Originally Posted by Electric Machinery Fundamentals by Chapman Flux weakening c

Question

Originally Posted by Electric Machinery Fundamentals by Chapman

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?

How the increased load will result in more flux weakening?

Thanks.

Explanation / Answer

as the load increases armature current decreases
hence IaRa value decreases speed increases from formula
N=K(V-IaRa)/

As the load increases back EMF increases

this back EMF will result in the generation of flux in the armature

hence the resultant flux decreases

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