How does magnetic flux affects the speed of a DC motor
How does magnetic flux affects the speed of a DC motor


It's all about how does current get to flow in the rotor...

Scenario 1

You have high flux and the rotor is spinning constantly at a moderate speed on no load. The rate of change of flux (\$\frac{d\Phi}{dt}\$) seen by the rotor is such that \$\frac{d\Phi}{dt}\$ induces an equal and opposite voltage in the rotor coil to that which is coupled from the battery via the commutation. This means that virtually no current flows into the rotor because, no torque is needed as speed is constant and there is no mechanical load.

Scenario 2

If the magnets then move further away, in order to induce that same equal voltage, the rotor has to speed up. In other words \$\frac{d\Phi}{dt}\$ becomes maintained once the rotor has increased its speed. The rotor has no choice but to speed up because the induced voltage is not equal to the applied voltage and hence there is a significant current flow and that creates an acceleration torque.

So, the rotor speeds up because a higher current is drawn (via the commutation brushes) and that creates acceleration. Once in equilibrium (\$\frac{d\Phi}{dt}\$ is back to where it was) the induced voltage and applied voltage are equal but, of course, now, the rotor is spinning faster.

Friction

To overcome friction, there must be a constant torque generated by the rotor and, that means that the induced voltage is not quite what the applied voltage is. This causes current to flow into the rotor via the commutation.

Mechanical load

To overcome mechanical load (and friction) there must be a bigger torque generated by the rotor and that means that the induced voltage is a little more not quite what the applied voltage is.