I am getting illegal characters when I converted the data type of name column

This is quite a frequent issue in data conversion. The probable reason can be-

You might be trying to convert some string related columns to integer or decimal. What that means is, when we convert data to a data type that can not store that data then some garbage value can result.

For example, text column to Integer, Date related column to decimal, etc.

Please check if you're not doing it. You may convert an integer related data to string but vice versa is not true.