If ac power is applied to the field winding, the inductive reactance causes high impedance. This impedance in an ac circuit has the same effect as high resistance. The result is a low current in the winding producing a weak magnetic field and low torque on the armature. As a result, the motor will not run efficiently.
As the motor is DC so the field and armature winding are made for DC supply. The winding which is made for DC always has more resistance than the winding which is made for AC. So, when we give AC supply self-inductance will be produced in the winding which opposes the current which further decreases the production of flux. For this reason, the motor will be run at a very less speed on the AC supply. When DC motor Runs on an AC supply more loss and heat will occur in the motor.
But one thing remembers is that separately excited or permanent magnet DC Shunt motor will not run if the field is connected with DC Supply The motor will vibrate.
If a DC series motor is connected to an ac supply same current will flow through the series field and armature field after being rectified by the commutator. So, the machine will run but with high sparking at the brush, poor pf, low speed, and low efficiency.