Question

State whether true or false- The speed of a DC series motor is directly proportional to armature current.
A. True
B. False

Answer 1

Remember that in a DC series motor, $N \propto \dfrac{{{E_b}}}{\phi }$ . Further, flux is directly proportional to armature current. Trace back the relations and you will get the exact relation between speed of the DC series motor and the armature current.

Complete step by step answer: The given statement is false.
The speed of a DC motor is inversely proportional to the armature current.
The armature current is given by :
${I_a} = \dfrac{{(V - \omega {K_g})}}{{{R_a}}}$
Where, ${I_a}$ = armature current
${R_a}$ = armature resistance,
${K_g}$ = Motor generator constant,
$\omega$ =Motor speed in radians/second,
V=line voltage.
As the motor turns faster, the armature current decreases. If the motor has no load or friction, the motor will speed up to the point that $V = \omega {K_g}$ .
${K_g}$ is proportional field current, so with no load or friction, motor speed varies inversely with field current. In the motor, the field current and armature current are the same, hence, motor speed varies approximately inversely with armature current.
And the answer is FALSE.

Note: One should remember that DC motors can be of different types depending upon the alignment of coil with the rotor. The major two types of DC motor series wound DC motor and shunt wound (parallel) DC motor. The DC shunt motors are self-regulatory i.e. there is no much change in both speed and flux across them. So, in a shunt DC motor speed remains constant, while in DC series motor speed is variable.