What happens if a DC motoris connected to AC supply?

If a DC motor is connected to an AC supply, the motor will not rotate properly.

The motor will either move in a to and fro motion or apply a break on the rotor.

The coil will generate heat and may burn if the supply is applied continuously. This is because the AC motor has a pair of slip rings that do not change the direction of the current.

The rotating magnetic field that drives the motor will not be generated. However, if the field winding is connected in series with the armature winding, the motor is like a universal motor and will probably operate normally.

A universal motor is similar to a DC motor with the armature connected in series with the field. If DC is supplied it works like any DC series motor.

If AC is supplied, the stator and rotor field strengths will vary exactly in phase with each other.

When the AC current changes direction, the stator and rotor fields will reverse at the exact same instant.

Therefore, the torque will always be in the same direction even though the fields change direction because the stator and rotor fields always have the same relationship with each other.

When connected to AC, torque magnitude pulses at twice the line frequency.