(a) Suppose the windings og the armature in the d.c. motor or Example 4 connot tolerate a current of more than 20 amp. WhaT do you think will happen if the armature gets jammed and cannot rotate when the motor is connected to the supply ?
(b) If the supply connection of the d.c. motor in Ex. are removed and the motor is used as a generated by connecting the shaft of its armature to an external mechanical rotor of speed 3000 r.p.m., how much e.m.f. will be generated ?

Explanation

(a) When the armature gets jammed and cannot rotate, back e.m.f. `E = 0`.
`:.` When connected to the supply.
current `I = (V)/(R ) = (200)/(8.5) = 23.53` amp.
Which exceeds the safe limit of 20 amp. Therefore. Armature will burn out.
(b) The e.m.f. generated is obviously equal to the back e.m.f. `E = 157.5` volt, as calculate in Example 8.