Does weakening shunt filed current at any one load setting t

Does weakening shunt filed current, at any one load setting to maintain a constant speed, increase or decrease armature current? WHY?

Solution

Field weakening (decreasing the field current in a shunt wound DC motor) decreases the field flux, which decreases the back EMF generated as the armature spins through the field. Since a DC motor can\'t run any faster if the back-EMF equals the applied motor voltage, decreasing the field allows the motor to run faster by decreasing the back-EMF at any given speed. To a large degree the achievable speed is inversely proportional to field current. So if the motor can run up to 2000 RPM with full rated field current it can run up to 4000 RPM with 1/2 the rated field current applied. There are other loss mechanisms that may prevent getting all the way there, however.

However, a DC motor genrates torque due to the interaction of the shunt field and the armature field. If you decrease the shunt field current the armature current must increase to maintain the same amount of torque. So for a given load there is a practical limit to the amount you can decrease the field current.