the main pole axis which is load dependent.
 is in the same direction as the main poles.
 is in direct opposition to the main poles.

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Electrical Machines
Consider the following statements regarding the speed control of dc motors : 1. ward-leonard method is suitable forconstant-torque drives 2. Ward-leonard method is suitable for constant-power drives 3. field-control method facilitates speed control below base speed 4. Armature-resistance control method is more efficient than ward-leanard method 5. Field-control method is suitable for constant-torque drives 6. Armature-resistance control method is suitable for constant-torque drives From these, the correct answer is

 1, 3, 5
 1, 2, 5
 1, 2, 6
 1, 4, 6

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