Question

Why does a dc motor sometime spark on light load?

Answer 1

I didn't get how much the load is. DC motors as others motors have coil which are magnatized and the field produce interact with the other field of coil or Permanent magnet (stator) and thus produce torque and rotation. when you cease the rotor the coil will become fully magnitized. And as its said Inductor are short circuted when magnitized. the coil will behave as inductor and produce spark when you conect them with battery. Also the charging take place very fast and the strall current is very high.
cheers !!!