# In field controlled dc motor, why does the armature current remain unchanged even if it is affected by field flux?

Basically in a field controlled dc motor, the armature current is kept constant (via some constant voltage source) while the field current is varied which will vary the field flux and interacts with the armature of motor and produce rotation. But why does the armature current not change when flux interaction is there with the armature and voltages are flowing around the motor. How does the armature current remain constant. Does the speed of rotor changes to bring the armature current back to the constant value?? What causes the armature current to not change from its constant value?

• The armature current doesn't change because it's supplied by a constant current source.
– Chu
Aug 20 '15 at 7:19
• But for instance assuming that the constant current source supplies 5A and now the field flux acts on the armature and armature current rises due to the field flux and now assume it is 7A. So the armature current is not constant now. So how is this problem countered Aug 20 '15 at 12:05
• If the current changed from 5A to 7A, it wouldn't be constant, would it? Constant current means the current is fixed! The current cannot vary from it's set value, no matter how the load changes. So the armature flux will be a constant magnitude no matter what happens to the flux generated by the field windings.
– Chu
Aug 20 '15 at 13:24
• It may help if you first consider a constant voltage source of, say, 10V. If a $100\Omega$ resistor is connected across the source the current is $0.1A$; if a $1\Omega$ resistor is connected the current is $10A$. So the voltage is constant regardless of the load current. Now consider a constant current source of, say $5A$. If a $1\Omega$ resistor is connected the current will be $5A$ and the voltage across the resistor will be 5V; if a $100\Omega$ resistor is connected the current will be $5A$ and the voltage will be 500V. So the voltage varies to maintain the rated current.
– Chu
Aug 20 '15 at 13:49

It's all embedded in Faraday's induction equation: $$\\text{emf} = -N\dfrac{d\Phi}{dt}\$$. In other words, to make emf the right value to drive load and friction, the motor can run slower when the flux is higher because, what is lost in the rate of change of flux is regained by the flux actually being higher.