If the field of a DC shunt motor is opened while running, what will happen to the speed?

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ALP CBT 2 Electrician Previous Paper: Held on 22 Jan 2019 Shift 1
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  1. Speed of the motor will remain constant
  2. The motor will get locked
  3. Speed of the motor will reduce
  4. Speed of the motor will become dangerously high

Answer (Detailed Solution Below)

Option 4 : Speed of the motor will become dangerously high
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In a DC shunt motor, \({E_b} \propto N\phi\)

For a constant back emf, flux is inversely proportional to the speed of the motor.

If field winding is disconnected accidentally, the speed would dangerously increase in order to maintain the back emf of the motor. For a constant back emf, flux is inversely proportional to the speed of the motor.

In the case of a series machine if field winding disconnected, then the motor circuit is open and no current will pass through the armature, for the operation of machine interaction of two fluxes is necessary thus the machine won't operate.

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