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Shunt DC motor - confusing - pls help!

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powersys

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Electric Machinery and Power System Fundamentals by Chapman said:
Flux weakening causes problems in both generators and motors. In generators, the effect of flux weakening is simply to reduce the voltage supplied by the generator for any given load. In motors, the effect can be more serious... when the flux in a motor is decreased, its speed increases. But increasing the speed of a motor can increase its load, resulting in more flux weakening. It is possible for some shunt DC motors to reach a runaway condition as a result of flux weakening, where the speed of the motor just keeps increasing until the machine is disconnected from the power line or until it destroys itself.
Would someone pls enlighten me why, when the speed of a motor increase, the motor load increases?
Is the load increase here due to the increase of friction and windage when speed increases?
How the increased load will result in more flux weakening?

Thanks.
 

E-design

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Weakening the flux will decrease the counter-emf Eo. This will cause the armature current to increase to a much higher value because the difference between Es (supply) and Eo. Only a small difference is needed for a great increase in armature current. The motor will accelerate due to the higher armature current until Eo ≈ Es again. It is important to understand that the motor will have a higher torque despite the weakened field. So for this reason allowing the flux to fall below safe levels will cause the motor to speed up to try and get Eo near Es again. This flux may be so low that the motor needs to speed up too much to get Eo near Es again that centrifugal forces can damage the motor by pulling windings out of the armature slots or seize the bearings or bushes. Normally a safety system prevents such high speed conditions.

Hope this explains your question
 

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