Question

In DC motor, flux is directly proportional to armature current.

Hint:

Remember the distinct behaviors of DC motors:

• DC Shunt Motor: Flux is constant ($\Phi \approx \text{const}$), so Torque is linear with current ($T \propto I_a$).
• DC Series Motor: Flux is variable ($\Phi \propto I_a$ before saturation), so Torque is quadratic with current ($T \propto I_a^2$).

Answer 1

Step 1: Analyzing DC Shunt Motors:
In a DC shunt motor, the field winding is connected directly across the constant input voltage source. The shunt field current ($I_{sh} = V/R_{sh}$) remains constant regardless of changes in load. This means the field flux ($\Phi$) remains constant and is independent of the armature current ($I_a$).

Step 2: Analyzing DC Series Motors:
In a DC series motor, the field winding is in series with the armature winding. This means the armature current flows directly through the field windings ($I_{se} = I_a$). Here, the flux is proportional to the armature current ($\Phi \propto I_a$) only up to the point of magnetic saturation. Beyond saturation, the iron core of the pole pieces cannot absorb more flux, and the flux ($\Phi$) remains practically constant despite any further increases in $I_a$.

Step 3: Conclusion:
Since flux is constant in shunt motors and only partially proportional to armature current in series motors before saturation, the statement cannot be applied as a general rule. Therefore, it is False.