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

 2 .1 Ω
 1.79 Ω
 3.1 Ω
 1.8 9 Ω

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Electrical Machines
The 3 - Φ induction motors are, by a very considerable margin, the most widely used AC motors as

  these are cheaper in cost, simple and rugged in construction.
  they have very good speed regulation.
  they have maximum efficiency and maximum starting torque.
  they are capable of operation under range of leading and lagging pf.

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