If a shunt motor running with a load has its shunt field opened, how would this affect the speed of the motor?

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Multiple Choice

If a shunt motor running with a load has its shunt field opened, how would this affect the speed of the motor?

Opening the shunt field collapses the magnetic flux in a DC shunt motor. With much less flux, the speed must rise to generate enough back electromotive force to oppose the applied voltage, so the motor speeds up. Because torque is proportional to flux times armature current, the torque capability drops as the field weakens, so the loaded motor tends toward very high speed until some limit or protection stops it. In short: the speed increases rapidly when the field is opened.

Opening the shunt field collapses the magnetic flux in a DC shunt motor. With much less flux, the speed must rise to generate enough back electromotive force to oppose the applied voltage, so the motor speeds up. Because torque is proportional to flux times armature current, the torque capability drops as the field weakens, so the loaded motor tends toward very high speed until some limit or protection stops it. In short: the speed increases rapidly when the field is opened.

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