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Consider the following statements regarding the starting of a dc shunt motor: a. External field resistance should be minimum b. External field resistance should be maximum c. Reduced voltage should be applied to the armature d. Increased voltage should be applied to the armature Which of the above statements are correct?

Consider the following statements regarding the starting of a dc shunt motor: a. External field resistance should be minimum b. External field resistance should be maximum c. Reduced voltage should be applied to the armature d. Increased voltage should be applied to the armature Which of the above statements are correct? Correct Answer a and c are correct

At the time of starting, reduced voltage should be applied to the armature to limit the starting current.

The armature resistance is kept at maximum position to control and reduce the high starting current and field rheostat at minimum position to ensure that speed does not go beyond rated value and to have high starting torque.

Important Points:

At starting, the field excitation of a dc shunt motor is kept at its maximum value due to following reasons.

  • As field current is kept maximum permissible value, the armature current during starting would be minimum for a given load torque. This minimum starting current would further result improved commutation during motor and load acceleration i.e. in other words, sparking at brushes and voltage drop in supply reduced.
  • A large field current would result in low operating speed. Consequently the time required from stand still to this low speed is less and, therefore there is less heating of the armature during starting.

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