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What will happen when field circuit gets opend,while the machine is running?(its related to OCC of DC Shunt Motor)

What will happen when field circuit gets opend,while the machine is running?(its related to OCC of DC Shunt Motor)

Grade:12th pass

1 Answers

Saurabh Kumar
askIITians Faculty 2411 Points
6 years ago
The DC motor will continue its rotation if it is at NO LOAD. If the load is more than the no load ( torque required to overcome friction and windage losses) condition the motor stops and a chance of high current flow may damage the winding if the voltage applied is high (Close to rated voltage if it is there).
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As mentioned above if the field circuit is opened, the back emf will drop to zero. With full armature voltage applied this will cause a very large armature current to flow which should normally blow the fuses. While the field current decays the motor will speed up as if in a field weakening mode.

The motor will try to speed up to develop a back-emf and with no field it requires a very large armature current. That is why one never starts a shunt motor without the field winding excited. Normally the armature fuses just blow as the current drawn can be very high depending upon the applied armature voltage. In this case the motor will accelerate and run away until the fuses blow.

If you have a large armature power supply so the fuses did not blow, on light load the motor would speed up and self destruct due to centrifugal forces. Similar to a DC series motor when the load is disconnected from the shaft. There is enough torque produced due to remnant flux in the field and a large armature current to accelerate the motor. If there is a large load it will decelerate but the armature current will rise to Varm/Ra..

Good Luck......................

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