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
 2 .1 Ω
 1.79 Ω

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Electrical Machines
The mmf produced by rotor currents of a 3 - Φ induction motor

  rotates at synchronous speed with respect to rotor.
  is at standstill with respect to stator mmf.
  rotates at slip speed with respect to stator mmf.
  rotates at the speed of rotor in the air gap.

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Electrical Machines
In a DC machine the armature mmf is

 rotating with respect to field poles.
  stationary with respect to armature.
  stationary with respect to field poles.
  rotating with respect to field poles as well as armature.

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