DC series motor started at no load: true/false

False. A DC series motor must never be started (or run) at no load.

Justification:

• The series field carries the armature current, so $\Phi\propto I_a$. At no load $I_a$ is very small, so the flux is very small.
• Speed $N\propto E_b/\Phi$, and $E_b\approx V$, so as $\Phi\to$ residual value the speed rises to a very high, dangerous value (racing).
• The excessive centrifugal force can damage the armature winding, commutator and bearings.
• Torque $T\propto I_a^2$ is small at no load, so there is no load to restrain the speed.

So the series motor should always be started with a load coupled directly or through gears, never with a belt (the belt may slip or break).