Speed/field current relationship of a dc machine

[cabellos]

Im having trouble explaining this relationship...from my graph i know that if armature voltage is held constant then the field current will decrease exponentially for increasing speed. But why?

 

[Cliff_J]

The windings have inductance and as you increase the speed the more that inductance plays a role in the impedance.

 

[ravenprp]

The speed/field current relationship of a DC machine is a crucial aspect in understanding the behavior and performance of the machine. This relationship can be explained by the basic principles of electromagnetism.

In a DC machine, the field current is responsible for creating a magnetic field in the stator, which interacts with the armature current to produce torque and rotation. The speed of the machine is directly proportional to the armature voltage, which is held constant in this scenario.

Now, as the speed of the machine increases, the back EMF (electromotive force) also increases. This back EMF opposes the armature voltage and reduces the net voltage across the armature. As a result, the armature current decreases, and so does the torque produced by the machine.

Since the torque is reduced, the required field current to maintain a constant speed also decreases. This is because the torque produced by the machine is directly proportional to the product of armature current and field current. Therefore, as the armature current decreases, the field current also decreases exponentially.

In simpler terms, the decrease in field current is a consequence of the decrease in armature current, which is caused by the increase in back EMF at higher speeds. This relationship is crucial in controlling the speed and torque of a DC machine and ensuring its stable operation.

In conclusion, the speed/field current relationship in a DC machine can be explained by the basic principles of electromagnetism, where the increase in speed leads to an increase in back EMF, which in turn decreases the armature current and subsequently the required field current to maintain a constant speed.