Answer:
Because the shunt winding consist of a large number of turns,
Step-by-step explanation:
It is nearly impossible to obtain satisfactory performance from a shunt motor connected to an ac power source because the shunt winding consist of a large number of turns, due to the high number of turns that the DC shunt motor has it develops a high impedance when connected to an ac power source. and due to this high impedance the amount of current that flows through the field will be very low making it nearly impossible for the shunt motor to operate properly