Question

State whether true or false
If the flux of DC motor approaches zero, its speed will also approach zero.
- True
- False

Answer 1

The speed of an DC motor is inversely proportional to the magnetic flux cutting through its armature core.
I.e. $N \propto \dfrac{1}{\phi }$
Where, N is the speed of the motor in rpm and $\phi$ is the magnetic flux through it.
So, if we increase the flux, the speed of the motor will decrease and vice-versa.
Therefore, when flux approaches zero speed of the motor will approach infinity.

Complete step by step answer:
A DC motor is an electromechanical device which converts electrical energy to mechanical energy. It works on the principle of force experienced by a current carrying conductor in a magnetic field.
Which is given by: $\overrightarrow F = I\left( {l \times \overrightarrow B } \right)$, where $I$ is the current through the conductor and l its length and B is magnetic field across it.
Now, there is an armature winding in a DC motor which is placed in a magnetic field which leads to develop force in two opposite directions at the arms of winding and a couple is created, starting the rotatory motion.
Now, speed of a DC motor is given by following relation:
$N = K\dfrac{{{V_{winding}}}}{\phi }$
Where, N is the speed of the motor in rpm.
K is a constant depending on the physical parameters of the motor.
${V_{winding}}$ is the potential drop across the winding.
And $\phi$ is the magnetic flux across the armature.
$\therefore N \propto {V_{winding}}{\text{ or }}N \propto \dfrac{1}{\phi }$
Therefore, by increasing or decreasing ${V_{winding}}$, the speed of DC motors increases or decreases respectively.
And by increasing or decreasing flux, the speed of the DC motor decreases or increases respectively.
Hence, when flux approaches zero, the speed of the motor approaches infinity.
Therefore, the given statement is false.

Note:
Speed of a DC motor can also be regulated by:
By varying armature circuit resistance: As the armature resistance is increased the speed of the motor decreases and vice-versa.
By varying the applied voltage: As the applied voltage increases the speed of the motor increases and vice-versa.