Why a shunt motor cannot be used as an universal motor?

Answer 1

the shunt field winding is usually very low current

if you change polarity of the field magnet 60 times a second you must beef up the winding considerably to handle the magnetizing/demagnetizing current

and it usually involves not just the poles but the entire motor housing

poles on dc motors are usually solid and must be laminated for ac so the entire outside must be laminated also

since the series motor the field winding takes full armature current its already pretty heavy windings

it will run on AC it just gets a lot hotter and has less torque

and a whole lot hotter if the poles have solid iron supports or case

universal motors run a lot better on DC too