What Rain said above is true. Bad foot controllers can get very HOT. I was using a metal Singer controller as a substitute for the Bakelite controller on my newly acquired Mrs. Crinkle the other day. Plugged it in ran the machine for a bit then walked away to do something. Came back less in less than an hour and that controller was almost hot.
They can be adjusted, so I did. All better now, but even so it's best not to leave sewing machines plugged in.
Joe