Electrical Machines
For a 3-phase, 4 pole, 50 Hz synchronous motor the frequency, pole number and the frequency are halved. The motor speed will be

 3000 rpm
 75 rpm
 375 rpm
 1500 rpm

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Electrical Machines
Armature reaction in a DC machine

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

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