Explain what happens when a d.c. motor is connected across an a.c. supply ?

DC Motor

1. Since on a.c. supply, reactance will come into the picture, the a.c. supply will be offered impedance (not resistance) by the armature winding.  Consequently, with a.c. supply, current will be much less.  The motor will run but it would not carry the same load as it would on d.c. supply.
2. There would be more sparking at the brushes.
3. Though motor armature is laminated as a rule, the field poles are not. Consequently, eddy currents will cause the motor to heat up and eventually burn on a.c. supply.

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