Electrical Machines
A 220 V, DC shunt motor is operating at a speed of 1440 rpm. The armature resistance is 1.0 Ω and armature current is 10 A. of the excitation of the machine is reduced by 10%, the extra resistance to be put in the armature circuit to maintain the same speed and torque will be

 3.1 Ω
 1.8 9 Ω
 1.79 Ω
 2 .1 Ω

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Electrical Machines
In a synchronous motor when the rotor speed exceeds the synchronous speed during hunting

  the damper bars develop induction generator torque.
  the damper bars develop synchronous motor torque.
  negative sequence currents are generated.
  the damper bars develop induction motor torque.

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