No experience with the s28 but i do know they dont like 48v. If its like the c40 300 mag then it sounds like you have over volted the s28 and possibly burnt out the brush springs which could result in arcing and serious damage. taking out the brush and filing it has worsened the problem and bang goes your motor..........i say all this as a complete guess. without looking at the motor its hard to give a solution to whats happened.

Personal advice would be to not use a mag motor and use something that will give you the rpm at the right voltage....maybee a little more but not double.

Others are more knowledgable in this field and i am sure will post.

regards

Shane