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

 5.7 Ω
 1.9 Ω
 2.2 Ω
 6 Ω

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Electrical Machines
The synchronous motor can be started by

  providing damper winding in its rotor circuit.
  coupling it with a DC compound motor.
  coupling it with a small AC induction motor called the pony motor.
  any of the above methods.

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