Permanent Magnet DC Motors

July 18, 2011  |  Articles

Permanent magnet (PM) DC motors were introduced in the 19th century but did not earn widespread acceptance due to the poor quality of magnetic materials (e.g., steel and tungsten steel) that were then available. 1 So, early motor designers turned to electromagnetic field excitation, which became the standard until recently. Advances in magnetic technology, such as rare earth magnets, demonstrated improvements in a PM motor’s steady state performance and power density. As a result, the permanent magnet DC motor has seen broad adoption in today’s global marketplace. 2 PM motors are used by vendors of computer peripherals, office equipment, medical instruments, automobiles, robots and others. 3

Benefits

The benefits 4 of PM field-excited motors over electromagnetically-excited motors include:

  • Higher efficiency since no electrical energy is used or losses incurred for developing or maintaining the motor’s magnetic field.
  • Higher torque and power density.
  • Linear torque speed charcteristics. 5 that are more predictable.
  • Better dynamic performance due to higher magnetic flux density in air gap.
  • Simplified construction and essentially maintenance-free.
  • More compact size. 6

Construction

Permanent magnet DC motors are much more efficient, lighter and compact than comparably sized wound DC motors because the permanent magnets replace the field windings of wound DC motors. 7 PM DC motors are constructed in two broad categories: brushed/commutator and brushless. The PM DC commutator motor uses a rotating armature winding with a stationary field of permanent magnets; a PM DC brushless motor has a reverse construction: a rotating field of permanent magnets and a stationary armature winding that is externally commutated by an electronic control. 8 (Sales of permanent magnet DC commutator motors are steadily decreasing while sales of PM DC brushless motors are increasing due to the absence of brushes and the associated maintenance of the brushes and commutator. 9 Subsequent discussion in this article will refer to the PM DC brushless motor.)

The field PM magnets have two configurations: surface-mounted or interior-mounted. 10 Surface-mounted magnets are less expensive but are not suited to high speeds. Interior-mounted, also called flux concentrating machines, overcome the shortcoming 11 of surfaced mounted machines in terms of air gap flux density, harmonics shielding and, in some cases, structural integrity.

In the 19th century, magnets were made of iron but it was known that 12 alloys of copper, silver and gold made superior magnets. 13 In 1932, Alnico (alloy of AL, CU, Fe NI and Co) was developed and reawakened interest in permanent magnet field excitation. In the past 20 years, other magnetic materials have been developed: rare earth magnets, which are samarium-cobalt alloys and are the highest performing magnetic materials. 14 Rare earth magnets are expensive but their price is decreasing. 15 Another material is neodymium-iron-boron alloy, which performs 30 % better than samarium cobalt alloys. The only drawback of neodymium is its poor corrosion resistance; however, protective coatings have been developed to overcome this deficiency. 16

Ceramic (barium ferrite and strontium ferrite) magnet motors are widely used in the world today. 17 They have much higher coercive forces than alnico and are better able to resist demagnetization. 18

Characteristics

Permanent magnet DC motors have similar characteristics to DC shunt wound motors in terms of torque, speed, reversing and regenerative braking characteristics. 19 However, PM DC motors have starting torque several times that of shunt motors and their speed load characteristics are more linear and predictable. 20 Torque varies a lot with speed, ranging from maximum (stall torque at zero speed) to zero torque at maximum (no load speed). An increase in torque requires a decrease in angular velocity and vice versa.

Maintenance

Reduced maintenance is one of the primary advantages of permanent magnet DC motors over wound DC motors. 21 Since the commutator and brush assemblies of wound motors are not used in PM DC motors, all the maintenance and related-costs associated to servicing these motor components is eliminated. The maintenance amounts to cleaning, ensuring clear ventilation pathways and bearing replacements, as appropriate.

Applications

Permanent magnet DC motors have been used in power ranges from the milliwatts (mW) to megawatts (MW), 22 but are primarily known for fractional horsepower applications. Brushless DC motors are gaining the most market share. This is the result of advances in control electronics as well as PM quality. The automotive industry uses a large number of PM DC commutator motors, which can vary from a few in an inexpensive car to about 100 in a luxury car. PM brushless DC motors are now recognized as the best propulsion motor for electric hybrid road vehicles. 23 For industrial applications, permanent magnet DC motors are seeing market adoption in applications, such as pumps, fans, blowers, compressors, centrifuges, mills, hoists, handling systems, machine tools, servo drives, elevators, light railways, missiles, radar, satellites, dentist drills, electric wheel chairs, artificial heart motors and power tools. 24

  1. Gieras, Jacek F. and Wing, Mitchell. Permanent magnet motor technology: design and applications. Marcel Dekker, Inc. 2002. Page 1.
  2. Gieras, Jacek F. and Wing, Mitchell. Permanent magnet motor technology: design and applications. Marcel Dekker, Inc. 2002. Page 2.
  3. Gieras, Jacek F. and Wing, Mitchell. Permanent magnet motor technology: design and applications. Marcel Dekker, Inc. 2002. Page 16.
  4. Gieras, Jacek F. and Wing, Mitchell. Permanent magnet motor technology: design and applications. Marcel Dekker, Inc. 2002. Page 1.
  5. C. Elanchezhan, G. Shanmuga Sundar and et al. Computer Aided Manufacturing. 2nd ed. Laxmi Publications 2007. Page 226.
  6. C. Elanchezhan, G. Shanmuga Sundar and et al. Computer Aided Manufacturing. 2nd ed. Laxmi Publications 2007. Page 226.
  7. G. K. Dubey. Fundamentals of electrical drives. 2nd ed. Alpha Science International. 2001. Page 65.
  8. Peter Campbell. Permanent magnet materials and their application. The Press Syndicate of the University of Cambridge. 1994. Page 161.
  9. Gieras, Jacek F. and Wing, Mitchell. Permanent magnet motor technology: design and applications. Marcel Dekker, Inc. 2002. Page 1.
  10. H. Wayne Beaty, James L. Kirtley. Electric motor handbook. 2nd ed. McGraw-Hill. 1998. Page 237.
  11. H. Wayne Beaty, James L. Kirtley. Electric motor handbook. 2nd ed. McGraw-Hill. 1998. Page 237.
  12. Gieras, Jacek F. and Wing, Mitchell. Permanent magnet motor technology: design and applications. Marcel Dekker, Inc. 2002. Page 1.
  13. Hamid A. Toliyat, G. B. Kliman. Handbook of electric motors. 2nd ed. Marcel Dekker, Inc. 2004. Page 349.
  14. Gieras, Jacek F. and Wing, Mitchell. Permanent magnet motor technology: design and applications. Marcel Dekker, Inc. 2002. Page 2.
  15. Gieras, Jacek F. and Wing, Mitchell. Permanent magnet motor technology: design and applications. Marcel Dekker, Inc. 2002. Page 2.
  16. Hamid A. Toliyat, G. B. Kliman. Handbook of electric motors. 2nd ed. Marcel Dekker, Inc. 2004. Page 349.
  17. Hamid A. Toliyat, G. B. Kliman. Handbook of electric motors. 2nd ed. Marcel Dekker, Inc. 2004. Page 349.
  18. Hamid A. Toliyat, G. B. Kliman. Handbook of electric motors. 2nd ed. Marcel Dekker, Inc. 2004. Page 349.
  19. Seth Leitman. Build Your Own Plug-In Hybrid Electric Vehicle. McGraw-Hill. 2009. Page 94.
  20. Seth Leitman. Build Your Own Plug-In Hybrid Electric Vehicle. McGraw-Hill. 2009. Page 94.
  21. Sivanagaraju, S and et al. Power Semiconductor Drives. PHI Private Learning Ltd. 2009. Page 333.
  22. Gieras, Jacek F. and Wing, Mitchell. Permanent magnet motor technology: design and applications. Marcel Dekker, Inc. 2002. Page 16.
  23. Gieras, Jacek F. and Wing, Mitchell. Permanent magnet motor technology: design and applications. Marcel Dekker, Inc. 2002. Page 16.
  24. Gieras, Jacek F. and Wing, Mitchell. Permanent magnet motor technology: design and applications. Marcel Dekker, Inc. 2002. Page 16.

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