ABSTRACT
The use of permanent magnets (PMs) in construction of electrical machines brings the following benefits:
• no electrical energy is absorbed by the field excitation system and thus there are no excitation losses which means substantial increase in efficiency,
• higher power density and/or torque density than when using electromagnetic excitation,
• better dynamic performance than motors with electromagnetic excitation (higher magnetic flux density in the air gap),
• simplification of construction and maintenance, • reduction of prices for some types of machines. The first PM excitation systems were applied to electrical machines as early as the 19th century, e.g., J. Henry (1831), H. Pixii (1832), W. Ritchie (1833), F. Watkins (1835), T. Davenport (1837), M.H. Jacobi (1839) [35]. Of course, the use of very poor quality hard magnetic materials (steel or tungsten steel) soon discouraged their use in favor of electromagnetic excitation systems. The invention of Alnico in 1932 revived PM excitation systems; however, its application was limited to small and fractional horsepower d.c. commutator machines. At the present time most PM d.c. commutator motors with slotted rotors use ferrite magnets. Cost effective and simple d.c. commutator motors with barium or strontium ferrite PMs mounted on the stator will still be used in the forseeable future in road vehicles, toys, and household equipment.
