The reason you're confused is because you're looking at the API reference, not LINQ's documentation . You'd have the same problem with any language, any API.
The assignment text says nothing about multiple classes:
Compare list of "existing Database Motors" to list of "New Excel Motors" by comparing the name property of each list only.
If you were comparing Motor objects you'd only need a single Motor class. In this case though you're comparing only names. You're checking to find the Motor objects whose names don't appear in a List of new motor names.
Except isn't useful here. As the description says :
Produces the set difference of two sequences.
This isn't idle wording. Just like SQL's EXCEPT the type/shape of the two sets must be the same. That's why the method only works with items of the same type.
What you want can be done using Where and List<string>.Contains

List<Motor> motors=ReadDatabase(DBpath.txt); 
List<string> new_names=ReadExcel(...);

var old_motors=motors.Where(m=>!new_names.Contains(m.Name))
                     .ToList();

You can speed this up if you use a HashSet

...
var names_set=new HashSet<string>(new_names);
var old_motors=motors.Where(m=>!names_set.Contains(m.Name))
                     .ToList();

如果我没理解错的话，你能不能通过将每个电机的名称与excel电机名称列表进行比较来过滤数据库电机，并返回没有匹配项的条目？

List<motor_Database> dbMotors = ReadDatabase(DBpath.txt); 
List<motor_Excel> xlMotors = ReadExcel(Excelpath.txt);

List<motor_Database> uniqueDbMotors = dbMotors
  .Where(dbMotor => xlMotors.All(xlMotor => xlMotor.MotorName != dbMotor.iName))
  .ToList();

此外，在“选项”列表中，您省略了使用一个简单的for循环。上面的代码可以写为：

var uniqueDbMotors = new List<motor_Database>();

foreach(var dbMotor in dbMotors)
{
    var foundMatch = false;

    foreach (var xlMotor in xlMotors)
    {
        if (dbMotor.iName == xlMotor.MotorName)
        {
            foundMatch = true;
            break;
        }
    }

    if (!foundMatch) uniqueDbMotors.Add(dbMotor);
}

您也可以使用“LEFT JOIN”来执行下列工作：

List<motor_Database> result = 
    (from ex in equipList_EX
    join up in equipList_Update on ex.iName equals up.MotorName into gj
    from up in gj.DefaultIfEmpty()
    where up == null
    select ex).ToList();