If we start DC series motor with out load the speed increases which in turn increases back EMF(proportional to speed) but due to increase in back EMF armature current decreases. Since field winding is in series the flux also decreases due to decrease in the armature current. We know that flux is inversely proportional to speed, so decrease in flux causes increase in speed. This process continues.
Theoretically speed reaches to infinity as flux tends to zero.
Practically due to high speeds the bearings,armature shaft,etc get damaged.
So keep in mind never start a DC series motor at no load.
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