Where does a permanent magnet have an advantage over an electromagnet?
Generally, the volume of space needed to produce a given static field will be less for permanent magnets when the working gap is small; electro magnets win in larger devices. However the economic cross over inches up each time a higher energy PM material becomes available. Electromagnet limiting factors are the space consumed by the windings and the power supply, and the heat generated during operation. Permanent magnets do not require a power supply, so they are space and energy efficient. An adjustable power supply allows the magnetic field of an electromagnet to be adjusted easily by simply adjusting the input current. However, adjustable permanent magnets can be used if the field does not have to be adjustment frequently.
