On NO-LOAD condition the current drawn by the motor is small.
θ ∝ I
As a result flux produced is also small
And as we know from emf equation of dc motor- Eb= ΦP NZ/60A N= 60AEb/ΦPZ
As we keep Eb constant N ∝ 1/ Φ
So on NO-LOAD condition speed of dc series motor will increase dangerously, which will give mechanical harm to motor.
So its advised to run dc motor on LOAD.