Question

why commercial motors use electromagnet in place of a permanent magnet?

Answer 1

Use of electromagnet in place of permanent magnets in motors has lots of advantages.
1. Electromagnets are stronger than permanent magnets in terms of strength of magnetic field. Also the strength of the field can be altered as per necessity.
2. Electromagnets do not have to scope of losing magnetic strength until current is stopped. A permanent magnet loses its magnetic strength with time.